Photic Preference of the Short-Tailed Opossum (Monodelphis domestica)

Abstract

The gray short-tailed opossum (Monodelphis domestica) is a nocturnal South American marsupial that has been gaining popularity as a laboratory animal. However, compared to traditional laboratory animals like rats, very little is known about its behavior, either in the wild or in a laboratory setting. Here we investigated the photic preference of the short-tailed opossum. Opossums were placed in a circular testing arena and allowed to move freely between dark (0 lux) and light (∼1.4, 40, or 400 lux) sides of the arena. In each of these conditions opossums spent significantly more time in the dark than in the illuminated side and a greater proportion of time in the dark than would be expected by chance. In the high-contrast (∼400 lux) illumination condition, the mean bout length (i.e., duration of one trip on the light or dark side) was significantly longer on the dark side than on the light side. When we examined the number of bouts greater than 30 and 60 s in duration, we found a significant difference between the light and dark sides in all light contrast conditions. These data indicate that the short-tailed opossum prefers the dark to the light, and can also detect very slight differences in light intensity. We conclude that although rats and opossums share many similar characteristics, including ecological niche, their divergent evolutionary heritage results in vastly different behavioral capabilities. Only by observing the behavioral capabilities and preferences of opossums will we be able to manipulate the experimental environment to best elicit and elucidate their behavior and alterations in behavior that can arise from experimental manipulations.

Full text

PHOTIC PREFERENCE OF THE SHORT-TAILED OPOSSUM
(MONODELPHIS DOMESTICA)
A. M. H. SEELKE,
a
J. C. DOOLEY
a
AND
L. A. KRUBITZER
a,b
*
a
Center for Neuroscience, University of California, Davis,
1544 Newton Court, Davis, CA 95618, United States
b
Department of Psychology, University of California, Davis,
1544 Newton Court, Davis, CA 95618, United States
Abstract—The gray short-tailed opossum (Monodelphis
domestica) is a nocturnal South American marsupial that
has been gaining popularity as a laboratory animal. How-
ever, compared to traditional laboratory animals like rats,
very little is known about its behavior, either in the wild or
in a laboratory setting. Here we investigated the photic pref-
erence of the short-tailed opossum. Opossums were placed
in a circular testing arena and allowed to move freely
between dark (0 lux) and light (1.4, 40, or 400 lux) sides
of the arena. In each of these conditions opossums spent
significantly more time in the dark than in the illuminated
side and a greater proportion of time in the dark than would
be expected by chance. In the high-contrast (400 lux) illu-
mination condition, the mean bout length (i.e., duration of
one trip on the light or dark side) was significantly longer
on the dark side than on the light side. When we examined
the number of bouts greater than 30 and 60 s in duration, we
found a significant difference between the light and dark
sides in all light contrast conditions. These data indicate
that the short-tailed opossum prefers the dark to the light,
and can also detect very slight differences in light intensity.
We conclude that although rats and opossums share many
similar characteristics, including ecological niche, their
divergent evolutionary heritage results in vastly different
behavioral capabilities. Only by observing the behavioral
capabilities and preferences of opossums will we be
able to manipulate the experimental environment to best
elicit and elucidate their behavior and alterations in
behavior that can arise from experimental manipulations.
Ó2014 IBRO. Published by Elsevier Ltd. All rights reserved.
Key words: marsupial, behavior, evolution, vision, nocturnal,
photic preference.
INTRODUCTION
One of the most fundamental behavioral characteristics of
any living species is diel pattern. Yet, for mammals there
is relatively little comparative data on basic aspects of
behaviors associated with the diel pattern such as
photic preference. Diel pattern varies dramatically within
orders, families, and genera, and can even vary within a
species depending on climate and environmental
context. For example, in the wild, a pig’s (Sus scrofa)
diel pattern can be dependent upon climate, such that in
temperate regions pigs are diurnal while in tropical
regions they are nocturnal. However, North American
and European domestic pigs show a strong tendency
toward diurnality, due to a temperate climate and the
environmental constraints imposed through domesticity
(Ruckebusch, 1972; Campbell and Tobler, 1984; Robert
and Dallaire, 1986; Robert et al., 1987). Behavioral pat-
terns can even vary within the same animal depending
on the amount of illumination present, as in the case of
nocturnal desert rodents (Lockard and Owings, 1974;
Price et al., 1984; Wolfe and Summerlin, 1989; Daly
et al., 1992; Longland, 1994). In the current study we
examine the photic preference of what is becoming a
more commonly used animal model, the gray short-tailed
opossum (Monodelphis domestica).
The gray short-tailed opossum is a South American
marsupial that has been gaining popularity as a
laboratory animal. In the wild, these semi-arboreal
opossums are found in the dry forest landscapes of
Brazil, Bolivia, Argentina, and Paraguay where they
consume insects and other invertebrates, fruits, and
small vertebrates (Streilein, 1982; Wilson and Reeder,
1993; MacDonald, 2001). They are primarily nocturnal,
with their most active period occurring within 1–3 h of sun-
down (Streilein, 1982). In the laboratory, these animals
have proven to be useful for a wide range of research
questions, particularly studies of development, due in no
small part to the early stage at which their offspring are
born and the lack of a pouch, which makes offspring
accessible (Saunders et al., 1989; Karlen et al., 2006;
Karlen and Krubitzer, 2009). Furthermore, the short-tailed
opossum was the first marsupial to have its genome
sequenced, opening the door to many evolutionary and
genetic studies (Mikkelsen et al., 2007).
We recently assessed the visual acuity of opossums
using the optokinetic test, which relies on the reflexive
head movements that follow a moving stimulus (Dooley
et al., 2012). On average, opossums exhibited a visual
acuity of 0.58 cycles per degree, which is similar to the
acuity of albino rats determined using the same method-
ology (Prusky et al., 2002). Recent analysis of the short-
tailed opossum genome has indicated that their retinas
contain two classes of cones in addition to rods (Hunt
http://dx.doi.org/10.1016/j.neuroscience.2014.03.057
0306-4522/Ó2014 IBRO. Published by Elsevier Ltd. All rights reserved.
*Correspondence to: L. A. Krubitzer, Center for Neuroscience,
University of California, Davis, 1544 Newton Court, Davis, CA
95618, United States. Tel: +1-530-757-8868.
E-mail address: lakrubitzer@ucdavis.edu (L. A. Krubitzer).
Neuroscience 269 (2014) 273–280
273

et al., 2009), although this has yet to be anatomically ver-
ified. Likewise, the proportion of rods to cones has yet to
be determined in this species. Thus, while there has been
a great deal of recent progress in understanding their
visual capabilities, much remains to be learned.
Although the behavior of opossums in the wild is
qualitatively similar to that of eutherian mammals of a
similar size that occupy a similar niche (Kimble, 1997),
behavior of any animal in the laboratory is a different mat-
ter entirely. One challenge in working with opossums is
that, contrary to what many might assume, they do not
behave like more traditional laboratory animals, such as
mice or rats (Kimble and Whishaw, 1994; Ivanco et al.,
1996; Pisula et al., 2012). Differences in motivation, as
well as memory capacity, may require researchers to
devise opossum-specific behavioral tasks. Just as
researchers developed novel behavioral tasks based on
the ecological preferences of rats and mice, proper
behavioral studies in opossums can only be conducted
once their basic behavioral preferences have been identi-
fied. In this experiment, we tested the photic preference of
opossums by placing them in a round testing arena and
allowing them to freely move between the light and dark
sides of the arena. The results from this experiment could
inform the next generation of behavioral tests designed
explicitly for opossums.
EXPERIMENTAL PROCEDURES
Subjects
Ten adult South American short-tailed opossums (M.
domestica) were used in these experiments. See
Table 1 for age, weight, and sex. Animals were housed
in standard laboratory cages with ad libitum access to
food and water and were maintained on a 14:10-h
light:dark cycle with the lights on at 7 am. All
experiments were performed under National Institutes of
Health guidelines for the care of animals in research
and were approved by the Institutional Animal Care and
Use Committee of the University of California, Davis.
Testing apparatus
The testing apparatus consisted of a 76-cm LCD monitor
(LG, Seoul, South Korea) oriented parallel to the ground
through which stimuli were presented (Fig. 1). One half
of the monitor was obscured with an opaque panel of
black Plexiglas, and the other half of the monitor was
covered with a clear panel of Plexiglas. One large sheet
of clear Plexiglas was placed above those to create a
smooth surface. A large cylinder (55.9 cm tall, 35.6 cm
internal diameter) formed the walls of the testing arena
and was positioned so that half the arena was over the
dark stimulus and half was over the light stimulus.
The light intensity of both the light and dark sides of
the arena was measured before each trial using a light
meter (Digital Illuminance/Light Meter LX1330B; Dr.
Meter, Union City, CA). The animals were exposed to
three levels of light intensity: low intensity, which ranged
from 1.2 to 1.7 lux (average 1.36 lux), medium intensity,
which ranged from 36.0 to 46.0 lux (average 43.22 lux),
and high intensity, which ranged from 327 to 443 lux
(average 387.90 lux). The light intensity of the dark side
always measured 0 lux.
Behavioral test
Behavioral testing was performed in a dark room
illuminated by a dim red light. All testing occurred during
the first half of the animal’s light period. At the beginning
of each test, animals were taken from their home cage
and placed in the center of the testing arena, at the
boundary between the light and dark sides. An infrared
video camera (IR bullet camera; TelPix, Los Angeles,
CA, USA) was suspended over the testing arena to
capture the animals’ behavior, and the video was
recorded and digitized (Pinnacle Studio; Corel Inc.,
Mountain View, CA, USA). The animals were allowed to
explore the testing arena for 10 min. At the end of the
testing period they were removed, the arena was
cleaned with ethanol, and the stimulus was changed.
Each day the animal was tested on each of the three
light intensities, and testing occurred on three
consecutive days. Stimuli were presented in a semi-
random order without replacement.
Data analysis
Behavior was scored using the open-access video coding
program OpenSHAPA (datavyu.org). The 10-min
exploration period began when the animal was first
placed in the middle of the arena. The animal was
considered to have crossed from one side of the arena
to the other when the front half of its body moved over
the center line. A single visit to the light or dark half of
the arena was defined as a bout. The time of each
crossing event was recorded. These times were
exported into Excel (Microsoft Corp., Redmond, WA,
USA), converted into milliseconds, and the duration of
each bout spent in either the light or dark side was
determined. From there, for each lighting condition we
calculated: (1) the total amount of time spent on the
light and dark sides, (2) the percentage of time spent on
the light and dark sides, (3) the mean bout length within
the light or dark side, (4) the number of times the animal
crossed from the light to the dark side, and (5) the
number of bouts in both light and dark that were longer
than 10, 30, and 60 s.
Data analysis was performed using the JMP software
package (SAS, Cary, NC, USA). Comparisons between
Table 1. Subject information
Animal # Sex Weight (g) Age (d)
20633 M 110 230
10636 F 84 299
20628 M 100 238
10653 F 67 257
10639 F 69 293
20653 M 91 209
10649 F 102 538
10696 F 87 219
20656 M 104 245
20670 M 112 210
274 A. M. H. Seelke et al. / Neuroscience 269 (2014) 273–280

(i.e., low, medium, and high-contrast conditions) and
within light conditions (i.e., time spent in light vs. dark in
the high-contrast condition) were performed using
Analysis of Variance (ANOVA). The percentage of time
spent on the light and dark sides was compared against
chance using a two-sample t-test.
RESULTS
Each animal was tested over the course of three days.
We examined the proportion of time spent in the light
side and the dark side of the arena during each of the
three days. The data collected from three days were
averaged together for each animal. As part of our initial
reanalysis of the data, we divided the animals by sex
and compared the results using paired t-tests. We found
no differences between males and females in any of the
analyzed data. Thus, we analyzed the data from males
and females together.
In order to gain an understanding of the overall
behavior of these animals, we first calculated the total
amount of time spent in the light side of the arena and
compared it against the total amount of time spent in
the dark side of the arena (Fig. 2A; Table 2). In the
high-contrast light condition, opossums spent
significantly more time on the dark side (437 s) than on
the light side (194 s; Table 2). This was true for the
medium- and low-contrast light conditions as well
(p< .0001). Neither the time spent on the light side of
the arena nor the time spent on the dark side of the
arena significantly differed between the high-, medium-,
and low-contrast conditions.
We next calculated the mean length of time the animal
spent in either the light or the dark side of the arena
before crossing to the other side of the arena (i.e., bout
length; Fig. 2B; Table 2). In the high-intensity light
condition, the mean bout length was significantly longer
in the dark half of the arena (42.99 ± 6.36 s) than in the
light half of the arena (13.71 ± 2.83 s; F
1,19
= 17.68,
p< .0005). In the medium-intensity light condition the
mean amount of time spent in the dark side of the arena
(13.91 ± 2.48 s) was not significantly longer than the
light side of the arena (62.81 ± 24.31 s; F
1,19
= 4.00,
p= .06). Finally, in the low-intensity light condition, the
mean bout length was significantly longer in the dark
half of the arena (14.60 ± 4.45 s) than in the light half
of the arena (39.87 ± 10.03 s; F
1,19
= 5.31, p< .05).
We then calculated the proportion of time in each trial
spent on the light and dark sides of the arena (Fig. 2C;
Table 2). This calculation allowed us to determine
whether opossums spent more time on a given side of
the arena than would be expected by chance (50%).
Across all contrast light conditions, opossums spent
significantly less time on the light side (32.56%) than on
the dark side of the arena (67.44%; see Table 2).
Further, neither the time on the light side of the arena
(F
2,29
= .797, p= NS) nor the time on the dark side of
the arena (F
2,29
= .797, p= NS) significantly differed
between the high-, medium-, and low-light intensity
conditions. Thus, in all light conditions, animals spent
significantly more time on the dark side and less time on
the light side of the arena than would be expected by
chance (t
9
P4.04, p< .005).
We next calculated the number of times the opossums
crossed from the light side to the dark side each minute
BA
C
Fig. 1. Schematic of the testing apparatus and stimuli. (A) A top view of the testing arena. The floor consisted of an LCD monitor through which the
visual stimuli were presented, and a large, gray, opaque cylinder formed the walls. (B) The logarithm of the light intensity in lux of the visual stimuli
on the light side of the arena is represented on the Xaxis, and the illumination level is represented on the Yaxis. The mean light intensity in the high-
contrast condition was 388 lux, the mean light intensity in the medium-contrast condition was 43 lux, and the mean light intensity in the low-contrast
condition was 1.4 lux. (C) Illustrations of the appearance of high-, medium-, and low-contrast visual stimuli.
A. M. H. Seelke et al. / Neuroscience 269 (2014) 273–280 275

for each light intensity (Table 2;Fig. 3). This analysis was
done to determine whether the overall activity level of the
opossum changed in the different light intensities. There
was no significant difference between the number of
crossings per minute in the high-, medium-, or low-light
intensity conditions (F
2,29
= 0.38, p= NS).
The above analyses revealed significant differences in
the amount of time spent in the light and dark halves of
the arena, but they did not provide any insight into how
individual bout lengths differed between conditions. In
order to determine this, we examined the length of
individual bouts to determine the number of bouts that
were greater than 10, 30, and 60 s in both the light and
dark sides of the arena in all light conditions. The
number of bouts was counted for each 10-min session
in the arena and the values for each condition were
averaged across days. We found no difference between
any conditions in bouts longer than 10 s (Fig. 4A;
Table 3). However, when we examined bouts that were
greater than 30 s we found significantly more long bouts
on the dark side than on the light side in all light
intensity conditions (F
1,19
> 11.76, p< .005)(Fig. 4B;
Table 3). Similarly, when we examined bouts greater
than 60 s we found a significantly greater number of
long bouts on the dark side than the light side in all light
intensity conditions (F
1,19
> 5.19, p< .05) (Fig. 4C;
Table 3). Furthermore, a planned comparison revealed
a significantly higher number of bouts greater than 60 s
on the dark side in the high-intensity light condition than
on the dark side in the low-intensity light condition
(p< .05).
Together, these results indicate that in all light
intensity conditions opossums spent approximately twice
the amount of time on the dark side of the arena than
on the light side of the arena, accounting for almost
70% of the total time that they spent in the arena.
Furthermore, in the high-contrast light condition the
mean bout length was longer on the dark side than the
light side. Finally, in the medium- and high-contrast light
conditions, the number of long bouts (i.e., greater than
30 or 60 s in duration) was higher on the dark side than
the light side. Importantly, these differences were not
due to a change in the frequency of crossings from light
to dark, demonstrating that the overall amount of
exploratory behavior did not change between the light
conditions.
DISCUSSION
Summary of results
In this experiment we assessed the photic preference of
short-tailed opossums by placing them in a testing
arena, which was half dark and half illuminated, and
determining how much time they spent on each side.
The animals were tested under three illumination levels:
high, which averaged around 388 lux, or about the same
light level as indoor office lighting or sunrise on a clear
day; medium, which averaged about 43 lux, or about the
same light level as sunrise on a cloudy day; and low,
which averaged around 1.4 lux, or about the same light
level as the full moon on a clear night. In each of these
conditions, the light side of the arena was bounded by a
dark side that measured 0 lux. In all illumination
conditions the opossums spent significantly more total
time and a significantly greater percentage of time on
the dark side of the arena than the illuminated side.
The results presented here demonstrate that short-
tailed opossums exhibit a strong and significant
preference for dark environments compared to light
A
C
B
Fig. 2. Time spent in the light (white bars) and dark (black bars)
halves of the arena. (A) The total amount of time spent in the light and
dark halves of the arena in high-, medium-, and low-contrast light
conditions. In all light conditions, opossums spent significantly more
time on the dark side of the arena than the light side of the arena. (B)
The mean bout length in the light and dark halves of the arena in high-
, medium-, and low-contrast light conditions. The mean bout length on
the dark side of the arena was significantly longer than on the light
side in the high- and low-contrast conditions. There was a trend for
mean bout length to be longer on the dark side of the arena in the
medium-contrast condition. (C) The percentage of time spent in the
light and dark halves of the arena in high-, medium-, and low-contrast
light conditions. The dashed line indicates chance. In all light
conditions, opossums spent a significantly larger proportion of their
time on the dark side of the arena than on the light side of the arena,
and a significantly greater proportion of their time on the dark side,
and less on the light side than would be expected by chance.
Mean + SE. ⁄– differs from dark. # – differs from chance.
276 A. M. H. Seelke et al. / Neuroscience 269 (2014) 273–280

environments, defined as spending more time in the dark
half than the light half of the testing arena. This
preference is robust, existing in high, medium, and low
illumination conditions. In each illumination condition,
opossums spend a greater amount of time in the dark, a
larger percentage of time in the dark, have longer mean
bout durations in the dark, and a higher number of very
long bout durations (greater than 30 or 60 s) in the dark.
These differences in bout duration are not due to a
change in overall activity level, as the number of times
opossums crossed from the light side to the dark side of
the arena did not differ between light conditions.
Furthermore, the fact that opossums spent the majority
of their time in the dark, even in the low illumination
condition, indicates that they have the ability to
discriminate between very low light conditions and
complete darkness, an ability that could be exploited in
the design of future behavioral tasks. To our knowledge,
this is the first time the photic preference of short-tailed
opossums has been experimentally determined.
Short-tailed opossum vision
Opossums are classified as nocturnal animals, although
they are most active in the few hours following sunset
and their activity is not decreased by the presence of
lights (Streilein, 1982). This description of their natural
behavior is consistent with our observations, in that the
opossums distinctly preferred the dark side of the arena
over the light side, but still ventured into the light. The
opossums also showed evidence of sensitivity to small
differences in low light levels, in that they spent more time
in the dark side of the arena even in the lowest intensity
light condition. This behavior, in part, may be due to their
retinal composition, which includes rods and two sets of
Table 2. Results
High contrast Medium contrast Low contrast
Light Dark Light Dark Light Dark
Total time (s) 194.35 ± 23.19 436.94 ± 23.68 191.66 ± 29.27 439.64 ± 28.10 230.31 ± 21.41 398.00 ± 19.94
% Time 30.81 ± 3.71 69.19 ± 3.71 30.28 ± 4.61 69.72 ± 4.61 36.58 ± 3.32 63.42 ± 3.32
Bout Length (s) 13.71 ± 2.83 42.99 ± 6.36 13.91 ± 2.48 62.81 ± 24.31 14.60 ± 4.45 39.87 ± 10.03
# Crossings per minute 2.08 ± 0.67 1.90 ± 0.59 2.61 ± 0.53
Fig. 3. The number of crossings from the light side to the dark side of
the arena per minute did not differ between light intensity conditions,
indicating that these results were not due to a change in the overall
activity level of the animal. Mean + SE.
A
B
C
Fig. 4. The number of bout lengths that was greater than 10 (A), 30
(B), and 60 (C) s in duration in the light (white bars) and dark (black
bars) halves of the arena in high-, medium-, and low-contrast light
conditions. (A) There was no difference in the number of bouts longer
than 10 s in the light and dark halves of the arena in any light contrast
condition. In the high-, medium-, and low-contrast light conditions,
there were significantly more bouts longer than 30 s (B) and 60 s (C)
in the dark half of the arena than in the light half of the arena.
Additionally, there were significantly more bout lengths that were
greater than 60 s in duration on the dark side of the arena in the high-
contrast condition compared to those in the low-contrast condition.
Mean + SE. ⁄– differs from dark. – differs from high contrast.
A. M. H. Seelke et al. / Neuroscience 269 (2014) 273–280 277

color-sensing cones, which respond to wavelengths
within the UV (335–445 nm) and green (500–570 nm)
light range (Hunt et al., 2009). While, to our knowledge,
the ratio of rods to cones has yet to be characterized for
the short-tailed opossum, other nocturnal marsupial spe-
cies, such as the Virginia opossum (Didelphis virginiana)
(Kolb and Wang, 1985) and Big-eared opossum (Didel-
phis auritis)(Ahnelt et al., 1995) have retinas with a high
rod/cone ratio.
Comparison with rat vision
Rats (Rattus norvegicus) are commonly used in
behavioral experiments, and in many cases behavioral
tasks that have been designed for rats have been
adapted for use in other species. Like short-tailed
opossums, rats are nocturnal, prefer dark to light
environments, and the proportion of time spent in
illuminated areas is inversely proportional to the
intensity of the illumination (Johnson, 1964a;
Woodhouse and Greenfeld, 1985). As in the short-tailed
opossum, the retina of rats contains three photoreceptors:
rods and two varieties of cones, which respond to UV and
green wavelengths (Jacobs et al., 2001; Ortin-Martinez
et al., 2010). And, like nocturnal marsupials that have
been studied, albino rats have a high rod to cone ratio
(approximately 1:100) (Walls, 1934; Szel and Rohlich,
1992).
While laboratory short-tailed opossums are an
outbred population, rats have been selectively inbred to
produce several distinct strains, including multiple albino
strains. Because of this, it is important to consider the
strain of rat when comparing them to different species.
Albino and pigmented rats differ on many sensory
measures, including vision. Like Monodelphis, albino
strains have a stronger preference for dark areas over
light areas (Matsuo and Tsuji, 1989) and relatively low
visual acuity compared to pigmented strains (Prusky
et al., 2002, report values of 0.536 cycles per degree vs
1.113 cycles per degree, respectively) (Birch and
Jacobs, 1979; Prusky et al., 2002). However, albino rats
have a number of anatomical anomalies, including a
decreased number of rods and decreased cell density
within the retina (Ilia and Jeffery, 2000), lower density
and abnormal distribution of cone types within the retina
(Ortin-Martinez et al., 2010), an incomplete decussation
of the optic nerve (Lund et al., 1974), and increased col-
licular activity in response to light stimulation (Thomas
et al., 2005). Thus, while some aspects of visual behavior
are similar in albino rats and short-tailed opossums, ulti-
mately, due to their normal pigmentation, short-tailed
opossums likely share more common features of the
visual system with pigmented rats. However, even
pigmented rats are far from an ideal comparison with
Monodelphis due to many other factors, including their
history of inbreeding, phylogeny, development, and differ-
ing natural habitats. This highlights the importance of
carefully selecting species for comparisons, and the
necessity of considering all aspects of the animal, includ-
ing ecological niche, evolutionary history, and diel pattern,
when making cross species comparisons.
Comparisons with other nocturnal animal models
As described above, short-tailed opossums are gaining
popularity as a model species for many topics, including
cortical development (Kahn and Krubitzer, 2002; Karlen
et al., 2006; Seelke et al., 2013), motor development
(Saunders et al., 1998; Cabana, 2000; Lavallee and
Pflieger, 2009), and genetics (Goodstadt et al., 2007;
Samollow, 2008). However, little is known about their
behavior, especially in comparison to more traditional
model species. There have been only a few studies that
directly compared the spatial behavior of short-tailed
opossums and other animals, specifically rats.
For example, comparisons of spatial memory using
the Morris water maze and radial arm maze (Kimble and
Whishaw, 1994) indicate that opossums exhibit different
behavioral patterns than rats. While rats readily learned
to find both visible and hidden platforms in the Morris
water maze task, opossums took much longer to learn
to find visible platforms and never successfully learned
to find hidden platforms (Kimble and Whishaw, 1994). In
the radial arm maze opossums required more attempts
to find food rewards and were much more likely to reenter
arms that had been previously searched (Kelly and
Masterton, 1977). When compared in an open field maze
(Wesierska et al., 2003) and elevated plus maze
(Wesierska and Turlejski, 2000), opossums exhibited a
higher amount of overall activity, and they switched from
defensive to exploratory behavior more quickly than rats,
which the researchers attributed to the hunting abilities of
the opossum. Similar results were observed when opos-
sums and rats were exposed to novel objects (Pisula
et al., 2012).
Finally, comparisons of forelimb movements of the
two species indicates that both rats and opossums
could use a single limb to grasp the prey and bring it to
their mouths (Ivanco et al., 1996), but rats displayed more
complex movements of their forepaws and used their dig-
its more than opossums. The authors concluded that the
greater complexity of the rats’ movements is related to the
greater anatomical and functional complexity of their
motor systems compared to that of opossums.
Table 3. Number of bouts >10, 30, or 60 s in each light condition
High contrast Medium contrast Low contrast
Light Dark Light Dark Light Dark
>10 s 15.7 ± 1.6 19.4 ± 1.8 11.2 ± 2.9 18.3 ± 2.3 16.2 ± 2.2 22.6 ± 3.2
>30 s 2.5 ± 0.5 7.0 ± 0.6 2.2 ± 0.4 5.3 ± 0.7 2.2 ± 0.6 5.8 ± 0.8
>60 s 0.9 ± 0.3 5.4 ± 1.5 0.7 ± 0.2 3.1 ± 0.4 1.2 ± 0.4 2.6 ± 0.5
278 A. M. H. Seelke et al. / Neuroscience 269 (2014) 273–280

Photic preference in other nocturnal species
The phenomenon of how behavior changes in light and
dark environments, especially in relation to circadian
activity, has been very thoroughly studied, but only in a
few animal models such as mice, rats, and fruit flies
(e.g. Mendoza et al., 2005; Allada and Chung, 2010;
Tapia-Osorio et al., 2013). However, there have been sur-
prisingly few studies that examine a nocturnal animal’s
affinity for a given level of illumination. The studies that
have been performed measure an animal’s photic prefer-
ence either indirectly, by using activity levels as a proxy
for preference, or directly, by allowing the animal to
choose between areas with different illumination levels,
as in the present study.
Several studies have examined how the behavior of
desert rodents (including Peromyscus polionotus,
Dipodomys merriami,Dipodomys nitratoides, and
Chaetodipus baileyi, to name a few) changes in
response to the phases of the moon in their natural
environments (Lockard and Owings, 1974; Price et al.,
1984; Wolfe and Summerlin, 1989; Daly et al., 1992;
Longland, 1994). The changing phases of the moon add
another variable to nocturnal behavior, in that the amount
of light available changes from night to night. It is brighter
during a full moon (1 lux) than during a new moon
(0.001 lux), and when the moon was full, animals spent
less time in open spaces, preferring to stay under the
cover of vegetation or within their burrows. On the other
hand, when the moon was new, the animals spent more
time foraging in the open. These studies demonstrate that
these nocturnal species can differentiate between low
light levels, and prefer the darker area. Such an ability
would be useful for assessing the risk of predation during
different illumination conditions.
Other studies have examined illumination preferences
under more controlled conditions. In two studies rats were
placed in an apparatus that allowed them to choose
between different illumination levels. In both cases, rats
preferred either complete darkness or the lowest
illumination condition (0.1 lux) over higher illumination
conditions (Johnson, 1964b; Johnson, 1965). The behav-
ior of non-rodent mammals has also been investigated.
The slow loris (Nycticebus coucang), a nocturnal prosim-
ian native to Southeast Asia and Indonesia, is more active
during periods of low illumination (0.7–1.3 lux) and shows
more behavioral quiescence during periods of higher illu-
mination (4.1–43.1 lux) (Trent et al., 1977). Similarly,
galagos (Galago crassicaudatus), another nocturnal pro-
simian, also show behavioral changes that are related to
illumination levels (Randolph, 1971). Galagos exhibited
significantly more locomotor activity during the lowest illu-
mination conditions than during higher illumination condi-
tions. Furthermore, galagos showed a strong preference
for low levels of illumination, which is likely one way that
these animals avoid predation.
Together, these studies demonstrate that the behavior
of nocturnal animals can vary significantly based upon
relatively small changes in illumination levels. Further,
the description of an animal as ‘‘nocturnal’’ provides
somewhat limited information about a particular species’
behavioral patterns. It is clear that photic preference is
an important factor that contributes to specific behaviors
that will be generated in a particular illumination context.
Acknowledgments—Thanks to Cindy Clayton, DVM, and the rest
of the animal care staff at the UC Davis Psychology Department
Vivarium. Additional thanks to Hoang Nguyen for assistance in
building the behavioral arena. This work was supported by grants
to Leah Krubitzer from NINDS (R21 NS071225) and NEI (R01
EY022987) and funding from the NEI (T32 EY015387) to James
Dooley and Adele Seelke.
REFERENCES
Ahnelt PK, Hokoc JN, Rohlich P (1995) Photoreceptors in a primitive
mammal, the South American opossum, Didelphis marsupialis
aurita: characterization with anti-opsin immunolabeling. Visual
Neurosci 12(5):793–804.
Allada R, Chung BY (2010) Circadian organization of behavior and
physiology in Drosophila. Annu Rev Physiol 72:605–624.
MacDonald D (2001) American Opossums. In: Macdonald D, Norris
S, editors. The new encyclopedia of mammals. Oxford,
UK: Oxford University Press. p. 808–814.
Birch D, Jacobs GH (1979) Spatial contrast sensitivity in albino and
pigmented rats. Vision Res 19(8):933–937.
Cabana T (2000) The development of mammalian motor systems:
the opossum Monodelphis domestica as a model. Brain Res Bull
53(5):615–626.
Campbell SS, Tobler I (1984) Animal sleep: a review of sleep duration
across phylogeny. Neurosci Biobehav Rev 8:269–300.
Daly M, Behrends PR, Wilson MI, Jacobs LF (1992) Behavioral
modulation of predation risk: moonlight avoidance and
crepuscular compensation in a nocturnal desert rodent,
Dipodomys merriami. Anim Behav 44(1):1–9.
Dooley JC, Nguyen HM, Seelke AM, Krubitzer L (2012) Visual acuity
in the short-tailed opossum (Monodelphis domestica).
Neuroscience 223:124–130.
Goodstadt L, Heger A, Webber C, Ponting CP (2007) An analysis of
the gene complement of a marsupial, Monodelphis domestica:
evolution of lineage-specific genes and giant chromosomes.
Genome Res 17(7):969–981.
Hunt DM, Chan J, Carvalho LS, Hokoc JN, Ferguson MC, Arrese CA,
Beazley LD (2009) Cone visual pigments in two species of South
American marsupials. Gene 433(1–2):50–55.
Ilia M, Jeffery G (2000) Retinal cell addition and rod production
depend on early stages of ocular melanin synthesis. J Comp
Neurol 420(4):437–444.
Ivanco TL, Pellis SM, Whishaw IQ (1996) Skilled forelimb movements
in prey catching and in reaching by rats (Rattus norvegicus) and
opossums (Monodelphis domestica): relations to anatomical
differences in motor systems. Behav Brain Res 79(1–2):163–181.
Jacobs GH, Fenwick JA, Williams GA (2001) Cone-based vision of
rats for ultraviolet and visible lights. J Exp Biol 204(Pt
14):2439–2446.
Johnson RN (1964a) Illumination preference of albino rats in a closed
hexagonal maze. Percept Motor Skills 19:827–830.
Johnson RN (1964b) Illumination preference of albino rats in a closed
hexagonal maze. Percept Motor Skills 19(3):827–830.
Johnson RN (1965) Illumination preference of albino rats in a tilt box
as a function of age and illumination intensity. Percept Motor Skills
21(2):535–543.
Kahn DM, Krubitzer L (2002) Massive cross-modal cortical plasticity
and the emergence of a new cortical area in developmentally blind
mammals. Proc Natl Acad Sci U S A 99(17):11429–11434.
Karlen SJ, Krubitzer L (2009) Effects of bilateral enucleation on the
size of visual and nonvisual areas of the brain. Cereb Cortex
19(6):1360–1371.
Karlen SJ, Kahn DM, Krubitzer L (2006) Early blindness results in
abnormal corticocortical and thalamocortical connections.
Neuroscience 142(3):843–858.
A. M. H. Seelke et al. / Neuroscience 269 (2014) 273–280 279

Kelly JB, Masterton B (1977) Auditory sensitivity of the albino rat. J
Comp Physiol Psychol 91(4):930–936.
Kimble DP (1997) Didelphid behavior. Neurosci Biobehav Rev
21(3):361–369.
Kimble D, Whishaw IQ (1994) Spatial behavior in the Brazilian short-
tailed opossum (Monodelphis domestica): comparison with the
Norway rat (Rattus norvegicus) in the Morris water maze and
radial arm maze. J Comp Psychol 108(2):148–155.
Kolb H, Wang HH (1985) The distribution of photoreceptors,
dopaminergic amacrine cells and ganglion cells in the retina of
the North American opossum (Didelphis virginiana). Vision Res
25(9):1207–1221.
Lavallee A, Pflieger J-F (2009) Developmental expression of
spontaneous activity in the spinal cord of postnatal opossums,
Monodelphis domestica: an anatomical study. Brain Res
1282:1–9.
Lockard RB, Owings DH (1974) Moon-related surface activity of
Bannertail (Dipodomys spectabilis) and Fresno (D. nitratoides)
kangaroo rats. Anim Behav 22(February):262–273.
Longland WS (1994) Effects of artificial bush canopies and
illumination on seed patch selection by heteromyid rodents. Am
Midl Nat 132(1):82–90.
Lund RD, Lund JS, Wise RP (1974) The organization of the retinal
projection to the dorsal lateral geniculate nucleus in pigmented
and albino rats. J Comp Neurol 158(4):383–403.
Matsuo M, Tsuji K (1989) Strain differences of the light-dark
preference in inbred rats. Behav Genet 19(3):457–466.
Mendoza J, Graff C, Dardente H, Pevet P, Challet E (2005) Feeding
cues alter clock gene oscillations and photic responses in the
suprachiasmatic nuclei of mice exposed to a light/dark cycle. J
Neurosci 25(6):1514–1522.
Mikkelsen TS, Wakefield MJ, Aken B, Amemiya CT, Chang JL, Duke
S, Garber M, Gentles AJ, Goodstadt L, Heger A, Jurka J, Kamal
M, Mauceli E, Searle SM, Sharpe T, Baker ML, Batzer MA, Benos
PV, Belov K, Clamp M, Cook A, Cuff J, Das R, Davidow L, Deakin
JE, Fazzari MJ, Glass JL, Grabherr M, Greally JM, Gu W, Hore
TA, Huttley GA, Kleber M, Jirtle RL, Koina E, Lee JT, Mahony S,
Marra MA, Miller RD, Nicholls RD, Oda M, Papenfuss AT, Parra
ZE, Pollock DD, Ray DA, Schein JE, Speed TP, Thompson K,
VandeBerg JL, Wade CM, Walker JA, Waters PD, Webber C,
Weidman JR, Xie X, Zody MC, Graves JA, Ponting CP, Breen M,
Samollow PB, Lander ES, Lindblad-Toh K (2007) Genome of the
marsupial Monodelphis domestica reveals innovation in non-
coding sequences. Nature 447(7141):167–177.
Ortin-Martinez A, Jimenez-Lopez M, Nadal-Nicolas FM, Salinas-
Navarro M, Alarcon-Martinez L, Sauve Y, Villegas-Perez MP,
Vidal-Sanz M, Agudo-Barriuso M (2010) Automated quantification
and topographical distribution of the whole population of S- and L-
cones in adult albino and pigmented rats. Invest Ophthalmol
Visual Sci 51(6):3171–3183.
Pisula W, Turlejski K, Stryjek R, Nalecz-Tolak A, Grabiec M,
Djavadian RL (2012) Response to novelty in the laboratory
Wistar rat, wild-captive WWCPS rat, and the gray short-tailed
opossum (Monodelphis domestica). Behav Process
91(2):145–151.
Price MV, Waser NM, Bass TA (1984) Effects of moonlight on
microhabitat use by desert rodents. J Mammal 65(2):353–356.
Prusky GT, Harker KT, Douglas RM, Whishaw IQ (2002) Variation in
visual acuity within pigmented, and between pigmented and
albino rat strains. Behav Brain Res 136(2):339–348.
Randolph M (1971) Role of light and circadian rhythms in nocturnal
behavior of Galago-Crassicaudatus. J Comp Physiol Psychol
74(1):115–120.
Robert S, Dallaire A (1986) Polygraphic analysis of the sleep-wake
states and the REM sleep periodicity in domesticated pigs (Sus
scrofa). Physiol Behav 37(2):289–293.
Robert S, Dancosse J, Dallaire A (1987) Some observations on the
role of environment and genetics in behaviour of wild and
domestic forms of Sus scrofa (European wild boars and
domestic pigs). Appl Anim Behav Sci 17(3–4):253–262.
Ruckebusch Y (1972) The relevance of drowsiness in the circadian
cycle of farm animals. Anim Behav 20:637–643.
Samollow PB (2008) The opossum genome: insights and
opportunities from an alternative mammal. Genome Res
18(8):1199–1215.
Saunders NR, Adam E, Reader M, Mollgard K (1989) Monodelphis
domestica (grey short-tailed opossum): an accessible model for
studies of early neocortical development. Anat Embryol
180:227–236.
Saunders NR, Kitchener P, Knott GW, Nicholls JG, Potter A, Smith
TJ (1998) Development of walking, swimming and neuronal
connections after complete spinal cord transection in the neonatal
opossum, Monodelphis domestica. J Neurosci 18(1):339–355.
Seelke AMH, Dooley JC, Krubitzer L (2013) Differential changes in
the cellular composition of the developing marsupial brain. J
Comp Neurol 52(11):2602–2620.
Streilein KE (1982) Behavior, ecology, and distribution of South
American marsupials. In: Mares MA, Genoways HH, editors.
Mammalian biology in South America. Linesville, PA: The
University of Pittsburgh. p. 231–250.
Szel A, Rohlich P (1992) Two cone types of rat retina detected by
anti-visual pigment antibodies. Exp Eye Res 55(1):47–52.
Tapia-Osorio A, Salgado-Delgado R, Angeles-Castellanos M,
Escobar C (2013) Disruption of circadian rhythms due to chronic
constant light leads to depressive and anxiety-like behaviors in
the rat. Behav Brain Res 252:1–9.
Thomas BB, Aramant RB, Sadda SR, Seiler MJ (2005) Light
response differences in the superior colliculus of albino and
pigmented rats. Neurosci Lett 385(2):143–147.
Trent BK, Tucker ME, Lockard JS (1977) Activity changes with
illumination in slow loris (Nycticebus coucang). Appl Anim Ethol
3(3):281–286.
Walls GL (1934) The visual cells of the white rat. Comp Psychol
18(3):363–366.
Wesierska M, Turlejski K (2000) Spontaneous behavior of the gray
short-tailed opossum (Monodelphis domestica) in the elevated
plus-maze: comparison with Long-Evans rats. Acta Neurobiol Exp
(Wars) 60(4):479–487.
Wesierska M, Walasek G, Kilijanek J, Djavadian RL, Turlejski K
(2003) Behavior of the gray short-tailed opossum (Monodelphis
domestica) in the open field and in response to a new object, in
comparison with the rat. Behav Brain Res 143(1):31–40.
Wilson D, Reeder D (1993) Mammalian species of the world: a
taxonomic and geographic reference. Washington,
DC: Smithsonian Institution Press.
Wolfe JL, Summerlin CT (1989) The influence of lunar light on
nocturnal activity of the old-field mouse. Anim Behav 37:410–414.
Woodhouse R, Greenfeld N (1985) Responses of albino and hooded
rats to various illumination choices in a six-chambered alleyway.
Percept Motor Skills 61(2):343–354.
(Accepted 27 March 2014)
(Available online 5 April 2014)
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