Seasonal variations in activity rhythms of male voles: mediation by gonadal hormones.

Adult male Microtus montanus were castrated or sham-castrated and housed under 16 hours of light (LD 16:8). Castrates showed increased preference for diurnal and decreased preference for nocturnal activity compared to shams. Castrates showed a trend toward increased crepuscular preference. Shams showed more total activity than castrates. Longterm castrates (under LD 8:16) displaying either predominantly diurnal or crepuscular activity were implanted with either empty Silastic capsules or capsules containing testosterone. Animals with testosterone implants displayed increased preference for nocturnal and decreased preference for crepuscular activity as compared to controls. No significant difference in preference for diurnal activity was found. Testosterone increased total activity. Testosterone appears to be an important, but not the only, cue in determining timing of running wheel use in male M. Montanus. Seasonal changes in reproductive function may be involved in the dramatic seasonal variations in activity timing observed in some field populations of microtine rodents. In all treatments, high interanimal variation occurred. This variation may be a function of the inherent flexibility of activity in microtine rodents.