Seasonal aggression independent of seasonal testosterone in wood rats.

Levels of inter-male aggression, both in laboratory encounters and in the field, rise dramatically during the breeding season, closely paralleling the seasonal rise in testosterone. However, post-pubertally castrated males also show the dramatic seasonal rise in aggression in laboratory encounters with castrated opponents and show no decrement in fighting ability when paired with intact opponents, clearly demonstrating the independence of seasonal aggression from the proximate modulating effects of testosterone in wood rats.