Short Days and Exogenous Melatonin Increase Aggression of Male Syrian Hamsters ( Mesocricetus auratus ) Aaron M. Jasnow,* Kim L. Huhman,* Timothy J. Bartness,* , † and Gregory E. Demas‡ ,1 Center for Behavioral Neuroscience, *Department of Psychology and †Department of Biology, Georgia State University, Atlanta, Georgia 30303; and ‡Center for the Integrative Study of Animal Behavior, Department of Biology, Indiana University, Bloomington, Indiana 47405 Received July 20, 2001; revised September 24, 2001; accepted December 14, 2001 Many nontropical rodent species rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behavior. Among these changes, some species of rodents demonstrate in- creased aggression in short, “winter-like” compared with long “summer-like” day lengths. The precise neu- roendocrine mechanisms mediating changes in aggres- sion, however, remain largely unknown. The goal of the present study was to examine the effects of photoperiod and exogenous melatonin on resident-intruder aggres- sion in male Syrian hamsters ( Mesocricetus auratus). In Experiment 1, male Syrian hamsters were housed in long (LD 14:10) or short (LD 10:14) days for 10 weeks. In Experiment 2, hamsters were housed in long days and half of the animals were given daily subcutaneous mel- atonin injections (15 g/day in 0.1 ml saline) 2 h before lights out for 10 consecutive days to simulate a short- day pattern of melatonin secretion, while the remaining animals received injections of the vehicle alone. Animals in both experiments were then tested using a resident- intruder model of aggression and the number of attacks, duration of attacks, and latency to initial attack were recorded. In Experiment 1, short-day hamsters under- went gonadal regression and displayed increased ag- gression compared with long-day animals. In Experi- ment 2, melatonin treatment also increased aggression compared with control hamsters without affecting cir- culating testosterone. Collectively, the results of the present study demonstrate that exposure to short days or short day-like patterns of melatonin increase aggres- sion in male Syrian hamsters. In addition, these results suggest that photoperiodic changes in aggression pro- vide an important, ecologically relevant model with which to study the neuroendocrine mechanisms under- lying aggression in rodents. © 2002 Elsevier Science (USA) Key Words: seasonal; photoperiod; pineal; agonistic; testosterone; dominance. Individuals of many nontropical rodent species un- dergo a variety of physiological and behavioral re- sponses across the seasons of the year (reviewed in Bronson and Heideman, 1994; Nelson, Badura, and Goldman, 1990). Although a variety of environmental factors (e.g., ambient temperature, humidity, food availability) also fluctuate on a seasonal basis, photo- period (day length) appears to be the primary envi- ronmental cue used by most nontropical mammalian species to coordinate behavioral and physiological re- sponses with the optimal time of year (Bronson, 1989). Animals maintained in short “winter-like” days (i.e., 12 h of light/day) undergo a variety of physiological and behavioral changes, including regression of the reproductive system, as well as changes in body mass, pelage, thermoregulation, and general activity (re- viewed in Bartness, Bradley, Hastings, Bittman, and Goldman, 1993). These physiological and behavioral changes in response to changes in photoperiod are mediated by a multisynaptic pathway that conveys photic information received by the retina to the pineal gland and results in changes in the pattern of secretion of the pineal indolamine hormone, melatonin. Inter- ruption of this pathway at any point or removal of the pineal gland blocks physiological and behavioral re- sponses to short day lengths (Elliot and Goldman, 1981; Tamarkin, Baird, and Almeida, 1985). 1 To whom correspondence should be addressed at Department of Biology, Indiana University, 1001 East 3rd Street, Bloomington, IN 47405. Fax: (812) 855-6705. E-mail: gdemas@bio.indiana.edu. Hormones and Behavior 42, 13–20 (2002) doi:10.1006/hbeh.2002.1797 0018-506X/02 $35.00 © 2002 Elsevier Science (USA) All rights reserved. 13