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Rodent activity in relation to moonlight in sandy and open habitats of the Great Basin Desert

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Rodents make foraging decisions by balancing energetic and reproductive demands with predator avoidance. To identify variations in the risk of predation, nocturnal rodents may use moonlight as an indirect cue of risk. Moonlight avoidance behaviors have been observed in many nocturnal rodents and are widely generalized to small mammals, but in no previous study has moonlight avoidance been evaluated in a systematic fashion in a naturally occurring community over several seasons. Here, a study in natural habitats was performed where the effects of moonlight on the activity patterns of desert rodents are examined in 62 separate localities across the Great Basin Desert of western North America from 1999 to 2006. Rodent activity is examined by live-trapping exclusively in sandy and open habitats throughout the Great Basin, using the presence of the sand-obligate kangaroo mouse (Microdipodops) as a habitat-indicator taxon. The activity patterns of this desert rodent community are assessed on 69 nights with clear skies (10,758 total trapnights) and examined in relation to corresponding values of moon phase and moon brightness to assess the frequency of moonlight avoidance. No relationship is found overall between the total abundance of the rodent community and moonlight. However, significant moonlight avoidance is revealed throughout the rodent community when nights of trapping around the new moon are removed from the analysis. Bipedal and quadrupedal rodent guilds within the rodent community are found to respond dissimilarly to moonlight; only bipedal rodents, but not quadrupedal rodents, display significant moonlight avoidance overall and during waxing moon phases. A further analysis during waxing moon phases indicates that quadrupedal rodents may in fact increase their activity on the same brightly moonlit periods avoided by bipedal rodents. Moreover, bipedal rodents are only found to avoid moonlight during the summer season, and not during the spring and fall seasons. Thus, moonlight avoidance may be an over-generalized phenomenon that actually occurs only in certain species and under specific circumstances. These results are discussed in the context of moonlight as a cue of predation risk, as well as the hypothesized antipredator adaptations of bipedal rodents.
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OCCIDENTAL COLLEGE
Los Angeles, California
RODENT ACTIVITY IN RELATION TO MOONLIGHT
IN SANDY AND OPEN HABITATS OF THE GREAT BASIN DESERT
by
Nathan S. Upham
A thesis submitted in partial fulfillment of the
requirements for the degree of Master of Arts in
Biology
August 2008
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The thesis of Nathan S. Upham is approved by:
____________________________________
John C. Hafner, Chairman
_____________________________________
H. Elizabeth Braker
_____________________________________
Donald R. Prothero
OCCIDENTAL COLLEGE
Los Angeles, California
Date __________________
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RODENT ACTIVITY IN RELATION TO MOONLIGHT
IN SANDY AND OPEN HABITATS OF THE GREAT BASIN DESERT
ABSTRACT
by Nathan S. Upham
Occidental College, 2008
Chairman: John C. Hafner
Rodents make foraging decisions by balancing energetic and reproductive demands with
predator avoidance. To identify variations in the risk of predation, nocturnal rodents may use
moonlight as an indirect cue of risk. Moonlight avoidance behaviors have been observed in
many nocturnal rodents and are widely generalized to small mammals, but in no previous study
has moonlight avoidance been evaluated in a systematic fashion in a naturally occurring
community over several seasons. Here, a study in natural habitats was performed where the
effects of moonlight on the activity patterns of desert rodents are examined in 62 separate
localities across the Great Basin Desert of western North America from 1999 to 2006. Rodent
activity is examined by live-trapping exclusively in sandy and open habitats throughout the Great
Basin, using the presence of the sand-obligate kangaroo mouse (Microdipodops) as a
habitat-indicator taxon. The activity patterns of this desert rodent community are assessed on 69
nights with clear skies (10,758 total trapnights) and examined in relation to corresponding values
of moon phase and moon brightness to assess the frequency of moonlight avoidance. No
relationship is found overall between the total abundance of the rodent community and
moonlight. However, significant moonlight avoidance is revealed throughout the rodent
community when nights of trapping around the new moon are removed from the analysis.
Bipedal and quadrupedal rodent guilds within the rodent community are found to respond
dissimilarly to moonlight; only bipedal rodents, but not quadrupedal rodents, display significant
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moonlight avoidance overall and during waxing moon phases. A further analysis during waxing
moon phases indicates that quadrupedal rodents may in fact increase their activity on the same
brightly moonlit periods avoided by bipedal rodents. Moreover, bipedal rodents are only found
to avoid moonlight during the summer season, and not during the spring and fall seasons. Thus,
moonlight avoidance may be an over-generalized phenomenon that actually occurs only in
certain species and under specific circumstances. These results are discussed in the context of
moonlight as a cue of predation risk, as well as the hypothesized antipredator adaptations of
bipedal rodents.
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ACKNOWLEDGMENTS
I would first of all like to thank Dr. J. C. Hafner who inspired me to undertake this
project on a clear full moon night after setting traps at one of the Great Basin localities described
here. The generous mentorship and intellectual guidance provided by Dr. Hafner throughout this
project and during my time at Occidental is deeply appreciated. I would like to thank Dr. H. E.
Braker for serving on my committee and being a helpful supporter of my academic goals at
Occidental College. I would also like to thank Dr. D. R. Prothero for graciously agreeing to
serve on my committee and providing a geosciences perspective to this project. Many thanks are
due to Dr. P. M. Hafner, E. Reddington, and C. W. Torres as important members of the
fieldwork team. This research was supported in part by the Nevada Department of Wildlife
(contracts 05-21 and 8-15 to John C. Hafner). Thanks to Dr. D. J. Webb for assistance with the
calculation of moon lux, and Dr. G. G. Martin, Dr. D. J. Pondella, and Dr. V. Carmona for
helpful conversations. My parents also deserve thanks for their unwavering support during this
project and throughout many other endeavors. Additional thanks go out to C. E. Edwards, C. A.
Abrams-Simonton, P. K. Gillispie, and E. C. Upham for providing needed distractions, and
N. M. Stewart for countless inspirational discussions. Last but not least, I would like to
acknowledge both the Biology Department and the Moore Laboratory of Zoology for providing a
positive working environment for the completion of this thesis.
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TABLE OF CONTENTS
Page
ABSTRACT………………………………………………………………………………….iii
ACKNOWLEDGEMENTS………………………………………………………………….v
LIST OF TABLES……………………………………………………...…………………..viii
LIST OF FIGURES………………………………………………………………...…….….ix
INTRODUCTION……………………………………………………………..………….…1
Moonlight avoidance behaviors…………………………………………..……………...2
Generality of moonlight avoidance?…………………………………..…………………4
Effects of moonlight on desert rodents in the Great Basin……...…………….…………6
MATERIALS AND METHODS…………….………………………………………………9
Kangaroo mice: an indicator of sandy and open habitats………….……………………..9
Study sites……………………………………………………………………….………10
Sampling of the rodent community……………………………………………….……..11
Rodent activity variables………………………………………………………….……..12
Moon variables…………………………………………………………………….…….13
Data analyses………………………………………………………………………….…15
RESULTS……………………………………………………………………………………18
Fieldwork and rodent activity……………………………………………………………18
Rodent community analyses……………………………………………………………..22
Rodent guild analyses……………………………………………………………………25
DISCUSSION………………………………………………………………………………..33
Design of study…………………………………………………………………………..34
Rodent activity in relation to moonlight……..…………………………………………..39
Conclusions and future research…………………………………………………………48
LITERATURE CITED……….……………………………………………………....……...50
APPENDIX A...………………….……………………………………………..……..…..…64
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LIST OF TABLES
Table Page
1. Comparison of fieldwork and rodent community composition statistics overall
and for each of the three seasonal periods…………...………………………….19
2. Abundance and characteristics of rodent species captured in sandy and open
Great Basin habitats…………………………………….…………………….…20
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LIST OF FIGURES
Figure Page
1. Map showing the distribution of sampling localities throughout the Great Basin
Desert of western North America…………………………………………….….8
2. Analysis of total rodent abundance in relation to moon lux………..……....…...24
3. Seasonal analysis of rodent community activity in relation to moonlight………27
4. Analysis of the bipedal and quadrupedal rodent guilds in relation to moon lux
over all nights of trapping…….…………………………………………………29
5. Analysis of bipedal and quadrupedal rodent guilds in relation to moon lux during
the waxing moon period……………………………………..……………….….32
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INTRODUCTION
The activity patterns of prey animals are determined by the concomitant need to avoid
predators while acquiring food and mates (McNamara and Houston 1987, Abrams 1993, Lima
1998a). Small mammal prey attempt to avoid encountering predators by limiting exposure to
risky situations, commonly reducing their activity or shifting to safer habitats in response to
stimuli of heightened risk (reviewed in Caro 2005). However, antipredator efforts must be
balanced with foraging efforts to meet the energetic demands of survival and reproduction
(Rosenzweig 1974, Brown and Kotler 2004, Griffin et al. 2005). In general, this type of
behavioral trade-off suggests that identifying situations of varying risk holds substantial adaptive
value for prey animals as they make decisions about when and where to forage (Sih 1987, Lima
and Dill 1990, Lima 1998b, Caro 2005). Yet the mechanisms by which most prey species
identify risk are not fully understood (Lima and Dill 1990, Blumstein and Bouskila 1996, Lima
1998b, reviewed in Stankowich and Blumstein 2005). Several rodent species are thought to
assess variations in the probability of encountering a predator and succumbing to a fatal attack
(i.e., the risk of predation; sensu Lima and Dill 1990) by utilizing indirect cues from the
environment (e.g., Thorson et al. 1998, Orrock et al. 2004). For nocturnal rodents in open desert
habitats, the avoidance of moonlight is often attributed to changes in the risk of predation from
visual predators such as owls (Kotler 1984b, Price et al. 1984, Brown et al. 1988, Kotler et al.
1991, Longland and Price 1991, Daly et al. 1992, Abramsky et al. 2002). Understanding the
extent to which moonlight influences the activity patterns of desert rodents may provide insight
on the role of predation risk in controlling foraging decisions (Brown et al. 1999, Meyer and
Valone 1999, Brown and Kotler 2004), microhabitat partitioning (Brown and Lieberman 1973,
Price 1978, Wondolleck 1978), and interspecific competition (Larsen 1986, Kotler and Brown
1988, Kotler and Holt 1989) in different rodent communities.
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Moonlight avoidance behaviors
In nocturnal habitats, regular fluctuation in the intensity and duration of moon
illumination over the lunar cycle provides an important source of environmental variation (Bond
and Henderson 1963). As a result, moonlight has been widely investigated for its possible role
as an environmental determinant of predation risk (reviewed in Rusak 1981, Lima and Dill 1990,
Lima 1998b, Caro 2005, Beier 2006). The moon’s nighttime illumination is thought to provide
small mammals with information regarding the likelihood of being detected and captured by
visually orienting predators. Indeed, evidence from laboratory and field enclosure studies
indicates that several species of owls are more efficient at capturing nocturnal rodents under
simulated full moon conditions (Dice 1945, Kaufman 1974, Clarke 1983, Kotler et al. 1988,
Kotler et al. 1991, but see Longland and Price 1991). Owls may be better able to detect prey
movement when dark shadows are cast against the substrate (Lockard and Owings 1974a,
Owings and Handa 1975, Kotler 1984b), resulting in reduced search times as illumination
increases (Clarke 1983). Additionally, mammalian predators such as red foxes (Vulpes vulpes)
are more successful at capturing prey on full moon nights (Kruuk 1964). This evidence suggests
that nocturnal illumination (i.e., moonlight or artificial illumination) increases the risk from
certain predators, and predicts that small mammal prey will alter their foraging activities to
reduce their exposure to predators during periods of bright moonlight.
Moonlight avoidance behaviors have been documented in a wide range of small
mammals including phyllostomid bats (Erkert 1974, Morrison 1978, Esbérard 2007), shrews
(Sorex; Vickery and Bider 1978), and hares (Lepus; Griffin et al. 2005), but are most extensively
described in nocturnal rodents. Rodent behavioral responses to moonlight have been studied for
well over a half-century, beginning with the “general impression” of Burt (1940:25) that
white-footed mice (Peromyscus leucopus) are more likely to be trapped on dark than moonlit
nights, and the early experimental observations of Blair (1943) that the activity of deer mice
(Peromyscus maniculatus) is inhibited by artificial light equivalent to half the intensity of the full
moon. Subsequent laboratory and field studies of have characterized moonlight avoidance
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behavior in rodents as generally including two responses: 1) reduced foraging and aboveground
activity (Lockard and Owings 1974a, b, Kotler 1984a, Wolfe and Summerlin 1989, Daly et al.
1992); and, 2) shifts in activity from open spaces (i.e., open microhabitats) to areas of greater
vegetative cover (i.e., bush microhabitats; Kaufman and Kaufman 1982, Kotler 1984b, Price et
al. 1984, Brown et al. 1988, Longland and Price 1991). Other behavioral responses to periods of
bright moonlight may include foraging in shorter but more frequent bouts (Longland and Price
1991, Vásquez 1994, Kramer and Birney 2001), increased vigilance while foraging (Vásquez
1994, Abramsky et al. 2002), and foraging closer to known refuges (Alkon and Saltz 1988, Daly
et al. 1992).
Diverse rodent taxa have been reported to display elements of moonlight avoidance.
These taxa principally include heteromyids rodents such as kangaroo rats (Dipodomys; Justice
1960, Schwab 1966, Lockard and Owings 1974a, b, O’Farrell 1974, Lockard 1975, Lockard
1978, Kaufman and Kaufman 1982, Kotler 1984a, b, Price et al. 1984, Bowers 1988, Brown et
al. 1988, Longland and Price 1991, Daly et al. 1992, Bouskila 1995), and cricetid rodents such as
Peromyscus and other peromyscines (Blair 1951, Hirth 1959, Pearson 1960, Falls 1968, Jahoda
1973, O'Farrell and Kaufman 1975, Clarke 1983, Travers et al. 1988, Wolfe and Summerlin
1989, Brillhart and Kaufman 1991, Falkenberg and Clarke 1998, Orrock et al. 2004).
Additionally, several other rodent taxa are thought to display moonlight avoidance behaviors,
including wood rats (Neotoma; Wiley 1971, Topping et al. 1999), South American
sigmodontines (Simonetti 1989, Vásquez 1994, Yunger et al. 2002), and murid rodents such as
gerbils (Gerbillus; Kotler et al. 1991, Kotler et al. 1993, Hughes et al. 1994). Even such
phylogenetically distinct rodent taxa as Old World porcupines (Atherurus and Hystrix; Emmons
1983, Alkon and Saltz 1988) and springhares (Pedetes; Brown and Peinke 2007) may avoid
periods of bright moonlight.
4
Generality of moonlight avoidance?
The bulk of evidence supporting moonlight avoidance in nocturnal rodents has resulted in
the conventional wisdom among mammalogists that small-mammal trapping success is greatly
reduced as the moon waxes, and particularly so on full-moon nights (see Lima 1998b, Brown
and Kotler 2004, Caro 2005). However, the general lore that moonlight reduces foraging activity
and the use of open microhabitats suggests a universal applicability to all nocturnal rodents that
may be misleading. A notable caveat to the generality of moonlight avoidance is that these
behaviors are largely dependent on the openness of the habitat examined. For instance,
red-backed voles (Myodes) living in areas of tall vegetation do not alter their activity in response
to changes in moonlight intensity (Getz 1968, Jensen and Honess 1995). Additionally, deer mice
and white-footed mice living in woodland areas show no differences in activity during full moon
nights (Falls 1953, Orr 1959), and may even increase their activity in response to illumination
(Barry and Francq 1982). Only a fraction of incident moonlight is able to penetrate dense forest
or shrub habitats (Smith 1996), so these responses are not surprising; indeed, changing
moonlight conditions are only expected to provide an environmental stimulus to rodents in open
habitats with sparse and widely spaced vegetation (Falls 1968, Beier 2006).
Accordingly, North American deserts and the associated desert rodent communities have
been the concentration of numerous investigations detailing the inhibition and alteration of
rodent activity during bright moonlight (e.g., Justice 1960, Schwab 1966, Lockard and Owings
1974a, b, O'Farrell 1974, Kotler 1984a, b, Price et al. 1984, Bowers 1988, Brown et al. 1988,
Longland and Price 1991, Daly et al. 1992, Bouskila 1995). Studies of North American desert
rodents are among the most commonly cited sources supporting the generality of moonlight
avoidance in nocturnal rodents (e.g., White and Geluso 2007). However, even among desert
rodents, there are several prominent exceptions to moonlight avoidance. Most notably, there is
considerable variation in the extent to which different species within the same rodent community
are affected by moonlight. Many studies of desert rodents detect responses to moonlight in only
one or a few species in a given community (e.g., Justice 1960, Kotler 1984b, Price et al. 1984,
5
Brown et al. 1988). Often only the most abundant species in the rodent community are
documented to avoid moonlight, which in many cases are the numerically dominant Merriam’s
kangaroo rat (Dipodomys merriami) or other species of Dipodomys (e.g., Price et al. 1984,
Bowers 1988, Bouskila 1995). Additionally, several studies have found that moonlight
avoidance is only displayed during certain seasons, and that desert rodent activity is unaffected
by moonlight during other times of the year (Lockard and Owings 1974b, Lockard 1978,
Bouskila 1995, Meyer and Valone 1999, see also Rosenzweig 1974). Taken together, these
caveats highlight the need for further investigation.
Previous field studies have used a variety of methods to assess rodent foraging activity in
relation to changing levels of moonlight. Yet several of the commonly employed experimental
designs have introduced complications such as artificial enclosures and habitat manipulations
(e.g., Brown et al. 1988, Wolfe and Summerlin 1989, Longland and Price 1991) that may alter
normal rodent activity, particularly when competing rodent species are excluded or are more
abundant than usual (see Falkenberg and Clarke 1998, Abramsky et al. 2004). Other potentially
complicating factors include artificial illumination to simulate moonlight (e.g., Kotler 1984b,
Brown et al. 1988, Longland and Price 1991), and the frequent presence of investigators in the
study area to check traps (e.g., O'Farrell 1974) or observe rodent activity (e.g., Justice 1960).
Additionally, studies supplying food to rodents ad libitum (e.g., Wolfe and Summerlin 1989) are
unlikely to provide realistic insights on how rodents in natural habitats are affected by
moonlight, since local and seasonal fluctuations in the availability of resources are expected to
substantially influence rodent foraging decisions (Rosenzweig 1974). Indeed, field studies
conducted in unnatural settings are concerned with the question of whether moonlight avoidance
is possible in nature, rather than the question of whether rodents actually avoid moonlight in their
natural habitats on a regular basis.
6
Effects of moonlight on desert rodents in the Great Basin
The present study seeks to investigate the prevalence of moonlight avoidance behaviors
in the naturally occurring, nocturnal desert rodent community in the Great Basin Desert of
western North America. The activity patterns of nocturnal desert rodents are assessed by
live-trapping at Great Basin localities over a multi-year period (Fig. 1; see also Appendix A).
Corresponding information pertaining to moon phase and brightness are obtained for each night
of trapping during the study period and analyzed to examine the effects of moonlight on rodent
activity. Data are collected throughout each year, which also enables the effects of moonlight to
be examined on a seasonal basis. Thus, desert rodent communities are treated as focal units
whose activity patterns can be evaluated in relation to two natural perturbations: changing
moonlight conditions and seasonal resource availability (i.e., a natural experiment sensu
Diamond 1986; see also Griffin 2005). The benefits of this design allow for moonlight
avoidance behaviors to be studied in unmodified natural habitats using large sample sizes and
sampling across a broad expanse of the Great Basin (Fig. 1). Consistent sampling of the
nocturnal rodent community across the Great Basin is accomplished by the use of a habitat
indicator: the presence of the sand-obligate kangaroo mouse (Microdipodops). Kangaroo mice
have narrow habitat affinities (Hall and Linsdale 1929, Hall 1941, Ghiselin 1970, Hafner 1981,
Hafner et al. 1996) and the use of this indicator taxon ensures that only sandy and open habitats
are sampled. Moreover, restricted sampling in open microhabitats should facilitate the detection
of rodent responses to moonlight, since it is here where the risk of predation from visual
predators is greatest on brightly moonlit nights (Kotler et al. 1988, Kotler et al. 1991, Longland
and Price 1991).
In the context of previous studies detailing moonlight avoidance behaviors in desert
rodents (e.g., Lockard and Owings 1974a, Kotler 1984b, Price et al. 1984, Daly et al. 1992), as
well as hypotheses that moonlight represents an indirect cue of predation risk for nocturnal
rodents (e.g., Brown et al. 1988, Vásquez 1994, Orrock et al. 2004), the nocturnal desert rodents
sampled in this study are predicted to respond to periods of bright moonlight with a combination
7
Figure 1. Map showing the distribution of 62 individual localities (white dots) where nocturnal
desert rodents were sampled to examine moonlight avoidance in the Great Basin Desert of
western North America (light gray). Sand-obligate kangaroo mice (Microdipodops; distribution
in dark gray) were utilized as a habitat-indicator taxon for this study: the presence of kangaroo
mice at a given locality allowed for specific sandy and open habitats to be identified across a
wide geographic area. The outline of the Great Basin Desert is modified from Cronquist et al.
(1972) as based on floristic data, and the distribution of Microdipodops is modified from Hafner
(2008) and Hafner and Upham (2009). See Appendix A for exact localities.
8
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of reduced overall aboveground activity, and reduced use of open microhabitats (i.e., shifts to
bush microhabitats). Because the study is restricted to open microhabitats, however, it is not
possible to differentiate between these two predicted effects of moonlight. Instead, the design of
this study facilitates the detection of these two predicted changes in rodent activity
simultaneously. Reduced overall rodent activity and shifts in activity from open to bush
microhabitats are both expected to result in proportionately lower abundances of rodents trapped
in open microhabitats during bright moonlight conditions. Thus, the central prediction of this
study states that if moonlight avoidance is indeed occurring on a community-wide basis in the
nocturnal desert rodents sampled, then there should be a negative relationship between the
proportionate abundance of rodents and increasing values of moon illumination over different
nights of trapping. Alternatively, if moonlight avoidance is occurring in only a few of the rodent
species, then there should be negative relationship between species diversity and increasing
levels of moonlight over the same period. In this study, the occurrence of moonlight avoidance
in Great Basin desert rodents is examined across the entire nocturnal rodent community, as well
as within the bipedal and quadrupedal rodent guilds.
MATERIALS AND METHODS
Microdipodops: an indicator taxon of sandy and open habitats
Kangaroo mice (Microdipodops) were utilized as indicators of habitat type for this study.
Since kangaroo mice have strong habitat affinities for areas of sandy soil and widely spaced
halophytic vegetation (Hall and Linsdale 1929, Hall 1941, Ghiselin 1970, Hafner 1981, Hafner et
al. 1996), the presence of one or more individuals of Microdipodops at a given trapping locality
was used to identify habitats that were both sandy and open (Fig. 1; Appendix A). Controlling
for habitat type in this manner ensured that desert rodent activity was assessed consistently in
similar environments at multiple localities. Any variation in the edaphic and floral composition
10
of habitats between sampling localities was assumed to be minimal and to have no confounding
effect on the amount of moonlight reaching different surface habitats.
Study sites
Seventy-one Great Basin localities were designated initially as sandy and open habitats
by the presence of the indicator taxon, Microdipodops. The trapping data from nights of
trapping at each of these localities were analyzed to assess the activity patterns of nocturnal
desert rodents. In situations where multiple localities were trapped on a single night, the
trapping data from localities less than 2 km apart were pooled. One or more nights of trapping at
each of the 71 localities yielded a total of 78 nights of trapping. Nights of trapping where local
cloud cover or precipitation was observed were removed from all analyses to control for possibly
altered or obstructed moon illumination. The remaining 69 nights of trapping with clear skies
included 62 localities throughout the Great Basin (Fig. 1; see also Appendix A). The data from
these 69 nights of trapping were used as a basis for all investigations of rodent activity in
response to moonlight.
All localities where rodent activity was sampled were within the Upper Sonoran Life
Zone, at elevations ranging from 1192 – 2127 m (mean 1560.29 m; Appendix A). These
sampling localities were generally located in valleys and basins where sand has accumulated
along ridges or around the past strandlines of dry lakes and been stabilized by the growth of
vegetation. A variety of halophytic shrubs were typically present at our sampling localities,
including Artemisia (sagebrush), Chrysothamnus (rabbitbrush), Atriplex (saltbush), Sarcobatus
(greasewood), Tetradymia (horsebrush), and Oryzopsis (ricegrass). Areas of vegetation were
generally low to the ground and well spaced, providing little cover from environmental variables
such as moonlight.
11
Sampling of the rodent community
Nocturnal desert rodents were sampled by live-trapping in Great Basin habitats from
September 1999 to September 2006. In general, sampling took place from March to October
each year (range 24 March to 30 October); no activity data were collected during the winter
months (November to February) when some members of the rodent community are inactive or
less active (Microdipodops, Perognathus, Chaetodipus; Kenagy 1973, O’Farrell 1974, Reichman
and Price 1993). All trapping data were collected originally for unrelated systematic studies of
Microdipodops (i.e., Hafner et al. 2008, Hafner and Upham 2009), so trapping activities were
timed without explicit concern for moon phase, moon position in the sky, or season.
Sherman folding aluminum live-traps (8 by 9 by 23 cm) were used to assess the activity
patterns of desert rodents on given nights. Between 200 and 400 traps were generally set per
locality in linear arrays of 50 – 100 traps running through the most uniform areas of open, sandy
habitat. All traps were baited with rolled oats and set to a hair-trigger. Traplines were set
generally one hour before sunset and checked the following morning within one hour after
sunrise. Notes on the microhabitat directly surrounding individual capture sites were not
recorded; however, traplines were always spread through open spaces between vegetation in the
following way: pairs of traps set at 9 m intervals between pairs (10 paces), with the two traps in a
pair set 2 – 3 m apart from each other and each trap at least 1 m away from the nearest shrub.
Thus, all trapping activities were deemed to be sampling rodent activity only in “open
microhabitats” (Price 1978, Thompson 1982, Price et al. 1984, Kotler et al. 1993). Captured
animals at each trapping site were identified to species and recorded along with the GPS
coordinates and elevation information for the locality. With the exception of animals retained
for unrelated phylogeographic studies, all animals were released at the site of capture. All
animals in this study were treated in accordance with the guidelines of the American Society of
Mammalogists (Gannon et al. 2007) and Occidental College’s Institutional Animal Care and Use
Committee. Field notes detailing locality information, trapping data, vegetation height and
12
spacing, cloud cover, precipitation, and morning temperature were recorded during all trapping
activities and are available upon request.
Rodent activity variables
Four rodent activity variables were examined in this study. These characters included
total abundance, species richness, and two community diversity indices: Shannon’s diversity
index (H’) and Simpson’s index of diversity (D). Rodent activity variables were used to assess
the amount of aboveground activity on given nights of trapping in each locality sampled. Thus,
only captures of nocturnal desert rodents were included in the calculation of rodent activity
variables. Any diurnal animals present during nights of trapping (e.g., least chipmunks [Tamias
minimus] and white-tailed antelope squirrels [Ammospermophilus leucurus]) were likely
captured during crepuscular periods and were removed from all analyses. Total abundance was
calculated as the proportionate number of traps that captured rodents on a given night of
trapping, and is thus analogous to the classical small-mammal trapping statistic of “trap success.”
Species richness (S), the total number of species captured on a given night of trapping, was
calculated to assess rodent community diversity. The two diversity indices, Shannon’s H’ and
Simpson’s D, were calculated as H’ = -Σ pi ln pi , and D = 1 - Σ pi2 , respectively, where pi is the
relative abundance of each species present in the community from 1 to S (Shannon and Weaver
1949, Simpson 1949). Both diversity indices describe the equitability (or evenness) of different
species within a rodent community, in addition to the number of different kinds of species
present and active in that community, under the moonlight conditions of a given night.
Potentially biasing correlations between any of the four rodent activity variables and the
number of traps set on given nights of trapping were identified by calculating Spearman’s
rank-order correlation coefficients rs. Both species richness S and Shannon’s diversity H’ were
found to be significantly correlated with the number of traps set (both rs > 0.30, P < 0.01). Thus,
these two variables were discarded in favor of total abundance and Simpson’s D, which were
13
independent of trapping regime (all P > 0.05). For all subsequent analyses of the rodent
community, only two rodent activity variables were retained: total abundance and Simpson’s D.
Moon variables
Local moon phase and position were obtained for all nights of trapping using MICA 2.0
computer software (U.S. Naval Observatory 2005). The dates of trapping, latitude and longitude,
and elevation were entered into the software for each locality to output information on moonlight
conditions and the length of the nighttime period for respective nights of trapping. The nighttime
period was defined here as the period between “end civil twilight” (post-sunset when the sun is
6˚ below horizon) and “begin civil twilight” (pre-sunrise when the sun is 6˚ below horizon).
While ambient illumination is generally still about 5 lux (lx, lumens per meter squared) at the
end of civil twilight, solar illumination rapidly declines to a faint glow on the horizon in the
minutes following civil twilight (Bond and Henderson 1963). Thus, the nighttime period defined
here refers conservatively to the portion of the night when the atmosphere is sufficiently free of
solar illumination to allow for the effects of moonlight, if present, to be experienced in surface
habitats (Janiczek and DeYoung 1987).
A total of five moon variables were calculated to describe the duration and intensity of
moonlight on given nights of trapping. The duration of moonlight on given nights was examined
using two variables: night-moon hours, and night-moon proportion. Night-moon hours describes
the actual length of the nighttime period with the moon above the horizon, while night-moon
proportion describes the length of the same period relative to the length of the nighttime period.
The intensity of moonlight on given nights was also examined using three variables: moon
fraction illuminated, proportionate moon illuminated, and moon lux. Moon fraction illuminated
describes the moon’s phase during the course of each night on a continuous scale of 0 to 1.0,
with 0 as a new moon, 0.5 as a first- or last-quarter moon, and 1.0 as a full moon. This variable
was calculated as the arithmetic mean of values generated at 15-minute intervals during
night-moon hours. Proportionate moon illuminated follows Stokes et al. (2001) as the product of
14
the night-moon proportion and the moon fraction illuminated. This variable describes the extent
of the moon’s illuminated fraction on a given night in a single quantity ranging from 0 to 1.0,
with 1.0 representing a full moon above the horizon for the entire nighttime period. The third
moonlight intensity variable, moon lux, provides information on the moon’s illuminance (Ev) as
measured in lux (luminous flux per unit area) for each night of trapping. Moon lux relates the
amount of visible moonlight reaching surface habitats under different moon conditions, and was
calculated for each night of trapping by using the following illuminance equation: Ev = [I0 0.5 (1
– cos(θ)) (R0/R)2] sin(ρ) , where I0 is a constant equal to 0.215 lx (illuminance of zenith full
moon at the mean earth-moon distance), and R0 is a constant equal to the mean earth-moon
distance of 384,400 km (D. J. Webb in litt., Austin et al. 1976, Janiczek and DeYoung 1987,
Krisciunas and Schaefer 1991). The moon’s elongation θ (angle between the sun and moon
viewed from earth, in radians), the earth-moon distance R (in km), and the moon’s altitude ρ
(angle between the moon and the horizon, in radians) were entered into the illuminance equation
as mean values calculated for each night of trapping during night-moon hours (arithmetic mean
taken at 15-minute intervals). On nights near the new moon when the moon did not rise (altitude
< 0°), a value of zero was entered for moon lux.
Pairwise comparisons between each of the five moon variables were performed to
identify redundant (highly correlated) variables. Spearman’s rank-order correlations revealed
that all five of the moon variables were highly correlated with each other (all rs > 0.90, P <<
0.001). Since these variables all describe slightly different aspects of the same moon phases on
the same nights of trapping, three variables (night-moon hours, night-moon proportion, and
proportionate moon illuminated) were eliminated to reduce redundancy in the data set. For all
subsequent analyses, moon fraction illuminated and moon lux were retained because they
provide the most complete description of moonlight in terms of the moon’s phase and lux
illuminance in given habitats on given nights of trapping.
15
Data analyses
SYSTAT computer software (version 9, Wilkinson 1998) was used for the statistical
analysis of data unless otherwise noted. A one-sample Kolmogorov-Smirnov test (with
Lilliefors modification) was performed on all variables to test for normality; the non-normal
distribution of nearly all variables prompted the use of nonparametric statistics throughout the
study. Specifically, nonparametric linear regression techniques were carried out to assess the
functional relationships between rodent activity variables and moon variables. Functional trends
were identified between pairs of variables using Quenouille’s (1952) ordering test, as executed
by the methodologically equivalent Kendall’s coefficient of rank correlation (tau; Sokal and
Rohlf 1995: 539). Kendall’s rank-order correlations were used to determine the significance of
regressions and the presence of functional relationships between pairs of variables (monotonic
increasing or decreasing trends). Regression equations of functional trends between variables
were obtained using Kendall’s robust line-fit method (Kendall and Gibbons 1990). Throughout
the study, no more than five comparisons were included in a single statistical experiment.
Although experiment-wise error rates are known to be affected by two or more simultaneous
comparisons (Rice 1989, Chandler 1995), simultaneous inference techniques may adjust type I
error rates too conservatively for small numbers of comparisons (Rice 1989, Rothman 1990,
Chandler 1995, Ludbrook 1998, Feise 2002). Thus, all test statistics in this study were evaluated
at an individual α of 0.05. However, results at this initial significance level were interpreted
conservatively; test statistics were considered to be indicative of strong significance only with an
individual α of 0.01.
Rodent community analyses.—The relationship between rodent community activity and
moonlight was examined by comparing rodent activity variables and moon variables over all
nights of trapping (N = 69). Rodent activity and moon variables were also examined separately
during waxing moon phases (N = 44) and waning moon phases (N = 25). These portions of the
lunar cycle were analyzed to address the observations of previous researchers (e.g., Alkon and
Saltz 1988, Wolfe and Summerlin 1989, Daly et al. 1992, Kotler et al. 1994) that desert rodents
16
display differential responses to moonlight during waxing and waning moon phases. In addition,
data were examined according to season, with nights of trapping divided into three periods
roughly corresponding with the seasonal periods of spring (N = 21), summer (N = 22), and fall
(N = 26). These seasonal periods were established by trisecting the 24 March to 30 October
trapping date range into three periods: the spring period (24 March to 5 June), the summer period
(6 June to 18 August), and the fall period (19 August to 30 October). Seasonal periods such as
these are expected to differ from each other substantially in terms of the amount and type of food
available in the Great Basin (Rosenzweig 1974), as well as the seasonal timing of rodent
behaviors such as reproduction and torpor (O'Farrell 1974, Burt and Grossenheider 1976).
Rodent guild analyses.—The rodent community was divided into two separate guilds
based on shared locomotive traits between rodent species. Since modes of locomotion have
ecological consequences for how different rodent species utilize habitat space and exploit
resources (Reichman and Price 1993), groups defined by shared locomotive traits can be
properly viewed as guilds (sensu Root 1967). Two main types of locomotion were identified
from among the different rodent species sampled: bipedal ricochet and quadrupedal movement
(either saltation or diagonal limb coordination; Eisenberg 1963). However, most of the rodent
species in this study were capable of using either bipedal or quadrupedal locomotion at different
times depending on the nature of the activity (e.g., exploring a new environment, foraging, or
avoiding predators; Bartholomew and Caswell 1951, Bartholomew and Cary 1954, Eisenberg
1963). Thus, each rodent species was designated as either primarily bipedal or primarily
quadrupedal based on their documented predominant mode of locomotion while foraging. The
primarily bipedal rodent guild was defined as including all species of kangaroo rats Dipodomys
that were captured, while the primarily quadrupedal rodent guild was defined as including all
other rodent species captured at sampling localities. Although individuals of Microdipodops are
commonly categorized in ecological studies as bipedal rodents (e.g., Kotler 1985b),
Microdipodops was classified in this study as a primarily quadrupedal rodent following the
observations of Eisenberg (1963) and the discussion of Hafner (1993). Bipedal rodent
17
abundance and quadrupedal rodent abundance were calculated for each night of trapping as the
relative abundance of all rodents in each guild. Nonparametric statistics were used to assess the
presence of functional relationships between the abundances of each rodent guild and the moon
variables. Rodent guild analyses were performed first over all nights of trapping, and then
separately for the waxing and waning moon periods and each seasonal period.
Circular analyses.—Circular statistical analyses were used as an additional statistical tool
to evaluate rodent activity in relation to the 29.53-day lunar cycle. Angular estimations of moon
phase were obtained by converting the moon fraction illuminated for each night of trapping to a
corresponding value on a 360° lunar cycle. This lunar cycle begins at 0° for the new moon and
varied continuously in a clockwise direction (e.g., 270° corresponds to the last-quarter moon).
The resulting circular variable, circular moon phase, accounted for the location of each night of
trapping within the lunar cycle. Circular moon phase differentiated between waxing and waning
moon phases by its angular value, and so facilitated comparisons with the rodent activity
variables in respect to position within the lunar cycle (in contrast, a moon fraction illuminated
value of 0.5 could represent either a waxing first-quarter or waning last-quarter moon).
Circular-linear correlation coefficients r (Fisher 1993, Zar 1999, Mardia and Jupp 2000) were
calculated between circular moon phase and each of the rodent activity variables for both the
rodent community analyses and rodent guild analyses. Circular-linear correlation coefficients
range in value between 0 and 1, so negative correlations were not possible. Instead of providing
a direction, these correlations provided information on whether linear variables were distributed
independently from a given circular variable (Zar 1999). Circular analyses were performed over
all nights of trapping, as well as separately during each of the waxing moon, waning moon, and
seasonal periods. All circular statistics were calculated using Oriana software (Kovach, 2006).
18
RESULTS
Fieldwork and rodent activity
Trapping activities on 69 nights with clear skies and involving 62 localities in the Great
Basin yield a total of 10,758 trapnights and 2,433 animals captured (mean trap success 22.6%;
see Fig. 1 and Appendix A for locality details). Nights of trapping are spread throughout
different moon phases, with the moon fraction illuminated ranging from 0.000 to 0.995 (mean
0.397). Fieldwork and rodent community diversity statistics are presented in Table 1 for all
nights of trapping and each of the seasonal trapping periods. Live-trapping efforts are roughly
distributed equally between the seasonal periods, with no significant differences between the
mean number of animals, number of trapnights, total abundance, or any of the three diversity
measures (Table 1; Kruskall-Wallis tests, df = 2, all P > 0.05). However, there are differences in
the mean abundances between all three seasons for both bipedal rodents (Kruskall-Wallis test, H
= 6.812, df = 2, P = 0.033) and quadrupedal rodents (Kruskall-Wallis test, H = 6.100, df = 2, P =
0.047). Bipedal rodents are most abundant during the fall period, and have a significantly
different mean abundance between the fall and spring period (Table 1; Mann-Whitney U =
156.0, N = 47, P = 0.012). In comparison, quadrupedal rodents are most abundant in the summer
period and have a significantly different mean abundance between summer and fall (Table 1;
Mann-Whitney U = 404.5, N = 48, P = 0.014). There are no differences in the abundances of
bipedal or quadrupedal rodents between any other paired seasonal comparisons (Table 1;
Mann-Whitney U tests, P > 0.05).
The nocturnal desert rodents sampled in this study include 20 different rodent species
from 62 sampling localities (Table 2). The total number of animals captured from each species
is displayed in Table 2 along with information on body size and predominant mode of
locomotion. In general, the bipedal rodents sampled are much larger than quadrupedal rodents
(Table 2).
19
TABLE 1 – Comparison of mean values for fieldwork and rodent community composition statistics overall (N = 69), as well as for
each of the three seasonal periods, spring (N = 21), summer (N = 22), and fall (N = 26).
Category Overall Spring Summer Fall
Number of animals 35.3 31.9 39.4 34.5
Number of trapnights 155.9 161.8 164.1 144.2
Total abundance 0.223 0.192 0.251 0.225
Bipedal abundance 0.086 0.053 0.069 0.127 *
Quadrupedal abundance 0.137 0.139 0.182 0.098
Species richness S 4.899 4.905 4.864 4.923
Shannon’s diversity index H 1.118 1.143 1.164 1.060
Simpson’s index of diversity D 0.574 0.589 0.597 0.543
* Bipedal rodent abundance is significantly different between the fall and spring periods.
Quadrupedal rodent abundance is significantly different between the summer and fall periods.
20
TABLE 2 – Abundance and characteristics of rodent species captured in sandy and open habitats of the Great Basin. Body mass data
is from Burt and Grossenheider (1976). See text for details on categorizing the predominant mode of locomotion for each species.
Species Number of captures Body mass (g) Locomotion
Merriam’s kangaroo rat (Dipodomys merriami) 480 35-50 g Bipedal
Ord’s kangaroo rat (Dipodomys ordii) 236 42-72 g Bipedal
Chisel-toothed kangaroo rat (Dipodomys microps) 111 72-91 g Bipedal
Desert kangaroo rat (Dipodomys deserti) 50 85-147 g Bipedal
Panamint kangaroo rat (Dipodomys panamintinus) 14 64-94 g Bipedal
Deer mouse (Peromyscus maniculatus) 652 18-35 g Quadrupedal
Little pocket mouse (Perognathus longimembris) 360 7-9 g Quadrupedal
Dark kangaroo mouse (Microdipodops megacephalus) 177 9-18 g Quadrupedal
Pallid kangaroo mouse (Microdipodops pallidus) 131 9-18 g Quadrupedal
Pinyon mouse (Peromyscus truei) 70 19-31 g Quadrupedal
Great Basin pocket mouse (Perognathus parvus) 60 19-28 g Quadrupedal
Northern grasshopper mouse (Onychomys leucogaster) 56 25-40 g Quadrupedal
Harvest mouse (Reithrodontomys megalotis) 11 9-17 g Quadrupedal
Southern grasshopper mouse (Onychomys torridus) 11 20-24 g Quadrupedal
21
TABLE 2 (concluded)
Species Number of captures Body mass (g) Locomotion
Desert woodrat (Neotoma lepida) 8 94-170 g Quadrupedal
Long-tailed pocket mouse (Chaetodipus formosus) 2 14-24 g Quadrupedal
Long-tailed vole (Microtus longicaudus) 1 37-57 g Quadrupedal
Montane vole (Microtus montanus) 1 28-85 g Quadrupedal
Sagebrush vole (Lagurus curtatus) 1 23-37 g Quadrupedal
House mouse (Mus musculus) 1 11-22 g Quadrupedal
22
Rodent community analyses
General effects of moonlight.—There are no functional relationships between total
abundance and either moon lux or moon fraction illuminated when analyzed over all nights of
trapping (all P > 0.05). However, visual examination of total abundance plotted in relation to
moon lux suggests that there is a slight negative trend between these two variables, albeit
insignificant (Fig. 2a; Kendall’s tau = -0.089, P = 0.280). In Figure 2a, nights of trapping with
low values of moon lux (< 0.0022 lx, represented by black circles) appear to vary widely with
regard to the total abundance of the rodents sampled (total abundance ranges from 0.030 to
0.477). When these 18 nights of trapping around the new moon are removed, the resulting
reduced data set (N = 51) yields a significant negative relationship between total abundance and
moon lux (Fig. 2b; Kendall’s tau = -0.219, P = 0.023). However, analysis of this same reduced
data set does not find any relationship between total abundance and moon fraction illuminated (P
> 0.05). There are no correlations between circular moon phase and total abundance either for
all nights of trapping or when the data set is reduced (all P > 0.05).
In terms of rodent community diversity, there are marginally non-significant negative
relationships for all nights of trapping between Simpson’s D and both moon fraction illuminated
(Kendall’s tau = -0.144, P = 0.080) and moon lux (Kendall’s tau = -0.147, P = 0.073).
Removing the same 18 nights of trapping around the new moon weakens these marginal
relationships such that Simpson’s D is unrelated to either of the moon variables (all P > 0.30).
However, circular moon phase is highly correlated with Simpson’s D over all nights of trapping
(r = 0.284, P = 0.005), as well as for the reduced data set (r = 0.326, P = 0.006). These
correlations with circular moon phase support the marginal negative relationships between
Simpson’s D and both moon variables, and indicate that rodent community diversity decreases
with increasing moonlight over all nights of trapping.
Effects of waxing and waning moonlight.—Separate analysis of the rodent communities
on nights with waxing moon phases (N = 44) and waning moon phases (N = 25) yields no
significant relationships between total abundance and either one of the moon variables (all P >
23
Figure 2 – Analysis of total rodent abundance on given nights of trapping in relation to
corresponding values of moon lux. (a) There is no relationship between total abundance and
moon lux over all nights of trapping, but note that nights of trapping on or near the new moon
(> 0.0022 lx; black circles) have widely varying total abundances. (b) By removing 18 nights of
trapping around the new moon, the reduced data set yields a significant negative relationship
between total abundance and moon lux (Kendall’s tau = -0.219, P = 0.023).
24
25
0.05). Similarly, Simpson’s D is not related to either moon variable during the waxing or
waning moon periods (all P > 0.05). Circular moon phase is also uncorrelated to either rodent
activity variable during the waxing or waning moon periods (all P > 0.05).
Effects of seasonal moonlight.—The seasonal relationships between each of the rodent
activity variables and moon lux are depicted graphically in Figure 3. During the spring, summer,
and fall seasonal periods, there are no relationships between total abundance and moon lux (Fig.
3; all P > 0.05), and during the spring and fall periods there are no relationships between
Simpson’s D and moon lux (Fig. 3; all P > 0.05). It is only during the summer period where a
relationship between rodent activity and moonlight is observed, and that is between Simpson’s D
and moon lux (Fig. 3; Kendall’s tau = -0.488, P = 0.001). This highly significant negative
relationship is corroborated by a similar negative relationship between Simpson’s D and moon
fraction illuminated also during the summer period (Kendall’s tau = -0.339, P = 0.027).
Additionally, this relationship is supported by a highly significant correlation between circular
moon phase and Simpson’s D during the same period (r = 0.614, P < 0.001). During all other
seasonal periods, there no relationships between total abundance or Simpson’s D and either
moon fraction illuminated or circular moon phase (all P > 0.05).
Rodent guild analyses
General effects of moonlight.—The abundances of the bipedal and quadrupedal rodent
guilds are analyzed in relation to moon lux and plotted for all nights of trapping in Figure 4.
This analysis yields a significant negative relationship between bipedal rodent abundance and
moon lux (Kendall’s tau = -0.177, P = 0.032), and no relationship between quadrupedal rodent
abundance and moon lux (Fig. 4; P > 0.05). The non-significant relationship between quadruped
abundance and moon lux is apparent from the scattered data points in Figure 4 (open circles), but
it should be noted that, unlike the negative bipedal trends, there is a slightly positive Kendall’s
tau between these two variables (Kendall’s tau = 0.045, P = 0.582). There is also a significant
correlation between circular moon phase and biped abundance (r = 0.255, P = 0.013), but
26
Figure 3 – Seasonal analysis of rodent community activity in relation to moonlight. Total
abundance and Simpson’s D are examined in relation to moon lux during each of the three
seasonal periods: spring, summer, and fall. There is a significant negative relationship between
Simpson’s D and moon lux during the summer period (Kendall’s tau = -0.488, P = 0.001), while
all other seasonal comparisons are non-significant.
27
28
Figure 4 – Analysis of the relative abundances of bipedal rodents (closed circles) and
quadrupedal rodents (open circles) in sandy and open habitats of the Great Basin in relation to
corresponding values of moon lux for all nights of trapping (N = 69). There is a significant,
negative relationship between biped abundance and moon lux (Kendall’s tau = -0.177, P =
0.032), and a non-significant relationship between quadruped abundance and moon lux.
29
30
circular moon phase is uncorrelated with quadruped abundance (P > 0.05). Neither biped nor
quadruped abundance are related to moon fraction illuminated (all P > 0.05).
Effects of waxing and waning moonlight.—Analysis of the waxing moon period in
relation to rodent abundance yields contrasting results for bipedal and quadrupedal rodents (Fig.
5). Data and results from these analyses are displayed in Figure 5. During the waxing moon
period, there is a significant negative relationship between biped abundance and moon lux
(Kendall’s tau = -0.258, P = 0.014), but quadruped abundance and moon lux are not significantly
related (Fig. 5a; P > 0.05). Upon visual inspection of the data in Figure 5a, however, quadruped
abundance appears to be generally increasing with higher values of moon lux. This relationship
is not significant (Kendall’s tau = 0.107, P = 0.308), but a positive value of Kendall’s tau calls
attention to a few data points with unusual levels of rodent activity. Indeed, closer examination
reveals two separate nights during the waxing moon period with abnormal trapping
circumstances: the first night, 24 March 2002, has abnormally low abundances for both bipedal
rodents (Fig. 5a, closed square) and quadrupedal rodents (Fig. 5a, open square); the second night,
2 September 2001 (involving two localities), has abundances that are uncommonly high for
bipedal rodents (Fig. 5a, closed triangles) and uncommonly low for quadrupedal rodents (Fig. 5a,
open triangles). When these two abnormal nights of trapping are excluded from the analysis (N
= 41), there is a highly significant negative relationship between biped abundance and moon lux
(Fig. 5b; Kendall’s tau = -0.308, P = 0.005), and a significant positive relationship between
quadruped abundance and moon lux (Fig. 5b; Kendall’s tau = 0.239, P = 0.028).
The differential responses of bipedal and quadrupedal rodents to moonlight are similarly
displayed in relation to moon fraction illuminated. During the waxing moon period there is a
significant negative relationship between biped abundance and moon fraction illuminated
(Kendall’s tau = -0.221, P = 0.035) and a non-significant but positive value of Kendall’s tau
between quadruped abundance and moon fraction illuminated (Kendall’s tau = 0.092, P = 0.379).
Removing the same nights of trapping with abnormal rodent guild abundances strengthens both
of these opposing relationships such that both are significant (bipedal: Kendall’s tau = -0.285, P
31
Figure 5 – Analysis of bipedal and quadrupedal rodent abundances in relation to moon lux on
nights of trapping during the waxing moon period. (a) Examination of all waxing moon nights
shows a negative relationship between biped abundance and moon lux (closed symbols;
Kendall’s tau = -0.258, P = 0.014), and a non-significant relationship between quadruped
abundance and moon lux (open symbols). Triangles and squares represent two nights of trapping
(three localities) with abnormal abundances of both bipeds and quadrupeds. (b) Removal of
abnormal nights of trapping results in a highly significant negative relationship between bipedal
abundance and moon lux (closed circles; Kendall’s tau = -0.308, P = 0.005), and a significant
positive relationship between quadrupedal abundance and moon lux (open circles; Kendall’s tau
= 0.239, P = 0.028).
32
33
= 0.009; quadrupedal: Kendall’s tau = 0.218, P = 0.044). Similarly, comparisons with circular
moon phase during the waxing moon period initially yield a significant correlation with biped
abundance (r = 0.331, P = 0.011) and no correlation with quadruped abundance (P > 0.05), but
both correlations increase in significance following the removal of the abnormal nights of
trapping (bipedal: r = 0.285, P = 0.009; quadrupedal: r = 0.218, P = 0.044).
In contrast to the waxing moon period, the waning moon period yields no functional
relationships between bipedal or quadrupedal rodent abundances and either of the moon
variables (all P > 0.05). Additionally, circular moon phase is uncorrelated to either biped or
quadruped abundance during the waning moon period (all P > 0.05).
Effects of seasonal moonlight.—Seasonal analysis of bipedal and quadrupedal rodent
abundances yields no relationships between either of the moon variables during the spring and
fall periods (all P > 0.05). However, during the summer period there are marginally significant
negative relationships between biped abundance and both moon lux (Kendall’s tau = -0.283, P =
0.077) and moon fraction illuminated (Kendall’s tau = -0.263, P = 0.087). These marginal
relationships are supported by a significant correlation between circular moon phase and biped
abundance (r = 0.474, P = 0.013), suggesting that bipedal rodents may avoid moonlight during
only the summer period. Circular moon phase is uncorrelated to both biped and quadruped
abundance during all other seasonal periods (all P > 0.05).
DISCUSSION
The results of this study in sandy and open habitats of the Great Basin Desert provide
several unique insights on the nature of moonlight avoidance behavior in nocturnal desert
rodents. By focusing on desert rodents in their natural habitats and obtaining a large sample size
(62 clear nights involving over 10,000 trapnights), the present study is able to offer a revised
explanation for the prevalence of moonlight avoidance and generality of this behavior to all
34
desert rodent species. However, since a study of this type and magnitude has heretofore not been
attempted, a critical examination of its design and methodology in the context of previous studies
is warranted.
Design of study
Prior to this study, nearly all field studies of moonlight avoidance have measured rodent
activity at only a single field site (e.g., Lockard and Owings 1974a, O'Farrell 1974, Kotler 1984a,
b, Price et al. 1984, Brown et al. 1988, Longland and Price 1991, Daly et al. 1992, Bouskila
1995). The few studies that have examined moonlight avoidance across areas of habitat >10 km2
have concentrated on rodents in grassland areas (Kaufman and Kaufman 1982, Brown and
Peinke 2007), or on lagomorphs (Griffin et al. 2005), but not on desert rodents. Additionally,
many previous studies have only measured rodent activity during a single season (e.g., Brown et
al. 1988, Daly et al. 1992, Orrock et al. 2004) or over the course of only a few days (e.g.,
Kaufman and Kaufman 1982, Kotler 1984a, Price et al. 1984, Bowers 1988). Of the limited
number of field studies that have evaluated moonlight avoidance in desert rodents year-round,
almost all have relied upon artificial food sources either to measure giving-up densities (GUDs;
Bouskila 1995, Meyer and Valone 1999) or quantify aboveground activity (Lockard and Owings
1974b, Lockard 1978). O’Farrell (1974) represents the only field study to use live-traps to assess
desert rodent activity year-round, but that study only provides anecdotal support for moonlight
avoidance based on qualitative observations.
In contrast to previous field studies that have artificially manipulated both nocturnal
illumination and seed resources to evaluate moonlight avoidance (e.g., Kotler 1984b), the current
study takes advantage of perturbations that are already running in natural habitats, and examines
the impact that these perturbations have on the focal units in question (Diamond 1986). In this
case, changing moon phases and seasonal variability in resources provide natural perturbations
for nocturnal desert rodents sampled from 62 localities throughout the Great Basin Desert (Fig.
1). Each locality sampled resides in a specific type of sandy and open habitat for which all
35
aspects of the environment, with the exception of moonlight and seasonal date of sampling, are
assumed to be similar. Thus, only features of the environment related to the perturbations are
expected to differ between localities and affect rodent activity patterns. Importantly, the use of
Microdipodops as a habitat-indicator taxon enables these similar open habitats to be identified
across the Great Basin, and for moonlight avoidance to be investigated in habitats that are
unaltered by experimental modifications (e.g., enclosures) as in other studies (e.g., Brown et al.
1988, Longland and Price 1991). Additionally, the use of live-traps set once per night to capture
rodents and assess activity is minimally invasive compared to the unnatural factors incorporated
by previous studies to measure rodent activity, such as bait trays (e.g., Kotler 1984a) or other
artificial devices (e.g., Lockard and Owings 1974b). By sampling each locality on only a single
night (or on a few non-consecutive nights), the trapping regime of this study also escapes the
methodological problems associated with repeatedly performing capture-mark-recapture
techniques at one site that lead to trap-happy or trap-shy animals (Smith et al. 1975). Although
larger sample sizes are always welcomed, the present study encompasses 69 clear nights of
trapping culled from seven years of fieldwork in the Great Basin. Thus, broad sampling aids in
the investigational power of this study and offers a sharp contrast to previous moonlight
avoidance studies that have only measured rodent activity for a few days, or during a single
seasonal period.
While the novel design of the present study offers a unique opportunity to evaluate
moonlight avoidance in natural habitats, there are also several potential limitations associated
with this design. First, this study assumes that trap success in open microhabitats is directly
related to the amount of rodent foraging activity in those areas, and thus provides a measure of
moonlight avoidance. However, not all moonlight avoidance behaviors are expected to alter the
amount of foraging in open microhabitats. Rodents may also alter their activity patterns to
forage in shorter but more frequent bouts during moonlit periods, but not change the total amount
of time spent aboveground and foraging in open spaces (Longland and Price 1991, Vásquez
1994, Kramer and Birney 2001). Thus, changes in the length and frequency of foraging bouts
36
may go undetected by the methods of this study. A second limitation is that this study is unable
to distinguish seasonal changes in rodent density from seasonal changes in rodent responses to
moonlight. It is possible that high rodent densities during certain seasonal periods (e.g., bipedal
rodents during the fall period; Table 1) may obscure the observation of any decreases in rodent
abundance that would result from rodents avoiding moonlight. Additionally, changes in rodent
density from sampling locality to sampling locality may similarly influence the ability of this
study to detect moonlight avoidance behaviors. However, the narrow habitat affinities of the
indicator taxon, Microdipodops, are expected to reduce variation in the species composition of
the rodent communities sampled and thus minimize heterogeneity in rodent densities between
localities.
Sampling open microhabitats.—The use of Microdipodops as an indicator taxon for
sandy and open habitats is a key element in the experimental design of this study, as it allows for
the control of many potentially confounding variables between sampling localities. Most
notably, the use of this indicator taxon ensures that all habitats sampled are sufficiently open to
allow for moonlight to reach surface habitats unobstructed. It is important to note, however, that
the two recognized species of Microdipodops, M. pallidus and M. megacephalus, have slightly
different habitat requirements. M. pallidus is generally found at lower latitudes and elevations
than M. megacephalus (Hall 1941, Hafner 1981), and the two species are noted to differ in their
soil texture preferences and floral associations (Hafner et al. 1996). Yet despite these
differences, the narrow habitat requirements within vegetation-stabilized dune systems remain
for both species (Hall 1941, Ghiselin 1970, Hafner 1981, Hafner et al. 1996). Thus, for the
purposes of this study either species of Microdipodops is expected to act as a reliable indicator of
sandy habitats with minimal vegetative cover.
Since all trapping efforts were designed specifically to maximize the capture of
Microdipodops, all traps in this study were set in open microhabitats. However, many
investigators have defined microhabitats differently (see Jorgensen 2004), especially when
making a distinction between “open” and “bush” microhabitats. Open microhabitats are
37
generally described as open spaces away from perennial shrubs, but the distance away from
shrubs that is considered to be “open” has been variously defined as greater than 0.5 m
(Thompson 1982), at least 1 m in diameter (Price et al. 1984), 1 – 3 m (Kotler et al. 1993), 2 – 8
m (Kotler 1985a), and at least 3 m (Bowers 1988). In contrast, bush microhabitats are nearly
always defined as areas underneath the canopy of a shrub or at shrub margins (Thompson 1982,
Price et al. 1984, Kotler 1985a, Bowers 1988, Kotler et al. 1993). In the context of these
microhabitat definitions, the placement of traps in the present study (> 1 m away from shrubs in
open spaces) clearly falls into the category of open microhabitats. By sampling in the sandy and
open habitats preferred by kangaroo mice, the probability of local shadows masking the effects
of moonlight on rodent activity are greatly reduced.
Bipedal and quadrupedal rodents as guilds.—The grouping of rodent species into guilds
based on shared predominant modes of locomotion is appropriate for this study for several
reasons. The present study is most concerned with locomotion type as an identifier of
ecologically similar groups of species with comparable patterns of habitat use and resource
exploitation. In the desert rodent communities examined, bipedal species of Dipodomys forage
mostly in open areas between shrubs and move rapidly between widely dispersed seed clumps
using a ricochetal gait. In contrast, all remaining rodent species in the communities sampled are
quadrupedal and tend to forage on more evenly distributed seed resources spread under and near
shrubs (Brown et al. 1979, Thompson 1982, Reichman and Price 1993). Since all species in the
rodent communities sampled are at least partially granivorous, classifying guilds by bipedal or
quadrupedal locomotion is still within the bounds of the original definition of guilds as groups of
species exploiting the same class of resources in a similar way (Root 1967).
While it is common practice to include species of Microdipodops in the category of
bipedal rodents along with species of Dipodomys (e.g., Kotler 1984b, Kotler 1985a, b, Longland
and Price 1991, Pierce et al. 1992), there is some debate about the frequency of quadrupedal
versus bipedal locomotion in Microdipodops (Brylski 1993, see O’Farrell and Blaustein 1974a,
b). Hafner (1993: 300) points out that Microdipodops and Dipodomys are often lumped together
38
as bipedal rodents as a “kind of shorthand for describing their curious, kangaroo-like
morphology,” but that Microdipodops is in fact mostly quadrupedal. Indeed, the only detailed
observational study of Microdipodops locomotion to date was performed by Eisenberg (1963),
who reached the conclusion that “quadrupedal ricochet is the predominant mode of locomotion”
in Microdipodops (Eisenberg 1963:29). Not surprisingly, several ecological studies that have
categorized Microdipodops as a bipedal rodent have found “anomalous” results where
Microdipodops displays behaviors that are more similar to quadrupedal species than bipedal
species (e.g., Pierce et al. 1992, Djawdan 1993). For the sake of argument, if individuals of
Microdipodops in this study are included with Dipodomys in the “bipedal” rodent guild, the
significance of several relationships is reduced (test statistics and P values available upon
request). This subsequently suggests that the inclusion of Microdipodops with Dipodomys is an
unnatural grouping. Since the present study is most interested in identifying ecologically similar
groups of species with equivalent patterns of habitat use, species of Microdipodops were
categorized as members of the primarily quadrupedal rodent guild.
Indeed, this is not the first study to group bipedal species of Dipodomys together as a
guild separate from other species in the rodent community. Brown and Heske (1990) classified
three species of Dipodomys as consisting of a “keystone guild” due to their ecological dominance
and the succession of a Chihuahuan Desert habitat to grassland in the absence of these species.
Additionally, Hallett (1982) assigned quadrupedal cricetid rodents and Perognathus species to
one guild and bipedal Dipodomys species to a separate guild, as did Dayan and Simberloff
(1994) with bipedal and quadrupedal heteromyids (see also Simberloff and Dayan 1991). Body
size similarities between bipedal and quadrupedal rodents also provide support for the
classification of separate rodent guilds (Table 2). With the exception of large adult voles
(Microtus) and the infrequent occurrence of desert woodrats (Neotoma lepida) in the rodents
sampled, all quadrupedal rodent species range in size from 7 – 40 g. In contrast, bipedal species
of Dipodomys range from 35 – 147 g with animals generally weighing 50 g or more. Differences
in body size may influence competitive interactions between bipedal and quadrupedal rodents
39
(Bowers and Brown 1982) and affect how these groups of rodents respond to risk from predators
(Longland and Price 1991). Moreover, the dissimilarity in the body sizes of the two rodent
guilds may be expected to alter the actual risk of predation from visual predator such as owls
during times of bright moonlight (Kotler et al. 1988, Longland and Price 1991).
Rodent activity in relation to moonlight
Upon initial inspection of rodent community activity patterns for all nights of trapping,
the lack of relationship between total abundance and moon lux (Fig. 2a) indicates that rodent
activity is unaffected by moonlight or that the methods of this study are unable to detect changes
in activity. However, it is possible that the observed range of total abundance values on nights
near the new moon obfuscates the overall relationship between rodent activity and moonlight.
As pointed out by Bowers (1988), dark nights near the new moon should present less variation in
predation risk between different rodent microhabitats. As a result, rodents may adjust their
activity patterns randomly on new moon nights in the absence of moonlight-associated predation
risk. By removing nights of trapping around the new moon (Fig. 2b), activity in the rodent
community is examined exclusively on those nights when the moon was (1) present above the
horizon, and (2) of a specified minimum brightness 0.0022 lx that is approximately equivalent to
a young crescent moon 10° above the horizon. The resulting significant relationship (Fig. 2b)
suggests that the rodents sampled are only affected by moonlight above a certain threshold of
illumination. Thus, rodent activity is not moonlight-dependent when the moon is absent (new
moon) or of low illuminance. Rodent abundances may fluctuate on dark or moonless nights
independent of moon phases on previous nights, and instead in accord with local risks, resource
availability, and energetic demands.
Examining the activity patterns of bipedal and quadrupedal rodents in relation to
moonlight (Fig. 4) may also provide an explanation for the observed absence of moonlight
avoidance over all nights of trapping. Figure 4 indicates that there is a weakly significant and
gradually negative relationship between bipedal rodent abundance and moon lux, but a
40
non-significant and seemingly random association for quadrupedal rodent abundance in relation
to moon lux. The contrary responses of bipedal and quadrupedal rodent guilds to moonlight
likely confound the overall relationship between total abundance and moon lux for the rodent
communities sampled. In addition, the decreasing activity of bipedal rodents with increasing
moonlight is reflected in the corresponding strong correlation between Simpson’s D and circular
moon phase. This correlation, along with the marginally negative relationships between
Simpson’s D and both moon variables, indicates that reductions in rodent community diversity
on brightly moonlit nights are the consequence of moonlight avoidance by bipedal rodents.
Effects of waxing and waning moonlight.—Separating out the waxing moon phases from
the waning moon phases provides important insights about the interactions between bipedal and
quadrupedal rodent guilds. Analysis of the waxing moon period (Fig. 5a) supports the results
displayed in Figure 4 that the activity of bipedal rodents (as measured by relative abundance)
decreases with increasing moon lux. In contrast, the activity of quadrupedal rodents appears to
increase with increasing values of moon lux, although this positive relationship was found
initially to be non-significant (Fig. 5a). Removal of two nights of trapping and the
corresponding data points for each bipedal and quadrupedal rodents yields significant and
opposite relationships between the two rodent guild abundances and moon lux (Fig. 5b). This
differential response of bipedal and quadrupedal rodents to moonlight is surprising in the context
of hypothesized moonlight avoidance behaviors in desert rodents, and so must be examined
critically.
The removal of two nights of trapping from the waxing moon analyses is justified for
several reasons. Principally, these nights of trapping take place on two individual nights during
the study where there are abnormal abundances of both bipedal and quadrupedal rodents.
Trapping data from 24 March 2002 is the earliest date of trapping for the entire study, so activity
data from this night may not represent an accurate sample of the rodent community.
Quadrupedal rodents, especially individuals of Perognathus, may still be in torpor and are not
active in large numbers until mid-April (O'Farrell et al. 1975). Indeed, only three quadrupedal
41
rodents and no bipedal rodents were captured on this night. The lack of bipedal Dipodomys may
be explained by the unusually bright moon, a waxing gibbous moon that was relatively high
above the horizon (mean altitude 46.1°) for most of the night with an illuminance of 0.125 lx.
Since this is the second highest moon lux value in the study, it is not surprising that Dipodomys
species are less active given the negative functional relationship between bipedal rodents and
moon lux. Additionally, the two separate localities sampled on 2 September 2001 have unusual
abundances of both bipedal and quadrupedal rodents (high for bipedal, low for quadrupedal;
triangles in Fig. 5a). Since these two localities are approximately 30 km apart and were sampled
on the same night, it is possible that an unknown third variable is affecting the activity of bipedal
and quadrupedal rodents on this night. This night occurs during the fall period when bipedal
rodents are at their highest mean abundance among the localities sampled (Table 2), so it is also
possible that the aberrant activity patterns on this night are due to locally high abundances of
bipedal Dipodomys and not the effects of moonlight.
Why would differential responses to moonlight for bipedal and quadrupedal rodents exist
during waxing moon phases but not during waning moon phases? When the moon is waxing, the
moon fraction illuminated increases across the moon’s surface from east to west, as viewed from
Earth. During this period, the moon rises during daylight hours and is already above the horizon
when the sun sets and the nighttime period begins. The full moon culminates this waxing
phenomenon, rising on the eastern horizon at sunset and providing illumination all during the
night. This subtle aspect of the lunar cycle may have a prominent psychological effect, since
nearly all desert rodent species display a peak of activity either after sunset during twilight or in
the first few hours of the night (O'Farrell 1974, Hafner 1975, Kenagy 1976). Evidence suggests
that the presence of the moon above the horizon when rodents first become active influences
foraging patterns for the remainder of the night. Kotler et al (1994) found that two species of
gerbil, Gerbillus allenbyi and G. pyramidum, generally foraged less during waxing moon phases
than waning moon phases, although these results were not significant. Additionally, Alkon and
Saltz (1988) found that Indian crested porcupines (Hystrix indica) avoided moonlight during
42
waxing moon periods by preferentially foraging after the waxing moon had set. Also noteworthy
is the observation that waxing moon phases are generally about 20% brighter than corresponding
waning moon phases (Bond and Henderson 1963, Austin et al. 1976), which presumably results
from the uneven concentration of low-albedo lunar maria on the west side of the moon
(Krisciunas and Schaefer 1991). Thus, waxing moon phases may reasonably represent a brighter
and more dangerous portion of the lunar cycle for desert rodents as compared to waning moon
phases.
Lunar illumination and competitive release.—Why would a differential response to
moonlight exist within the rodents sampled? Bipedal Dipodomys species are significantly less
active as moon illumination increases, while corresponding quadrupedal species in the same
rodent community are significantly more active under the same moonlight conditions (Fig. 5b).
These results point to a possible competitive release in microhabitat use within the rodent
community as mediated by moonlight and predation risk. Wondolleck (1978) investigated
competition and microhabitat use in a Sonoran Desert rodent community and found that by
removing individuals of bipedal Dipodomys merriami, the quadrupedal Arizona pocket mouse
(Perognathus amplus) significantly shifted its foraging patterns to open microhabitat areas that
were previously occupied by D. merriami. This type of competitive release has been observed
by several workers who have noted that the ecological dominance of the bipedal and
large-bodied Dipodomys plays a substantial role in determining the microhabitat use of
quadrupedal species in different rodent communities (Brown and Lieberman 1973, Price 1978,
but see Larsen 1986). It appears likely that if Dipodomys species are reducing their activity on
nights with bright moonlight, that quadrupedal species may take advantage of the absence of a
main competitor for seed resources in open habitat areas. Indeed, both Kotler (1984b) and Price
et al. (1984) found that quadrupedal Perognathus longimembris were often captured relatively
more frequently in open areas than bush areas under bright moonlight, which was opposite of the
response observed for Dipodomys and contrary to their expectations of moonlight avoidance. In
the present study, the observed increase in quadrupedal abundance during brightly moonlit
43
periods is consistent with a hypothesis of competitive release that allows for quadrupedal rodents
to exploit previously unavailable resources.
Effects of seasonal moonlight.—The seasonal analysis of rodent community activity
patterns in relation to moonlight yields only one significant relationship, and that is during the
summer period between Simpson’s D and moon lux (Fig. 3). This highly significant relationship
indicates that with increased moon brightness, as measured in lux, the diversity and evenness of
the rodent communities decreases steadily. This result is supported by Simpson’s D being
similarly related to moon fraction illuminated and significantly correlated with circular moon
phase. There is no evidence for moonlight avoidance during the spring or fall seasonal periods,
so why is rodent diversity reduced during the summer period? The rodent guild analyses reveal
that the abundance of bipedal rodents is significantly correlated with circular moon phase during
the summer period, but since this correlation is undirected, it is initially unclear whether biped
abundance increases or decreases with brighter moonlight. However, when the marginally
significant negative relationships between biped abundance and both moon lux and moon
fraction illuminated are considered, these results suggest that bipedal rodents are avoiding bright
moonlight during the summer period only. Thus, the reduction in biped abundance during
summer moonlit periods is likely the driving factor behind the reduced values of Simpson’s D
during this period (Fig. 3). The strong, summer-only moonlight avoidance behavior of bipedal
rodents is also likely responsible for driving three previous results in this study: the
community-wide moonlight avoidance of rodents excluding nights around the new moon (Fig
2b), the moonlight avoidance of bipedal rodents across all nights of trapping (Fig. 4), and the
corresponding decrease in rodent community diversity over the same period. It should also be
noted that only 12 of the 22 nights of trapping within the summer period take place during the
waxing phases of the lunar cycle, so the observed summer-only response to moonlight is not
influenced disproportionately by the strong moonlight avoidance of bipedal rodents during the
waxing moon period (see Fig. 5).
44
Several other workers have documented seasonal moonlight avoidance in bipedal
Dipodomys species, but there is much variation in the season that moonlight is avoided. For
instance, Bouskila (1995) found that D. deserti and D. merriami avoided moonlight during the
fall season only, and that activity patterns were unaffected by moonlight during the summer.
However, this result was primarily attributed to a higher risk of predation in covered
microhabitats from seasonally active snakes during the summer. In contrast, Lockard and
Owings (1974b) and Lockard (1978) only found evidence for moonlight avoidance in
banner-tailed kangaroo rats (D. spectabilis) during the winter season, and observed no
relationship between rodent activity and moonlight during other seasons. Past studies have also
observed species of Dipodomys to avoid moonlight during the summer (Kaufman and Kaufman
1982, Kotler 1984a, Brown et al. 1988), but the sampling period of these studies was confined to
the summer season.
The summer moonlight avoidance observed in the present study is perhaps best explained
in the context of Rosenzweig’s (1974) model for the optimal aboveground activity of
banner-tailed kangaroo rats. This model discusses rodent foraging decisions in terms of the costs
and profits of additional foraging activity, such that rodents are expected to balance the costs of
predation with the energetic and reproductive profits of avoiding predators while foraging.
However, the net reproductive profit of activity is expected to fluctuate seasonally in accord with
seasonal changes in resource availability and the seasonal timing of reproduction. In the Great
Basin Desert, most rodents including Dipodomys focus all reproductive efforts to the spring
season when, among other factors, inter- and intraspecific competition is lowest following the
harsh winter. Thus, the reproductive profit of additional activity peaks during the spring season
and these profits are expected to outweigh any costs from predation risk during bright moonlight.
This model can additionally be extended to the summer and fall seasons. During the summer
season, the reproductive profits of activity are much lower and seed resources are expected to be
more abundant, so costs of predation during moonlit periods should outweigh the benefits of
additional activity such that rodents will avoid moonlight. The absence of moonlight avoidance
45
during the fall period may subsequently be explained by the need to store seed resources and add
body mass before the impending winter, and thus, an increase in the profitability of foraging
activity during this period. In this study, it is also observed that the mean abundance of bipedal
Dipodomys species is at a maximum during the fall period (Table 1), so interspecific competition
may be near its annual maximum in the fall.
Interestingly, the calculation of moon lux in this study enables further insight on the role
of moonlight in determining seasonal activity patterns. Seasonal changes in the moon’s altitude
above the horizon affect the brightness of moonlight that reaches surface habitats. This
difference in moon brightness, as measured in lux, is especially apparent when full moons are
compared between different seasons. For example, lunar data from the central Great Basin
shows that full moons in the fall period pass ~ 40° higher above the horizon, and are nearly three
times brighter, than full moons in the summer period (US Naval Observatory 2005). In this
context, less bright full moons during the summer period would be predicted to have less of an
impact on predation risk and rodent activity than brighter full moons during the fall period.
However, this prediction is not consistent with the summer moonlight avoidance observed in this
study. It is likely that even though the differences in moon brightness are substantial between
seasons, desert rodents are influenced to a greater extent by seasonal variations in interspecific
competition and foraging requirements.
Moonlight as a cue of predation risk?.—The open and nearly two-dimensional nature of
desert habitats has prompted Ylonen and Brown (2007) to hypothesize that desert rodent
communities represent fear-driven (µ-driven) systems where temporal and spatial variation in
risk from predators controls foraging and reproductive decisions (see also Brown et al. 1999).
The suites of aerial and terrestrial predators that patrol desert habitats, in combination with the
paucity of vegetative cover, have provided an ideal setting for studies investigating the role of
predation risk in structuring ecological communities (for review see Brown and Kotler 2004). In
the present study investigating the function of moonlight as a possible cue of predation risk in
desert rodents, there are several predators for which the presence of moonlight could have
46
influenced risk. Predators at the localities sampled likely included long-eared owls (Asio otus),
coyotes (Canis latrans), kit foxes (Vulpes macrotis), long-tailed weasels (Mustela frenata), and
badgers (Taxidea taxus). Great Basin rattlesnakes (Crotalus viridis lutosus) are not noted to
frequent sandy habitats in the Great Basin (Klauber 1982), and are thought to rarely venture into
sandy habitats associated with Microdipodops (Pierce et al. 1992). Thus, only visually-orienting
predators were thought to regularly visit the sampling localities in this study and influence the
risk of predation in open microhabitats.
However, if moonlight acts as a cue of predation risk from visual predators, then why do
not all members of the rodent community avoid moonlight in response to this cue? While
bipedal rodents appear to be concerned with avoiding the risky situations associated with
moonlight, interspecific competition may influence the foraging decisions of quadrupedal
rodents to a greater extent than predation risk. This result could be explained in two ways: 1)
bipedal rodents are actually more at risk from visual predators than quadrupedal forms due to
their larger body size, more conspicuous morphology, and erratic movement patterns; or, 2) the
competitive advantage of bipedal rodents that results from their larger body size, rapid ricochetal
locomotion, and ability to gather large amounts of seed resources provides the luxury of
exercising moonlight avoidance. In either case, the interactions between bipedal and
quadrupedal rodents may be dictated by the foraging decisions of bipedal Dipodomys species. It
is possible that the combination of higher risk for Dipodomys during periods of bright moonlight,
and the luxury of avoiding moonlight provided by the ecological dominance of this genus, results
in the observed moonlight avoidance in bipedal rodents. Thus, this study is likely observing the
interactions of interspecific competition between the two rodent guilds along with the influence
of moonlight-associated predation risk on bipedal rodents.
Kangaroo rats and lunar illumination.—The results of this study indicate that only
bipedal species of Dipodomys appear to display moonlight avoidance behaviors. The question is
subsequently raised, why are quadrupedal rodents seemingly less concerned than bipedal rodents
about moonlight-associated increases in predation risk on bright nights? This question is
47
particularly relevant in the context of a large body of research suggesting that the unique
morphology of Dipodomys (i.e., large hind feet, rapid bipedal ricochet locomotion, large eyes,
long tail, and inflated auditory bullae) is particularly well-adapted for detecting and escaping
predators (e.g., Bartholomew and Caswell 1951, Webster and Webster 1971, Kotler 1984a, b,
1985b). If these unique morphological traits together represent what is often referred to as the
classic “anti-predator morphology” of Dipodomys (e.g., Kotler 1985b), it would seem that
individuals of Dipodomys should be well equipped to detect and escape visual predators such as
owls during moonlit periods, and should not be forced to avoid moonlight at any period of time.
It follows that quadrupedal rodents “lacking morphological anti-predator adaptations” (Kotler
1985b:826) would be expected to be more sensitive to bright moonlight (Bowers 1988).
However, the results of the present study seem to provide contradictory evidence regarding the
assumed effectiveness of the supposed “anti-predator morphology” for kangaroo rats, at least
during the summer seasons and waxing moon phases.
Many other researchers have also found evidence for species of Dipodomys avoiding
bright moonlight (e.g., Justice 1960, Schwab 1966, Lockard and Owings 1974a, b, O'Farrell
1974, Kaufman and Kaufman 1982, Kotler 1984b, Price et al. 1984, Bowers 1988, Brown et al.
1988, Longland and Price 1991, Daly et al. 1992, Bouskila 1995). However, most of these
studies have either focused solely on the activity patterns of Dipodomys (Schwab 1966, Lockard
and Owings 1974a, b, Kaufman and Kaufman 1982, Bowers 1988, Daly et al. 1992, Bouskila
1995), or have found that species of Dipodomys are the only rodents in the community to display
marked moonlight avoidance (Justice 1960, O'Farrell 1974, Price et al. 1984). For instance,
O’Farrell (1974:821) mainly observed moonlight avoidance in the “larger, more conspicuous
species” in the rodent community (i.e., four species of Dipodomys), as compared to the “small,
cryptic species” (i.e., four quadrupedal species) that “did not exhibit as pronounced an effect”
during periods of bright moonlight. Yet Dipodomys-only evidence has not prevented some
workers from making general conclusions about moonlight avoidance in all desert rodents (e.g.,
Bowers 1988). The few studies that have detected moonlight avoidance in both Dipodomys and
48
quadrupedal rodents have found inconsistent responses to illumination between rodent species
(Kotler 1984b, Longland and Price 1991), and even between different experimental trials (Brown
et al. 1988). This study’s finding that quadrupedal rodent species respond to moonlight
differently, and perhaps oppositely, as compared to species of Dipodomys highlights the need to
re-evaluate the ecological role of Dipodomys in the Great Basin desert rodent community.
It is possible that the unique morphology of Dipodomys does not in fact represent a finely
tuned product of natural selection, but instead is the result of broader macroevolutionary events
that have occurred within the heteromyid and geomyid rodent families. Hafner (1993) proposed
the idea that the unique morphologies of Dipodomys, as well as those of Microdipodops and
pocket gophers (Geomys), are the result of heterochronic alterations in the timing of certain
developmental processes. Developmental heterochrony could explain how large-scale
morphological changes may have occurred and been co-opted for their current advantageous
function in exploiting dispersed seed resources in different habitat areas (Hafner 1993). If this
were the case, the distinctive morphology of Dipodomys may be the simple product of neoteny
rather than the gradual selection of adaptive traits (Gould 1977, Hafner 1993). Conversely, a
second hypothesis is that since Dipodomys is so well-adapted to living with the dangers of desert
habitats, and dominant in exploiting dispersed seed resources across large areas of habitat, that
individuals of Dipodomys do not need to risk being active during moonlit periods. It is possible
that a large quantity of accumulated seed resources provides Dipodomys with the luxury of
waiting for less dangerous portions of the night to do their foraging.
Conclusions and future work
The results of this study indicate that moonlight plays a substantial role in patterning the
aboveground activity of a variety of nocturnal rodent species in the Great Basin Desert.
However, data from this study also question whether the “generality of moonlight avoidance” is
in fact “clear” (Lima 1998b: 235) and applicable to the majority of small nocturnal mammals as
has been suggested (Brown and Kotler 2004, Caro 2005). Examining the activity patterns of the
49
natural rodent community in sandy habitats over a 7-year period has provided a unique
opportunity to investigate moonlight avoidance behavior among interacting rodent species across
an expansive geographic area. A study of this type and magnitude has heretofore not been
attempted, but results suggest that similar studies in other North American deserts should provide
a promising avenue for research. In particular, the results of this study should encourage future
studies of a similar nature: investigating perturbations in natural habitats with large sample sizes,
at multiple sites spread throughout a large geographical area, sampling at sites over several
seasons, and during all phases of the moon. Future studies of moonlight avoidance should focus
attention on open habitats with diverse rodent communities and attempt to use habitat indicators
to identify specific types of open habitat, such as the sandy habitats identified in this study.
While the present study examined rodent activity in the cool, high-elevation Great Basin Desert,
a fruitful area of research may involve comparing these observed patterns of moonlight
avoidance to other North American deserts. Specifically, the southern desert regions of North
America (Mojave, Sonoran, and Chihuahuan deserts) may provide an appropriate venue for such
research, but other deserts worldwide may also be suitable. A key question to be investigated in
the future should address whether large-bodied, bipedal, and ecologically dominant rodents
continue to be affected by moonlight differently than smaller-bodied, quadrupedal, and less
dominant rodents.
50
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APPENDIX A - Exact localities of study sites in the Great Basin. Localities taken verbatim
from specimens of the indicator taxon Microdipodops collected for Hafner (2008) and Hafner
and Upham (2009) and deposited in the Moore Laboratory of Zoology (Occidental College).
64
CALIFORNIA
INYO COUNTY – 1: 2.4 miles S, 2.3 miles W Deep Springs, 5050 feet. 2: 4.6 miles S, 3.9
miles W Deep Springs, 5000 feet. 3: 7.2 miles S, 4.0 miles W Deep Springs, 4920 feet.
LASSEN COUNTY – 4: 4.4 miles N, 13.6 miles E Ravendale, 5650 feet. 5: 4.7 miles N, 10.8
miles E Ravendale, 5350 feet. MONO COUNTY – 6: 5 miles N Benton, 5600 feet. 7: 0.2
miles S, 1.5 miles E Oasis, 5050 feet. 8: 1.0 miles S, 4.0 miles E Oasis, 5100 feet.
NEVADA
CHURCHILL COUNTY – 9: 4.3 miles N Fallon, 3900 feet. ELKO COUNTY – 10: 0.9 miles
S, 0.4 miles W Cobre, 5900 feet. 11: 10.9 miles S, 2.5 miles W Contact, 5700 feet. 12: 13.2
miles S, 0.6 miles E Ruby Valley, 6000 feet. ESMERALDA COUNTY – 13: 1.8 miles S, 5.3
miles E Coaldale, 4797 feet. 14: 12.0 miles N, 2.5 miles W Goldfield, 4860 feet. 15: 5.1 S, 1.1
miles E Silver Peak, 4300 feet. EUREKA COUNTY – 16: 22.8 miles N, 3.6 miles W Eureka,
5850 feet. 17: 6.2 miles N, 9.5 miles W Eureka, 6000 feet. HUMBOLDT COUNTY – 18: 13.8
miles N, 11.2 miles E Jungo, 4200 feet. LANDER COUNTY – 19: 6.2 miles S, 19.6 miles W
Austin, 6150 feet. LINCOLN COUNTY – 20: 5.3 miles S, 1.6 miles E Geyser, 5900 feet; 5.2
miles S, 1.9 miles E Geyser, 5900 feet; 5.1 miles S, 2.3 miles E Geyser, 5900 feet. 21: 31 miles
N, 1 mile W Hiko, 5100 feet. 22: 6 miles N, 31 miles W Hiko, 4800 feet. 23: 24 miles W
Panaca, 4600 feet. LYON COUNTY – 24: 11.7 miles S, 3.5 miles E Yerington, 4690 feet; 11.1
miles S, 2.8 miles E Yerington, 4640 feet. MINERAL COUNTY – 25: ¼ mile N Fletcher, 6100
feet. 26: 9.8 miles N, 10.8 miles E Luning, 5350 feet. 27: 12.7 miles N, 9.2 miles E Luning,
5050 feet. 28: 0.4 miles S, 0.5 miles E Marietta, 4950 feet. 29: 8.9 miles S, 1.2 miles E Mina,
4400 feet. 30: 7.3 miles N, 2.6 miles W Schurz, 4287 feet. NYE COUNTY – 31: 3.2 miles N,
4.2 miles E Belmont, 7000 feet. 32: 4.9 miles S, 28.2 miles W Currant, 6000 feet. 33: 6.1
miles S, 2.4 miles E Danville, 6800 feet. 34: 8.4 miles N, 17.5 miles W Duckwater, 6350 feet.
35: 4.6 miles S, 19.8 miles E Goldfield, 4950 feet. 36: 3.0 miles S, 4.3 miles E Gold Reed,
5330 feet. 37: 2.9 miles S, 3.1 miles E Gold Reed, 5350 feet; 2.9 miles S, 4.0 miles E Gold
Reed, 5330 feet. 38: 0.9 miles N, 10.3 miles E New Reveille, 4900 feet. 39: 3.7 miles N, 3.2
65
miles E San Antonio, 5600 feet. 40: 0.5 miles S San Antonio, 5400 feet. 41: 9.2 miles N, 8.1
miles W Tonopah, 4850 feet. 42: 0.5 miles N, 32.0 miles E Tonopah, 5600 feet. 43: 9.8 miles
S, 9.9 miles E Tonopah, 5200 feet; 11.0 miles S, 10.0 miles E Tonopah, 5200 feet; 10.6 miles S,
10.0 miles E Tonopah, 5200 feet. 44: 1.0 miles N, 8.5 miles W Tybo, 6200 feet. 45: 19.2 miles
N, 13.4 miles E Warm Springs, 6000 feet. 46: 7.7 miles N, 9.5 miles E Warm Springs, 5200
feet. 47: 5.9 miles N, 10.2 miles E Warm Springs, 5200 feet; 6.4 miles N, 10.1 miles E Warm
Springs, 5200 feet. 48: 12.7 miles S, 0.4 miles E Warm Springs, 6000 feet. PERSHING
COUNTY – 49: 2.5 miles N, 22.5 miles W Lovelock, 3950 feet. 50: 0.5 miles S, 11.5 miles W
Vernon, 4450 feet. WASHOE COUNTY – 51: 28.5 miles N, 27.8 miles W Gerlach, 4700 feet;
28.2 miles N, 27.6 miles W Gerlach, 4700 feet. 52: 24.5 miles N, 25.0 miles W Gerlach, 4800
feet; 24.0 miles N, 24.8 miles W Gerlach, 4800 feet. 53: 22.4 miles N, 23.6 miles W Gerlach,
4800 feet. 54: 6.4 miles N, 1.0 miles W Nixon, 4200 feet. 55: 1.0 miles N, 1.0 miles W
Wadsworth, 4200 feet. WHITE PINE COUNTY – 56: 6.0 miles S, 4.2 miles W Osceola, 5800
feet.
OREGON
HARNEY COUNTY – 57: 2.4 miles N, 3.4 miles E Fields, 4050 feet. LAKE COUNTY – 58:
36 miles N, 14 miles E Valley Falls, 4300 feet.
UTAH
BEAVER COUNTY – 59: 11.2 miles N, 39.6 miles W Milford, 5200 feet. 60: 4.2 miles S, 15.8
miles W Minersville, 5050 feet. MILLARD COUNTY – 61: 16.1 miles S, 19.6 miles E
Garrison, 5400 feet. 62: 19.3 miles S, 18.4 miles E Garrison, 5100 feet.
... These fluctuations could be explained since several approaches (resource foods, refuges, temperature, between others) nevertheless we suspect of the influence of moonlight on behavior of the marsupials and rodents, due to during the survey in the savanna the moon was in waxing crescent (November 31), likewise in the ecotonal area and forest that was full (December 10) to waning gibbous (December 14) (US Naval Oceanography, 2012). Effect of moonlight has been well documented for several nocturnal mammals by reducing their use of open space, or restricting their activity to darker periods of the night (Morrison, 1978;Gilbert & Boutin, 1991;Wolfe & Summerlin, 1989;Upham, 2008). ...
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Bahuaja Sonene National Park protects the unique sample of subtropical humid savannas in Peru, which are known as “Pampas del Heath” with 6,136 hectares of area. Many endangered species and/or endemic from savannas occur there, however studies about the diversity of mammals in Pampas del Heath are limited and only three assessments there have been carried out since mid-1970s. Therefore we surveyed mammals in three habitat types of the Pampas del Heath (savanna, ecotonal area and forest) during late 2011. We used several methods of record for the different mammal groups including 1) capture techniques with mist nets, snap traps, Sherman traps, Tomahawk traps and pitfall traps, 2) and detection techniques direct by means of camera traps, visualization of mammals during long walk, observation of tracks and interviews to local people. Total capture efforts totalized 6,033 trap/nights, 136 mist-net/nights and 108 cameras/nights. Sixty-nine species of mammals were recorded: 33 in savanna, 33 in ecotonal area and 38 in forest. Sixteen species are new records for the Pampas del Heath and three are new records from Peru (Cryptonanus unduaviensis, Rhogeessa hussoni and Rhogeessa io). Analyses on the sampling effort, relative density, diversity and community structure of small mammals were made for the three habitats types. Moreover eight species are Threatened and 24 are listed in CITES. The new records here presented elevated the previous known mammal species richness in Peru from 538 to 541, and show the importance to conduct inventories to describe the biodiversity in remote areas, like the Pampas del Heath.
... These fluctuations could be explained since several approaches (resource foods, refuges, temperature, between others) nevertheless we suspect of the influence of moonlight on behavior of the marsupials and rodents, due to during the survey in the savanna the moon was in waxing crescent (November 31), likewise in the ecotonal area and forest that was full (December 10) to waning gibbous (December 14) (US Naval Oceanography, 2012). Effect of moonlight has been well documented for several nocturnal mammals by reducing their use of open space, or restricting their activity to darker periods of the night (Morrison, 1978;Gilbert & Boutin, 1991;Wolfe & Summerlin, 1989;Upham, 2008). ...
Article
Bahuaja Sonene National Park protects the unique sample of subtropical humid savannas in Peru, which are known as “Pampas del Heath” with 6,136 hectares of area. Many endangered species and/or endemic from savannas occur there, however studies about the diversity of mammals in Pampas del Heath are limited and only three assessments there have been carried out since mid‑1970s. Therefore we surveyed mammals in three habitat types of the Pampas del Heath (savanna, ecotonal area and forest) during late 2011. We used several methods of record for the different mammal groups including 1) capture techniques with mist nets, snap traps, Sherman traps, Tomahawk traps and pitfall traps, 2) and detection techniques direct by means of camera traps, visualization of mammals during long walk, observation of tracks and interviews to local people. Total capture efforts totalized 6,033 trap/nights, 136 mist-net/nights and 108 cameras/nights. Sixty-nine species of mammals were recorded: 33 in savanna, 33 in ecotonal area and 38 in forest. Sixteen species are new records for the Pampas del Heath and three are new records from Peru (Cryptonanus unduaviensis, Rhogeessa hussoni and Rhogeessa io). Analyses on the sampling effort, relative density, diversity and community structure of small mammals were made for the three habitats types. Moreover eight species are Threatened and 24 are listed in CITES. The new records here presented elevated the previous known mammal species richness in Peru from 538 to 541, and show the importance to conduct inventories to describe the biodiversity in remote areas, like the Pampas del Heath.