Short-day increases in aggression are inversely related to circulating testosterone concentrations in male Siberian hamsters (Phodopus sungorus)

Many nontropical rodent species display seasonal changes in both physiology and behavior that occur primarily in response to changes in photoperiod. S hort-day reductions in reproduction are due, in part, to reductions in gona dal steroid hormones. In addition, gonadal steroids, primarily testosterone (T), have been implicated in aggression in many mammalian species. Some sp ecies, however, display increased aggression in short days despite basal ci rculating concentrations of T. The goal of the present studies was to test the effects of photoperiod on aggression in male Siberian hamsters (Phodopu s sungorus) and to determine the role of T in mediating photoperiodic chang es in aggression. In Experiment 1, hamsters were housed in long and short d ays for either 10 or 20 weeks and aggression was determined using a residen t-intruder model. Hamsters housed in short days for 10 weeks underwent gona dal regression and displayed increased aggression compared to long-day-hous ed animals. Prolonged maintenance in short days (i.e., 20 weeks), however, led to gonadal recrudescence and reduced aggression. In Experiment 2, hamst ers were housed in long and short days for 10 weeks. Half of the short-day- housed animals were implanted with capsules containing T whereas the remain ing animals received empty capsules. In addition, half of the long-day-hous ed animals were castrated whereas the remaining animals received sham surge ries. Short-day control hamsters displayed increased aggression compared to either castrated or intact long-day-housed animals. Short-day-housed T tre ated hamsters, however, did not differ in aggression from long-day-housed a nimals. Collectively, these results confirm previous findings of increased aggression in short-day-housed hamsters and suggest that short-day-induced increases in aggression are inversely related to gonadal steroid hormones. (C) 2000 Academic Press.