Effects of castration on open-field behaviour and aggression in male meadow voles (Microtus pennsylvanicus)

A series of three experiments tested the hypothesis that castration would result in altered open-field behaviour and aggression in male meadow voles. The open-field behaviour of laboratory-reared voles was not affected, and that of wild-captured laboratory-housed animals showed only one significant difference. However, in free-ranging voles, recaptured at intervals for testing, castration resulted in significantly reduced intermale aggression, and significantly decreased activity and increased urination in the open field. Thus, only the results from the experiment using free-ranging voles indicate a significant castration effect, and we suggest the lack of effect in laboratory-housed animals may be due to changes in factors including photoperiod, social milieu, and season.The changes in aggression and open-field behaviour in the wild animals are in agreement with interpretation of previous field studies of intact male voles, which demonstrated changes in these two behaviours at the onset of the breeding season. Among free-ranging meadow voles, therefore, the decreased hormone levels resulting from castration led to changes in these behaviours opposite to those observed in the spring when hormone levels increase with seasonal reproductive maturity. These behaviours thus appear to be at least partially dependent upon circulating testosterone levels in wild voles.