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Plant climbing behavior in the scorpion centruroides vittatus

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We examined plant climbing by the common striped scorpion Centruroides vittatus to determine which of two hypotheses (the predation avoidance hypothesis or the increased prey availability hypothesis) best explained this behavior. In the field we observed nocturnal scorpion activity for 16 mo during 1992-1993 to quantify climbing behavior and collected data on potential prey abundance on the ground and in vegetation for 5 mo. We also performed a laboratory experiment examining the effects of hunger level on scorpion climbing and activity. Individuals found up in vegetation accounted for 19.3% and 25.2% of scorpions active in 1992 and 1993, respectively. Juveniles were significantly more likely to be found in vegetation than were adults in both years, although juveniles and adults did not differ in the proportion found carrying prey in vegetation. We found significant seasonal variation in prey abundance, with prey density being greatest in September. Prey density was also significantly greater on the ground than in vegetation when all trapped invertebrates were included; however, when we excluded collembolans because of their small size, this difference was no longer significant. Laboratory results indicated that there were no within-treatment differences among age classes/sexes in any behavior (climbing frequency, activity level or maximum height climbed). When hungry, males (but not females or juveniles) climbed higher and juveniles (but not adults) were more active. Hunger level had no effect on climbing frequency for any age class/sex. In combination, the field and laboratory data are most consistent with the predation avoidance hypothesis as the main reason for plant climbing by scorpions.
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Plant Climbing Behavior in the Scorpion Centruroides Vittatus
Author(s): Christopher A. Brown and Daniel J. O'Connell
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American Midland Naturalist,
Vol. 144, No. 2 (Oct., 2000), pp. 406-418
Published by: The University of Notre Dame
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Am. Midi. Nat. 144:406-418
Plant Climbing Behavior in the Scorpion
Centruroides vittatus
CHRISTOPHER A. BROWN' AND DANIELJ. O'CONNELL
Department of Biology, Box 19498, University of Texas at Arlington, Arlington 76019
ABSTRACT.-We examined plant climbing by the common striped scorpion Centruroides
vittatus to determine which of two hypotheses (the predation avoidance hypothesis or the
increased prey availability hypothesis) best explained this behavior. In the field we observed
nocturnal scorpion activity for 16 mo during 1992-1993 to quantify climbing behavior and
collected data on potential prey abundance on the ground and in vegetation for 5 mo. We
also performed a laboratory experiment examining the effects of hunger level on scorpion
climbing and activity.
Individuals found up in vegetation accounted for 19.3% and 25.2% of
scorpions active in 1992 and 1993, respectively.
Juveniles were significantly more likely to be
found in vegetation than were adults in both years, although juveniles and adults did not
differ in the proportion found carrying prey in vegetation. We found significant seasonal
variation in prey abundance, with prey density being greatest in September. Prey density was
also significantly greater on the ground than in vegetation when all trapped invertebrates
were included; however, when we excluded collembolans because of their small size, this
difference was no longer significant. Laboratory results indicated that there were no within-
treatment differences among age classes/sexes in any behavior (climbing frequency, activity
level or maximum height climbed). When hungry, males (but not females or juveniles)
climbed higher and juveniles (but not adults) were more active. Hunger level had no effect
on climbing frequency for any age class/sex. In combination, the field and laboratory data
are most consistent with the predation avoidance hypothesis as the main reason for plant
climbing by scorpions.
INTRODUCTION
Habitat selection has important consequences for the fitness of an organism and both
the availability of food and the abundance of predators can influence this choice. Ideally,
individuals should choose habitats which both maximize food abundance and minimize the
risk of predation. In practice organisms must often trade off predator avoidance and food
acquisition to maximize fitness (Fraser and Huntingford, 1986; Sih, 1987; Lima and Dill,
1990). These trade-offs may cause animals to forage suboptimally or to engage in potentially
risky behavior (e.g., in different size classes of whelk; Rochette and Himmelman, 1996).
The relative importance of these two factors in influencing habitat selection is the subject
of some debate (see e.g., Kerfoot and Sih, 1987; Cowlishaw, 1997), in part because of diffi-
culties in assessing food availability and predation risk in the field.
Scorpions occur in a variety of terrestrial habitats and may be divided into two general
groups based on microhabitat preference (Polis, 1990): ground-dwelling species, which live
in burrows or under surface debris such as rocks or logs, and arboreal species, which may
be found at various heights in vegetation. This latter group includes the "bark scorpions"
(Stahnke, 1966), a term used to describe many species in the family Buthidae commonly
observed off the ground. In deserts and semiarid regions, where scorpions are common,
the majority of species are ground-dwellers (Polis, 1990; Polis and Yamashita, 1991). During
the day these animals are relatively inactive and remain in their shelter or burrow. Individ-
' Corresponding author and present address: Christopher A. Brown, Department of Biology, State
University of New York College at Fredonia, Fredonia 14063. e-mail: brownc@fredonia.edu
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BROWN & O'CONNELL: SCORPION CLIMBING IN PLANTS
uals emerge from these diurnal retreats shortly after sunset to forage or engage in other
activities, usually remaining on the ground surface near the burrow or shelter (e.g., Polis,
1979; Polis et al., 1985). However, some species climb into vegetation following emergence
(Williams, 1970; Tourtlotte, 1974; Maury, 1975; Polis, 1979; Bradley, 1988; Cao, 1993; Sku-
telsky, 1996), where they stay for all or part of the night.
Although the exact reasons for scorpion climbing are unclear, hypotheses linking either
predation risk or prey abundance to this behavior have been suggested (Bradley, 1988; Polis,
1990). The first hypothesis suggests that climbing is a predator avoidance behavior and,
thus, implicitly assumes that predation risk is reduced in vegetation compared to the
ground. Two observations are consistent with this hypothesis. First, plant climbing has been
seen most commonly in smaller species and juveniles of larger species (Polis, 1979; Bradley,
1988; Skutelsky, 1996). Since these species/individuals are probably preyed upon by a wider
range of ground-dwelling predators (including larger con- and heterospecific scorpions;
Polis and McCormick, 1987) than are larger species/individuals, climbing may reduce en-
counter rates with potential predators. Second, in a few species individuals have been ob-
served carrying prey caught on the ground into vegetation (Polis, 1979; Cao, 1993), perhaps
again due to a reduced chance of encountering a predator (and being killed or losing the
prey while escaping). The second hypothesis suggests that prey abundance is higher in
vegetation, so that foraging is more profitable in this microhabitat (Bradley, 1988; Polis,
1990). Although ground-dwelling scorpions have been observed eating in vegetation (Polis,
1979; Cao, 1993), evidence for active foraging in vegetation is sparse (see Polis, 1990; Sku-
telsky, 1996). To date, there has been no attempt to test either of these hypotheses.
Few quantitative data are available concerning plant climbing behavior of scorpions. For
the vaejovid Paruroctonus mesaensis,
Polis (1979) found that >25% of all scorpions climbed
into vegetation following prey capture to heights ranging from 2 to >75 cm. Of these
scorpions, juveniles were significantly more likely to climb than adults. In another vaejovid,
P utahensis, juveniles were observed in vegetation over four times as often as were adults
(Bradley, 1988). Juveniles of the buthid Buthus occitanus were found on bushes in >40%
of observations, a percentage more than ten times greater than that of adults (Skutelsky,
1996). In this study we examined climbing behavior in a west Texas population of the buthid
scorpion Centruroides
vittatus. Our first objective was to quantify the proportion of scorpions
climbing in vegetation and the proportion feeding on the ground or up in plants using
data collected in the field over 16 mo. Our second objective was to examine densities of
potential prey items on the ground and in vegetation using 5 mo of field-collected data.
Our third objective was to examine the effect of hunger level on climbing behavior and
activity level in a controlled laboratory experiment. We hypothesized that, in the absence
of predation pressure, hunger level should have no effect on scorpion activity or climbing
propensity if climbing is strictly an antipredator behavior. Conversely, if scorpions climb
strictly due to increased prey abundance in vegetation, we predicted that hungry scorpions
would be more likely to climb and would be more active than scorpions which had recently
eaten.
-METHODS
Study organism and field site.-Centruroides vittatus is a medium-sized scorpion found
from Louisiana west to eastern New Mexico and from southern Nebraska to northern Mex-
ico (Shelley and Sissom, 1995). Centruroides
vittatus is included among the "bark scorpions"
(Stahnke, 1966) because it is commonly found off the ground, on trees or fenceposts or in
buildings. However, at our study site the majority of individuals were found in depressions
under rocks and surface debris during the day. Individuals emerged after sunset and were
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THE AMERICAN MIDLAND
NATURALIST
observed both on the ground and in vegetation either actively wandering (e.g., adult males
during mating season) or waiting motionless.
Scorpions were observed at and collected from Chandler Independence Creek Preserve
(hereafter Independence Creek), a property of the Texas Chapter of The Nature Conser-
vancy. This site is located in west Texas on the northeastern edge of the Chihuahuan Desert,
at the confluence of Independence Creek and the Pecos River, 37 km south of Sheffield,
Terrell Co (30?26'30"N, 101?43'26'W). The terrain is extremely rocky in most areas with
dominant vegetation being plants <2 m in height. Medellin-Leal (1982) has characterized
vegetation in this region as microphyllous desertic brushwood, and plants at our site include
creosote bush (Larrea tridentata), catclaw (Acacia sp.), mesquite (Prosopis sp.), lechugilla
(Agave lechugilla), numerous cacti (primarily Opuntia sp.) and a variety of grasses and
herbaceous species. Along both Independence Creek and the Pecos River are stands of
larger trees, including several species of oak. These areas were not included in our field
sampling, although we did observe Centruroides vittatus climbing these trees on several
occasions. Potential predators of nocturnally active C. vittatus include seven species of scor-
pions, orb-weaving and cursorial spiders, solpugids, scolopendrid centipedes and several
reptiles and small mammals. Among the scorpions only two species (Diplocentrus sp. and
Vaejovis crassimanus) are potential predators of adult C. vittatus; however, adults of all
species are potential predators of juvenile C. vittatus.
Field methods.-During April 1992 we established eighteen permanent 100 m2 (10 m X
10 m) noncontiguous quadrats in haphazardly chosen locations at Independence Creek in
three groups of six quadrats each. Within groups quadrats were set up on either side of
narrow dirt roads, with 2-4 quadrats per side. Distances between quadrats in a group were
generally 5-50 m and distances between groups were 0.4-0.7 km. Quadrat corners were
marked with flagging tape, which was replaced as needed. Although we did not quantify
vegetation structure, the three areas differed somewhat in the kinds of larger plants found.
One area (quadrats 1-6) contained primarily lechugilla and creosote bush, a second area
(quadrats 7-12) had few lechugilla but more mesquite and catclaw and a third area (quad-
rats 13-18) had few large plants, being dominated by shrubs and grasses. Plant cover ap-
peared to be equivalent among the three areas surveyed.
We surveyed 10-18 separate quadrats for nocturnal scorpion activity each month from
April 1992 until October 1993, except during July, November and December 1992 (zero
quadrats done), June 1992 (four quadrats done) and April 1993 (five quadrats done). We
usually performed monthly surveys over two consecutive nights, although they were com-
pleted in one evening during several months. Each night we began surveys approximately
1 h after sunset and finished within 2-3 h. Quadrats were surveyed by 2-3 people concur-
rently, each working from the outside of the plot towards the middle. Scorpion activity
was
observed using flashlights equipped with ultraviolet bulbs (the exoskeleton fluoresces yel-
low-green under UV light and can be seen at distances of 1-10 m). For each Centruroides
vittatus we recorded age class/sex (male, female or juvenile), position (on the ground or
in vegetation) and height aboveground to the nearest cm. Scorpions were counted as being
in vegetation only if the entire body was off the ground. We also noted any predation events
by a scorpion and, if retrieved, preserved prey items for identification.
One night each month from June to October 1993 we estimated potential prey density
at ground level and at various heights above ground in two of the above quadrats, selected
randomly. To capture ground-dwelling prey we placed one coffee can lid (15.2 cm diameter)
at each corner and in the center of each quadrat (five lids/quadrat, total lid area/quadrat
= 907 cm2). Lids were placed flush with the ground and the exposed side covered with a
sticky adhesive. To determine prey distribution in vegetation we placed a square wooden
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BROWN & O'CONNELL: SCORPION CLIMBING IN PLANTS
stake (2.5 cm wide) near nine separate plants or clumps of vegetation per quadrat. Each
stake was marked in 10 cm intervals from 0-80 cm, where 80 cm represented the maximum
height previously measured for an arboreal Centruroides vittatus. One interval per stake
(e.g., 0-10 cm, 10-20 cm) was covered on all sides with adhesive, with a separate interval
done for each stake in a quadrat (total stake area with adhesive/quadrat = 800 cm2). We
used this method to obtain estimates of prey abundance at different heights throughout
the quadrat, rather than within a single stand of vegetation. Stakes and lids were placed <1
h before sunset and retrieved <1 h after sunrise and should indicate prey available to
foraging scorpions throughout the night. We wrapped collected lids and stakes in clear
plastic and returned them to the laboratory where potential prey items were counted and
identified to order. We did not survey quadrats for scorpion activity on nights when they
contained sticky traps.
Laboratory methods.-We examined the effect of hunger level on climbing behavior of
Centruroides
vittatus with an experiment during July and August 1994, using 52 scorpions
(16 juveniles, 20 males and 16 females) collected at Independence Creek from March-June
1994. Before the start of the experiment adults and larger juveniles were kept in plastic
containers (18.5 X 7.5 X 9 cm) with a sand substrate; a moistened paper towel served as
cover and a source of water. Smaller juveniles were kept in petri dishes (9 cm diam) with
a sand substrate and a moistened paper towel square. Scorpions were maintained in the
experimental room on a reverse light cycle (9 h dark: 15 h light) for at least 1 mo before
the start of trials. Food was offered once every 3 wk (one adult cricket, Acheta domestica,
for adult scorpions, 1-3 juvenile crickets for juvenile scorpions). Temperature in the room
was maintained at 23.5-25.5 C.
Nine aquaria (51 cm X 26.5 cm X 30.5 cm) were used as experimental chambers. To
simulate vegetation we used two square wooden stakes per aquarium (90 cm in height, 2.5
cm wide), marked off in 5 cm intervals. Although artificial, these stakes provided a simple
standardized climbing structure requiring no maintenance (as would be true for live plants)
and preliminary trials indicated that scorpions readily climb these stakes. We placed one
stake, nailed to a wooden platform, at either end of an aquarium at least 7.5 cm from any
side wall. Aquaria were then filled with sand to a depth of approximately 5 cm, sufficient
to cover the platforms. Sand was replaced after each trial run to avoid potential changes
in behavior caused by release of pheromones by females (as in Paruroctonus mesaensis;
Gaffin and Brownell, 1992). A tile (12 cm square) supported by pebbles, placed in the
middle of an aquarium, served as cover.
Each scorpion was observed in two trials, one after an extended period without food
(unfed) and one immediately after eating (recently fed). Unfed scorpions had last eaten
4-5 wk before beginning a trial run (1-2 wk longer than the normal feeding interval).
Recently fed scorpions had been offered a cricket 24 h before beginning a trial run. Both
sets of trials were run in an identical manner. We placed one scorpion in each aquarium
at the beginning of the light cycle and allowed it to acclimate for 15 h. Beginning 10 min
before the room went dark we observed all aquaria once every 15 min throughout the 9 h
dark cycle, using UV flashlights to minimize disturbance. At each observation we recorded
activity (moving or stationary), position (on the ground or climbing the stake) and height
aboveground, if applicable. Observations for each trial were made over two consecutive
dark cycles. All scorpions were initially run in the unfed trials, with recently fed trials oc-
curring approximately 4 wk later.
Statistical analysis.-For field data we used one-way ANOVA to examine the effect of age
class/sex on height climbed and to examine variation among quadrats in nocturnal activity.
We also used one-way ANOVA to assess monthly variation in potential prey abundance. To
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THE AMERICAN
MIDLAND
NATURALIST
examine variation in potential prey abundance between the ground and vegetation, we
combined all lid or stake counts within each quadrat and analyzed these data with a paired
t-test. Prey abundance data are given as number of prey per cm2, log-transformed to obtain
homogeneity of variances. Gtests of independence (Sokal and Rohlf, 1981) were used to
examine differences between juveniles and adults in numbers found climbing and numbers
found feeding in vegetation. Finally, we used correlation analysis (Pearson's r) to examine
the relationship between adult density and either juvenile density or the proportion of
juveniles found in vegetation. We included scorpions of undetermined age/sex in the anal-
ysis of among-quadrat variation only.
From laboratory data we defined three variables: activity level, climbing frequency and
maximum height climbed. Activity level and climbing frequency were defined as the total
number of observations during which a scorpion was seen moving or was seen up a stake,
respectively. These variables had a maximum value of 36 for one dark period. In these
analyses we used nonparametric statistics due to deviations from normality (even after trans-
formations) for most of the data. We examined the effect of age class/sex on activity level,
climbing frequency and maximum height climbed using Kruskal-Wallis
ANOVA, with a sep-
arate analysis done for each feeding treatment. A Wilcoxon matched-pairs test was used to
assess variation in these variables due to feeding treatment. For both analyses we combined
data from the two dark periods for activity level and climbing frequency. Maximum height
climbed was also determined from the combined data. All analyses were done using Statis-
tica for Windows version 4.5 (StatSoft, 1993).
RESULTS
Activity and climbing behavior in the field.-In our nocturnal surveys we observed 135
Centruroides vittatus (47 male, 48 female, 34 juveniles and 6 undetermined) during 1992,
and 131 C. vittatus (36 male, 37 female, 55 juvenile and 3 undetermined) during 1993 (see
Table 1 for the distribution of sampling effort among quadrats). This species accounted for
89% of scorpions active in 1992 and 93% of scorpions active in 1993. During most months
the density of active C. vittatus was 1.0-2.6 per 100 m2 (Fig. 1). Activity declined sharply
when nocturnal temperatures dropped below approximately 13 C, as in January, February
and October 1993. We found no significant variation in surface activity among quadrats
during either 1992 (F = 1.00, P > 0.05, df = 17, 52) or 1993 (F = 1.17, P > 0.05, df =
17, 108; Table 1). Juvenile density was not significantly correlated with adult density whether
we used data from all quadrats (r = 0.07, P > 0.05, n = 198) or averaged across months
for each quadrat (r = -0.15, P > 0.05, n = 18).
The monthly proportion of Centruroides vittatus found in vegetation ranged from zero
(January and October 1993) to one-half (June 1992 and April 1993; Fig. 2). Overall, climb-
ing scorpions represented 19.3% (26 of 135) and 25.2% (33 of 131) of observations in 1992
and 1993, respectively. Juveniles were more likely to be found in vegetation than adults
during both 1992 (G = 14.89, P < 0.0001) and 1993 (G = 3.84, P = 0.05; Table 2), although
the proportion of juveniles found climbing was uncorrelated with adult density (using quad-
rat averages; r = -0.07, P > 0.05, n = 18). Age class/sex had no significant effect on the
height climbed by scorpions (mean ? 1 SE: females, 34.7 ? 5.2 cm; males, 27.2 ? 11.7 cm;
juveniles, 29.3 ? 3.4 cm; F = 0.59, P > 0.05, df = 2, 56).
We observed three Centruroides vittatus carrying prey during August-September 1992
and an additional 10 carrying prey during April-July 1993. Twelve of these scorpions were
observed in vegetation, but in no instance did we see the scorpion capture the prey or
carry it into vegetation. When data from both years were combined the proportion of
climbing scorpions found carrying prey did not differ between adults and juveniles (G =
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BROWN & O'CONNELL: SCORPION CLIMBING IN PLANTS
TABLE 1.-Nocturnal surface density (number/100 m2) per quadrat for juvenile and adult Centru-
roides vittatus at Independence Creek, averaged over all months. The last columnn indicates the number
of months (of a possible 16) a quadrat was surveyed
Quadrat number Juvenile density Adult density Times surveyed
1 0.07 0.67 15
2 0.2 1.2 15
3 0.27 0.87 15
4 0.57 1.5 14
5 0.77 0.15 13
6 0.27 0.36 11
7 0.27 0.18 11
8 0.9 0.5 10
9 0.54 1.23 13
10 0.64 1 11
11 0.55 1.09 11
12 0.7 0.7 10
13 0.11 0.78 9
14 1 0.56 9
15 0.57 0.57 7
16 0.29 0.57 7
17 0.25 1.25 8
18 0.43 1.14 7
0.003, P > 0.05; Table 2). We were able to collect prey items from six scorpions feeding in
vegetation; these included four roaches (family Blattoidea), one orthopteran leg, and one
juvenile C. vittatus (being eaten by an adult female).
Our sticky traps captured prey from 12 orders of arthropods, with most taxa caught both
on the ground and in vegetation (Table 3). Collembolans compose almost half the inver-
tebrates caught at surface level, but are unlikely to be used as prey, except possibly by second
instar scorpions. Therefore, we analyzed variation in potential prey density both with and
without collembolans. Whether or not we included collembolans, potential prey density
exhibited significant variation among months (with collembolans: F = 10.41, P < 0.001, df
= 4, 84; without collembolans: F = 9.66, P < 0.001, df = 4, 84). A post hoc Scheffe's test
(Day and Quinn, 1989) indicated that prey density was significantly higher in September
than in any other month. When collembolans were included potential prey abundance on
the ground was significantly higher than in vegetation (t = 2.98, P < 0.05, df = 9). However,
we found no significant variation in potential prey density when we removed collembolans
(t = 1.54, P > 0.05, df = 9).
Climbing behavior in the laboratory.-One male and two juvenile scorpions did not eat
when offered prey before the recently fed trials, and one juvenile died during the recently
fed trials; data from these scorpions were excluded from all analyses. There was no influence
of age class/sex on climbing frequency (unfed: H = 0.36, P > 0.05; recently fed: H = 4.13,
P > 0.05), activity level (unfed: H = 0.38, P > 0.05; recently fed: H = 0.51, P > 0.05) or
maximum height climbed (unfed: H = 0.45, P > 0.05; recently fed: H = 4.63, P > 0.05;
df = 2, 48 for all tests) for either unfed or recently fed scorpions (Figs. 3a-c). Hunger level
did have some effects on behavior, but results were not consistent across the three age/sex
groups. There was no effect of hunger level on climbing frequency for any group (males:
Z = 1.85, P > 0.05, n = 19; females: Z = 0.65, P > 0.05, n = 16; juveniles: Z = 0.47, P >
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THE
AMERICAN
MIDLAND
NATURALIST
5 30
Juv Adult Total
4I - ~
I \ - 25
crEg
~ yg \ s~-20 ?
-15 u
~~~~~1-~~1
-5
0 n 0\ 0
AM J J A S OND J FMAM J J A S O
1992 1993
FIG. 1.-Surface densities of nocturnally active Centruroides
vittatus, April 1992-October 1993. No
data were obtained in July, November or December 1992. The line indicates the average temperature
at the completion of each monthly quadrat survey
0.05, n = 13; Fig. 3a). Males climbed significantly higher when hungry than after recently
eating (Z = 2.33, P < 0.05, n = 19; Fig. 3c), while hunger level did not affect maximum
height climbed by females (Z = 1.39, P > 0.05, n = 16) or juveniles (Z = 0.75, P > 0.05,
n = 13). Finally, hunger level had no significant effect on activity level in males (Z = 0.72,
P > 0.05, n = 19) or females (Z = 0, P > 0.05, n = 16), but juveniles were significantly
more active during the unfed trials compared to the recently fed trials (Z = 2.03, P < 0.05,
n = 13; Fig. 3b).
DISCUSSION
Our field data show that individuals climbing in vegetation were a substantial percentage
(approximately 20-25%) of all Centruroides vittatus observed nocturnally in 1992-1993. The
range of heights climbed by C. vittatus was similar to the range reported for Paruroctonus
mesaensis (Polis, 1979) and juvenile Buthus occitanus (Skutelsky, 1996). As has been found
in P mesaensis (Polis, 1979), P utahensis (Bradley, 1988) and B. occitanus (Skutelsky, 1996),
juveniles were more likely than adults to be found in vegetation. However, when observed
in vegetation juveniles were no more likely to be carrying prey than were adults. Juvenile
activity and climbing behavior did not appear to be affected by adult activity levels, as there
was no significant decrease in activity or increase in the proportion found in vegetation
when adult density increased. Such behavioral shifts by juveniles may occur only at higher
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BROWN
& O'CONNELL: SCORPION
CLIMBING
IN PLANTS
60
50 -
o 40-
-r
0
30-
20 -
10 -
0 AM J J A S O N D J F MAM J J A S O
1992 Month 1993
FIG.
2.-Monthly variation in the percentage of Centruroides vittatus (combining adults andjuveniles)
found climbing in vegetation, April 1992-October 1993. No data were obtained in July, November or
December 1992
adult densities than we have observed at Independence Creek, and so may be difficult to
detect at this site.
Potential prey abundance in the field varied seasonally, but we found significant differ-
ences between densities of ground-level and arboreal prey only when taxa unlikely to be
important prey items (collembolans) were included. (Note that we use ground-level and
arboreal to indicate only where prey were trapped, not their presumed microhabitat pref-
erence. It is possible that some animals were trapped at heights different from where they
normally occurred or were trapped on the ground after falling or being blown out of
vegetation.) Although we did not directly observe any prey captures, we suspect that many
TABLE 2.-The number of male, female and juvenile Centruroides
vittatus observed on the ground
(Ground), in vegetation (Climb) and carrying prey in vegetation (Climb/Prey) at Independence Creek
during 1992 and 1993. The category Climb includes scorpions observed with or without prey
1992 1993
Ground Climb Climb/Prey Ground Climb Climb/Prey
Males 43 4 0 31 5 1
Females 41 7 1 28 9 3
Juveniles 19 15 1 36 19 6
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THE AMERICAN
MIDLAND
NATURALIST
TABLE
3.-Densities (number/100 cm2) of potential invertebrate prey found on the surface and at
heights aboveground from 0-80 cm. Values shown are total number of trapped prey of each order,
combining data from all 5 mo
Height
aboveground
(cm)
Order Surface 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80
Acari 0.27 0 0 0 0 0 0.1 0 0
Araneae 0.04 0.1 0 0.1 0.1 0.1 0.1 0 0
Blattoidea 0.04 0 0 0.1 0 0.1 0.1 0 0.2
Coleoptera 0.04 0.1 0.1 0 0 0.1 0.3 0 0
Collembola 2.29 0.7 0.3 0 0 0 0 0 0
Diptera 1.42 0.6 0.6 0.9 0.2 0.6 0.6 0.2 0.6
Hemiptera 0.04 0.2 0.1 0.1 0 0 0.1 0 0.1
Homoptera 0.31 0.1 0.1 0 0.2 0.1 0.6 0.3 0.1
Hymenoptera 0.18 0.1 0.1 0 0.4 0.2 0.1 0.1 0.2
Lepidoptera 0.02 0 0 0 0 0 0 0 0
Orthoptera 0.08 0.3 0 0 0.1 0 0 0.1 0
Trichoptera 0 0 0 0 0 0 0 0 0.2
Unknown 0.36 0.4 0.3 0.2 0.3 0.1 0.3 0.1 0.2
Total 5.08 2.6 1.6 1.4 1.3 1.3 2.3 0.8 1.6
prey carried by arboreal Centruroides vittatus were captured on the ground, based on data
collected within and outside quadrats. Of the six prey items recovered from quadrat surveys,
five (four roaches and a juvenile C. vittatus) were normally observed on the surface. An
additional 14 prey items were recovered from scorpions in vegetation outside the quadrats,
of which nine (one solpugid, three juvenile C. vittatus and five roaches) were more com-
monly observed on the ground.
The laboratory experiment yielded few significant results, and these were not consistent
across all age classes/sexes. Only juveniles were more active and only males climbed higher
when hungry than after having recently eaten. Additionally, climbing frequency was unaf-
fected by hunger level for both juveniles and adults.
Before proceeding further, we address two potential difficulties with our study, the first
methodological and the second primarily statistical. First, in our experimental design all
scorpions were run initially in the unfed trial. In retrospect, we should have randomized
the trial order to avoid possible biases of trial sequence (e.g., learning that the stake was
devoid of prey). Despite this, we feel confident that our results were not artifacts. Enough
time passed between trials (4-5 wk) that retention of information about experimental cham-
bers by scorpions seems unlikely. Additionally, Shaffer and Formanowicz (1996) have dem-
onstrated in studies of sprint speed that C. vittatus do not show trial effects over a much
shorter period (1 wk).
The second difficulty concerns the possibility that some climbing scorpions were counted
multiple times in the field over the course of our study. As we did not mark scorpions, we
cannot assess the extent to which this occurred. From a statistical viewpoint, counting mul-
tiple occurrences of the same individual as independent data points is a type of pseudo-
replication (Hurlbert, 1984). This can lead to overestimation of the degrees of freedom in
parametric tests, increasing the risk of a Type I error. As most of our field analyses were
nonsignificant or strongly significant, we suggest that any potential pseudoreplication will
have minimal effect on our conclusions. The one exception is for climbing propensity of
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BROWN & O'CONNELL: SCORPION CLIMBING IN PLANTS
Male
(RF) Fem
(UF) Fem
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FIG. 3.-The effects of hunger level on the behavior of male, female (Fem) and juvenile (Juv)
Centruroides vittatus in the laboratory climbing experiment. Data represent means (+1 SE). Methods
for scoring climbing frequency and activity level are given in the text. UF = unfed, RF = recently fed.
Sample sizes: males, n = 19; females, n = 16; juveniles, n = 13. (a) Climbing frequency. (b) Activity
level. (c) Maximum height climbed
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Male
(UF)
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THE AMERICAN MIDLAND NATURALIST
juveniles versus adults in 1993, where we found a P-value of exactly 0.05. The significance
of this result therefore should be viewed cautiously.
Based on both field and laboratory data, we conclude that scorpion climbing behavior is
unlikely to be explained solely by increased prey abundance in vegetation. Potential prey
abundance on the ground was greater than, or not significantly different from, abundance
in vegetation and scorpions did not alter behavior by increasing climbing frequency in the
laboratory when hungry. Although we did not determine this, the possibility exists that
foraging in vegetation is more profitable because prey there have greater biomass or higher
levels of an important nutrient or are easier to subdue. However, since scorpions are op-
portunistic foragers and may emerge rarely from their retreats (Polis, 1990), it seems more
likely that foraging patch decisions would be based primarily on prey encounter rate (i.e.,
prey density). If this is correct, the choice of the surface as a foraging site is as good as or
better than vegetation.
The support for the predation avoidance hypothesis (Bradley, 1988; Polis, 1990), we be-
lieve, is slightly more compelling. The higher percentage of juveniles found climbing com-
pared to adults is consistent with this hypothesis, although we would also predict that ju-
venile climbing frequency should increase as adult surface activity increased (a conclusion
not supported by our results). We point out that the prediction of a greater proportion of
juveniles than adults found climbing is not unique to the predator avoidance hypothesis.
We also would predict that juveniles would be more likely to climb if prey availability was
higher in plants and younger scorpions required more frequent meals or if the abundance
of preferred prey sizes of juveniles was greater in vegetation. The high percentage of ar-
boreal scorpions carrying prey that we suspect were captured on the ground is also consis-
tent with the suggestion that the risk of encountering a predator is lower in vegetation. In
order to adequately test this hypothesis, one would need to quantify predation pressure (in
the field or laboratory) and then test to see how scorpion climbing behavior is related to
changes in this pressure. In the field this would involve estimating population densities of
all potential scorpion predators, as well as determining each predator's encounter rate with
and ability to capture various size classes of scorpions, a potentially daunting task.
Thus, we suggest that plant climbing by Centruroides vittatus is more likely to be a pred-
ator avoidance strategy than a foraging strategy, although support for either hypothesis was
relatively weak. Predation risk is well known to cause changes in behavior in a variety of
invertebrates (e.g., backswimmers: Sih, 1982; whelks: Rochette and Himmelman, 1996; grass-
hoppers: Schmitz, 1998), including scorpions (Polis and McCormick, 1986; Skutelsky,
1996),
and often leads at least some individuals to forage suboptimally. Given that these are usually
the individuals most vulnerable to predation (Sih, 1987; Lima and Dill, 1990), it may not
seem surprising that the behavior of juvenile C. vittatus offers the best support for the
predation avoidance hypothesis. However, it remains unclear based on our data whether
juveniles indeed are forced into a trade-off between foraging and predator avoidance. Po-
tential prey densities may not differ significantly between the surface and vegetation, so
that foraging in either location may be equally profitable. Even if prey abundance is higher
on the surface, juveniles may still be maximizing prey encounters (by foraging on the
ground) and minimizing predation risk (by carrying captured prey into vegetation). This
latter observation will depend on whether juveniles forage at the best times or in the best
locations on the surface (see Polis and McCormick, 1986).
We conclude by offering a third possible hypothesis, originally made by Jaeger (1978) to
explain plant climbing by the salamander Plethodon cinereus.
Jaeger suggested that sala-
manders climb plants as part of a movement pattern strategy (Kiester and Slatkin, 1974).
In these animals movement was in search of food and may have been due to a random
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BROWN & O'CONNELL: SCORPION CLIMBING IN PLANTS
walk, in which movement was not influenced by the topography of the area, or a "hill
climbing" movement, in which animals moved up an increasing resource gradient (i.e.,
potential prey). In the latter case, the movement strategy need not be influenced by prey
level only, but may be a response to other resources or cues. For example, increased scor-
pion activity has been correlated with increased water vapor pressure and wind velocity
(Skutelsky, 1996) and decreased moonlight (Hadley and Williams, 1968; Skutelsky, 1996).
Thus, for instance, scorpions may be climbing in reaction to a humidity gradient in order
to avoid desiccation, or their climbing may be simply a result of random movement over
the terrain. We suggest that this hypothesis should be considered as another alternative in
any future research on climbing behavior in scorpions.
Acknowledgments.-For assistance in the field, we thank Dan Formanowicz,John Davis, Paul Klawinski
and Josh "Slothor" Rose. JoBeth Holub graciously kept our field site free of rattlesnakes and helped
us enjoy our visits to west Texas. Regina Huse and Noel Gutierrez did yeoman's work tracking the
movements of scorpions in the laboratory. D. Formanowicz assisted in much of the initial statistical
and experimental design. The comments of three anonymous reviewers helped us sharpen our ideas
and analysis. This work was supported by a grant from the Texas Chapter of The Nature Conservancy
to D. Formanowicz.
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SUBMITTED
6 JULY
1999
418 144(2)
ACCEPTED
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2000
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... A temporal shift in activity between juveniles and adults can occur with lunar cycle in B. occitanus (Skutelsky, 1996), but temporal shifts between adults and juveniles do not always occur as in P. utahensis (Bradley, 1988). In earlier studies of the focal animal, Centruroides vittatus (Say, 1821) (Buthidae), more juveniles than adults were observed climbing in vegetation (Brown & O'Connell, 2000), and temporal differences in surface density occur in adults, juveniles and second instars (Yamashita, 2004). Parameters such as microhabitat use, taxa of prey in the diet, the height of scorpion in vegetation and seasonal and lunar activity have been examined in previous studies (see McReynolds, 2004McReynolds, , 2008, but the size of scorpion was not considered. ...
... Polis (1980b) hypothesized that S. mesaensis juveniles under (or in) vegetation were there to avoid cannibalism by the adults and Skutelsky (1996) hypothesized that B. occitanus juveniles avoided predators by climbing in vegetation. Juvenile C. vittatus in west Texas were climbing in vegetation at higher frequency than adults (Brown & O'Connell, 2000). Brown & O'Connell (2000) found more support for the predator avoidance hypothesis for climbing by C. vittatus. ...
... Juvenile C. vittatus in west Texas were climbing in vegetation at higher frequency than adults (Brown & O'Connell, 2000). Brown & O'Connell (2000) found more support for the predator avoidance hypothesis for climbing by C. vittatus. However, there is no support for smaller size classes of C. vittatus in south Texas climbing into vegetation to avoid larger size classes except perhaps size classes I and II using grass at a higher frequency. ...
... They are excellent predators, preying on a wide variety of populations of insects, spiders and even other scorpions, and in turn are prey, mostly of vertebrates (Williams 1987;McCormick & Polis 1990;Polis 1990;Brownell & Polis 2001). They occur in a variety of terrestrial habitats and may be divided into two general groups based on microhabitat preference: ground-dwelling species, which live in burrows or under surface debris such as rocks or logs, and arboreal species, found at various heights on the vegetation (Polis 1990;Brown & O'Connell 2000). In deserts and semiarid regions, where scorpions are common, the majority of species are ground-dwellers (Polis 1990;Polis & Yamashita 1991). ...
... The behavior and the substrate used by many species of scorpions are related, among other factors, with the sex-age class (Skutelsky 1996;Brown & O'Connell 2000;McReynolds 2004;Yamashita 2004). For example, walking behavior in several scorpion species is more associated with males than with females and juveniles (Polis & Farley 1979;Polis 1980;Yamashita 2004). ...
... We found the juveniles associated with feeding in vegetation. The behavior of climbing the vegetation has been previously observed in other species of scorpions (Williams 1970;Polis 1979;Bradley 1988;Cao 1993;Skutelsky 1996;Brown & O'Connell 2000;McReynolds 2004McReynolds , 2008. In Paruroctonus utahensis (Williams, 1968), more juveniles than adults were observed in the vegetation (Bradley 1988). ...
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Predation has long been implicated as a major selective force in the evolution of several morphological and behavioral characteristics of animals. The importance of predation during evolutionary time is clear, but growing evidence suggests that animals also have the ability to assess and behaviorally influence their risk of being preyed upon in ecological time (i.e., during their lifetime). We develop an abstraction of the predation process in which several components of predation risk are identified. A review of the literature indicates that an animal's ability to assess and behaviorally control one or more of these components strongly influences decision making in feeding animals, as well as in animals deciding when and how to escape predators, when and how to be social, or even, for fishes, when and how to breathe air. This review also reveals that such decision making reflects apparent trade-offs between the risk of predation and the benefits to be gained from engaging in a given activity. Despite this body of evidence, several areas in the study of animal behavior have received little or no attention from a predation perspective. We identify several such areas, the most important of which is that dealing with animal reproduction. Much work also remains regarding the precise nature of the risk of predation and how it is actually perceived by animals, and the extent to which they can behaviorally control their risk of predation. Mathematical models will likely play a major role in future work, and we suggest that modelers strive to consider the potential complexity in behavioral responses to predation risk. Overall, since virtually every animal is potential prey for others, research that seriously considers the influence of predation risk will provide significant insight into the nature of animal behavior.
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Indirect effects emerge when a change in the abundance of one species indirectly affects another by changing the abundances of intermediate species—called density‐mediated indirect effects—or they arise when one species modifies how two other species interact—called trait‐mediated indirect effects. I report on field experiments that evaluated how grass and herb biomass in old‐field interaction webs was influenced indirectly by a spider carnivore through its interactions with a generalist and a grass‐specialist grasshopper species. I manipulated interaction pathways between the spider and the plants using different combinations of the grasshopper species. I changed the modality of predator‐prey interactions to isolate density‐mediated from trait‐mediated effects using natural spiders (predation spiders) or spiders that were prevented from subduing prey by mouthpart manipulation (risk spiders). I found that indirect effects were stronger in speciose, reticulate food webs than in linear food chains owing to a trait‐mediated effect, a diet shift by herbivores in response to predation risk. Spiders alone did not have significant effects on grasshopper densities in the field experiments, removing any possibility of density‐mediated indirect effects. The study illustrates that ecologists should not underestimate the importance of behavioral ecology in determining community‐level interactions.
Article
Field and laboratory experiments demonstrated that the degree to which an aquatic insects, Notonecta hoffmanni, shows avoidance behavior towards an important predator is proportional to the magnitude of the risk of predation. Laboratory experiments showed that I, II, and III instar juvenile N. hoffmanni can suffer significant mortality due to predation from adult N. hoffmanni whereas IV and V instars suffered no mortality from adults. In particular, the relative risk of predation from adults was I > II > III > IV = V @? no risk. Based on these data I predicted that I, II, and III instars should avoid adults whereas IV and V instars should not, and that the relative degee of avoidance should be I > II > III > IV = V @? no avoidance. Two measures of predator avoidance were documented: alterations in the use of space, and reductions in movement. In both the laboratory and the field the effects of adults on the behavior of juvenile notonectids were determined by comparing the juvenile's behavior when adults were experimentally removed to that in controls where adults were present. Alterations in spatial patterns were as predicted, i.e., in both the laboratory and the field only I, II, and III instars avoided adults by altering their use of space, and the degree to which spatial patterns were altered was highly correlated with the relative risk of predation. In the laborabory, reductions in movement were also as predicted, i.e., only I, II, and III instars reduced their movement in the presence of adults and the reductions in movement was highly correlated with the risk of predation. However, in the field, only I and III instars avoided adults by moving less; i.e., II instars did not conform to expectations. Accordingly, in the field the correlation between the reduction in movement and the risk of predation was positive but not significant. In the laboratory, the effects of adults on juvenile feeding rates were also documented. Because I and II instars avoided adults by altering their spatial and movement patterns, they suffered a cost in reduced feeding rates. Unexpectedly, III instars altered their feeding behaviors but suffered no reduction in feeding rats. Partial correlation analysis showed that a decrease in the juvenile's use of the central region of water bodies (where prey were four times more dense) resulted in a significant decrease in their feeding rates, but that a reduction in their movement had no such effect. I then discuss the effects on the predictions of optimal foraging theory of some common mechanisms by which notonectids avoid their predators. Because the degree of avoidance is related to the risk of predation I also discuss the possibility that feeding behavior represents an adaptive compromise between the conflicting demands of feeding efficiently and avoiding predators. Finally, because the avoidance of predation reduces both the feeding rate of predators on foragers and of foragers on prey, I examine the effects of such reductions on population interactions among predators, foragers, and prey.
Article
Field and laboratory studies were conducted on the nocturnal surface activities any feeding behavior of scorpions, using ultraviolet light. Two species, Vejovis confusus and V. mesaensis, showed a decrease in surface occurrence as the evening progressed, while Centruroides sculpturatus showed a random occupation throughout the night. Increased intensity of moonlight resulted in a significant decrease in surface occurrence in V. confuses and V. mesaensis; C. sculpturatus showed no significant response to increased illumination. Species differences in the amount of surface activity and feeding behavior were noted. Potential prey, food preferences, and mechanisms used in prey detection are also discussed.
Article
Current reproductive effort may often be at the expense of future reproduction. One way in which future reproduction of viviparous animals may be affected is by increased. risk of predation resulting from decreased mobility associated with pregnancy. The common striped scorpion, Centruroides vittatus, may experience considerable risk of predation associated with reproduction because it is viviparous,;with an eight-month gestation period. C. vittatus also carries the newborn young on its back during their first instar. The purpose of this study was to establish a cost of viviparity and parental care in these scorpions by determining sprint speed at three reproductive stages: pregnant, carrying offspring and post-dispersal of offspring. Post-dispersal speed was used as a best estimate of non-pregnant speed. Pregnant speeds averaged 84% of post-dispersal speeds. Lower speeds were correlated with absolute and relative measures of litter size. Speed while carrying offspring averaged 61% of post-dispersal speed, and was correlated with mass of the litter and number of individuals in the litter. Sixty-five per cent of the females carrying young could not be induced to run; these females instead assumed a defensive posture. Results indicate that female scorpions experience a cost (in decreased running speed) to viviparity and parental care, and that some females may reduce this cost by using an alternative defensive strategy. (C) 1996 The Association for the Study of Animal Behaviour.
Article
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Article
The influence of moonlight on the foraging behaviour of the scorpionButhus occitanus israeliswas observed. In contrast to their visual predators, most scorpions use air and surface vibrations to locate their prey. Therefore, moonlight is not likely to have a direct effect on the scorpions' foraging behaviour, but rather an indirect effect through changes in risk from predation or changes in prey availability. Adult scorpions were less active on moonlit nights than on dark nights, and adults that were active on full moon nights tended to ambush under bushes. Juveniles, however, did not seem to alter their activity as a response to moonlight. Mass-to-size ratios of adult scorpions foraging on moonlit nights were significantly lower than those of scorpions foraging on dark nights, suggesting that scorpions that forage in bright moonlight are individuals with relatively low energy reserves. The results are consistent with the hypothesis that risk of predation is higher during periods of bright moonlight. It is suggested that the scorpions use environmental factors such as illumination and humidity as proximal cues, to evaluate levels of predation risk and food availability.
Article
Terrestrial salamanders (Plethodon cinereus) leave the forest floor during rainy and foggy nights and climb plants where they forage on plant-dwelling homopterans and hemipterans. Salamanders that climb plants at night ingest a significantly larger volume of food than do conspecifics that forage on the forest floor at night. Plants are not climbed on dry nights when desiccation of the salamanders occurs rapidly. Plant-dwelling insects may provide an important but periodic food resource for the salamanders.