Testosterone and territorial aggression in black redstarts

The sex hormone testosterone has been traditionally linked to aggressive behaviour, not only in humans but also in other animals. In birds, testosterone is involved in regulating aggressive behaviour during the breeding season, which is the time of the year when males typically defend a territory and attract mates.

We study territorial aggression in the black redstart (Phoenicurus ochruros), a songbird that is different from many species studied so far, because it establishes a territory not only during the breeding season, but also outside a breeding context in autumn or winter.

The results of our work suggest that testosterone is unrelated to territorial aggression in black redstarts. But why is that so? In birds, the testes show large seasonal differences in size: in small songbirds such as the black redstart, they have the size of small beans during the breeding season, but they are hardly visible during the rest of the year. And of course such tiny testes cannot produce much testosterone. But because black redstarts aggressively defend a territory during most of the year they must have “invented” other mechanisms to stimulate territorial aggression. We suspect that territorial aggression is hard-wired into the brain of black redstarts. Instead of using testosterone to boost territoriality, they may use a different hormone to suppress territoriality during the only time of the year when they are not territorial, and that is during migration.

Our work adds to the increasing evidence that we need a much more differentiated view on hormones and how they affect behaviour.

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Selected publications of the black redstart project