Neuroscience 269 (2014) 273–280 PHOTIC PREFERENCE OF THE SHORT-TAILED OPOSSUM (MONODELPHIS DOMESTICA) A. M. H. SEELKE, a J. C. DOOLEY a AND behaviors associated with the diel pattern such as a,b L. A. KRUBITZER * photic preference. Diel pattern varies dramatically within a Center for Neuroscience, University of California, Davis, orders, families, and genera, and can even vary within a 1544 Newton Court, Davis, CA 95618, United States species depending on climate and environmental b Department of Psychology, University of California, Davis, context. For example, in the wild, a pig’s (Sus scrofa) 1544 Newton Court, Davis, CA 95618, United States 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 Abstract—The gray short-tailed opossum (Monodelphis and European domestic pigs show a strong tendency domestica) is a nocturnal South American marsupial that has been gaining popularity as a laboratory animal. How- toward diurnality, due to a temperate climate and the ever, compared to traditional laboratory animals like rats, environmental constraints imposed through domesticity very little is known about its behavior, either in the wild or (Ruckebusch, 1972; Campbell and Tobler, 1984; Robert in a laboratory setting. Here we investigated the photic pref- and Dallaire, 1986; Robert et al., 1987). Behavioral pat- erence of the short-tailed opossum. Opossums were placed terns can even vary within the same animal depending in a circular testing arena and allowed to move freely on the amount of illumination present, as in the case of between dark (0 lux) and light (1.4, 40, or 400 lux) sides nocturnal desert rodents (Lockard and Owings, 1974; of the arena. In each of these conditions opossums spent Price et al., 1984; Wolfe and Summerlin, 1989; Daly significantly more time in the dark than in the illuminated et al., 1992; Longland, 1994). In the current study we side and a greater proportion of time in the dark than would examine the photic preference of what is becoming a be expected by chance. In the high-contrast (400 lux) illu- mination condition, the mean bout length (i.e., duration of more commonly used animal model, the gray short-tailed one trip on the light or dark side) was significantly longer opossum (Monodelphis domestica). on the dark side than on the light side. When we examined The gray short-tailed opossum is a South American the number of bouts greater than 30 and 60 s in duration, we marsupial that has been gaining popularity as a found a significant difference between the light and dark laboratory animal. In the wild, these semi-arboreal sides in all light contrast conditions. These data indicate opossums are found in the dry forest landscapes of that the short-tailed opossum prefers the dark to the light, Brazil, Bolivia, Argentina, and Paraguay where they and can also detect very slight differences in light intensity. consume insects and other invertebrates, fruits, and We conclude that although rats and opossums share many small vertebrates (Streilein, 1982; Wilson and Reeder, similar characteristics, including ecological niche, their 1993; MacDonald, 2001). They are primarily nocturnal, divergent evolutionary heritage results in vastly different behavioral capabilities. Only by observing the behavioral with their most active period occurring within 1–3 h of sun- capabilities and preferences of opossums will we be down (Streilein, 1982). In the laboratory, these animals able to manipulate the experimental environment to best have proven to be useful for a wide range of research elicit and elucidate their behavior and alterations in questions, particularly studies of development, due in no behavior that can arise from experimental manipulations. small part to the early stage at which their offspring are Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. 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 Key words: marsupial, behavior, evolution, vision, nocturnal, opossum was the first marsupial to have its genome photic preference. sequenced, opening the door to many evolutionary and genetic studies (Mikkelsen et al., 2007). We recently assessed the visual acuity of opossums INTRODUCTION using the optokinetic test, which relies on the reflexive One of the most fundamental behavioral characteristics of head movements that follow a moving stimulus (Dooley any living species is diel pattern. Yet, for mammals there et al., 2012). On average, opossums exhibited a visual is relatively little comparative data on basic aspects of acuity of 0.58 cycles per degree, which is similar to the acuity of albino rats determined using the same method- *Correspondence to: L. A. Krubitzer, Center for Neuroscience, ology (Prusky et al., 2002). Recent analysis of the short- University of California, Davis, 1544 Newton Court, Davis, CA tailed opossum genome has indicated that their retinas 95618, United States. Tel: +1-530-757-8868. contain two classes of cones in addition to rods (Hunt E-mail address: [email protected] (L. A. Krubitzer). http://dx.doi.org/10.1016/j.neuroscience.2014.03.057 0306-4522/Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. 273 274 A. M. H. Seelke et al. / Neuroscience 269 (2014) 273–280 et al., 2009), although this has yet to be anatomically ver- covered with a clear panel of Plexiglas. One large sheet ified. Likewise, the proportion of rods to cones has yet to of clear Plexiglas was placed above those to create a be determined in this species. Thus, while there has been smooth surface. A large cylinder (55.9 cm tall, 35.6 cm a great deal of recent progress in understanding their internal diameter) formed the walls of the testing arena visual capabilities, much remains to be learned. and was positioned so that half the arena was over the Although the behavior of opossums in the wild is dark stimulus and half was over the light stimulus. qualitatively similar to that of eutherian mammals of a The light intensity of both the light and dark sides of similar size that occupy a similar niche (Kimble, 1997), the arena was measured before each trial using a light behavior of any animal in the laboratory is a different mat- meter (Digital Illuminance/Light Meter LX1330B; Dr. ter entirely. One challenge in working with opossums is Meter, Union City, CA). The animals were exposed to that, contrary to what many might assume, they do not three levels of light intensity: low intensity, which ranged behave like more traditional laboratory animals, such as from 1.2 to 1.7 lux (average 1.36 lux), medium intensity, mice or rats (Kimble and Whishaw, 1994; Ivanco et al., which ranged from 36.0 to 46.0 lux (average 43.22 lux), 1996; Pisula et al., 2012). Differences in motivation, as and high intensity, which ranged from 327 to 443 lux well as memory capacity, may require researchers to (average 387.90 lux). The light intensity of the dark side devise opossum-specific behavioral tasks. Just as always measured 0 lux. researchers developed novel behavioral tasks based on the ecological preferences of rats and mice, proper Behavioral test behavioral studies in opossums can only be conducted once their basic behavioral preferences have been identi- Behavioral testing was performed in a dark room fied. In this experiment, we tested the photic preference of illuminated by a dim red light. All testing occurred during opossums by placing them in a round testing arena and the first half of the animal’s light period. At the beginning allowing them to freely move between the light and dark of each test, animals were taken from their home cage sides of the arena. The results from this experiment could and placed in the center of the testing arena, at the inform the next generation of behavioral tests designed boundary between the light and dark sides. An infrared explicitly for opossums. video camera (IR bullet camera; TelPix, Los Angeles, CA, USA) was suspended over the testing arena to EXPERIMENTAL PROCEDURES capture the animals’ behavior, and the video was recorded and digitized (Pinnacle Studio; Corel Inc., Subjects Mountain View, CA, USA). The animals were allowed to Ten adult South American short-tailed opossums (M. explore the testing arena for 10 min. At the end of the domestica) were used in these experiments. See testing period they were removed, the arena was Table 1 for age, weight, and sex. Animals were housed cleaned with ethanol, and the stimulus was changed. in standard laboratory cages with ad libitum access to Each day the animal was tested on each of the three food and water and were maintained on a 14:10-h light intensities, and testing occurred on three light:dark cycle with the lights on at 7 am. All consecutive days. Stimuli were presented in a semi- experiments were performed under National Institutes of random order without replacement. Health guidelines for the care of animals in research and were approved by the Institutional Animal Care and Data analysis Use Committee of the University of California, Davis. Behavior was scored using the open-access video coding program OpenSHAPA (datavyu.org). The 10-min Testing apparatus exploration period began when the animal was first The testing apparatus consisted of a 76-cm LCD monitor placed in the middle of the arena. The animal was (LG, Seoul, South Korea) oriented parallel to the ground considered to have crossed from one side of the arena through which stimuli were presented (Fig. 1). One half to the other when the front half of its body moved over of the monitor was obscured with an opaque panel of the center line.