Male and female cooperatively breeding fish provide support for the “Challenge Hypothesis”

The idea that territorial aggression is regulated by androgens and that aggression itself can modulate androgen levels is well established in males. In many species, females also display aggressive behavior, yet little work has been conducted on the effects of female aggression on hormone levels. In this study, we compared the effects of a simulated territory intrusion (a method for testing the Challenge Hypothesis) on males and females of the fish, Neolamprologus pulcher. This cichlid fish from Lake Tanganyika is a particularly useful species to examine sex differences in the behavioral mediation of hormones as breeding pairs remain in a territory year round and both sexes defend this territory against conspecific and heterospecific intruders. In our study, both sexes indeed aggressively defended their territory against a simulated territory intruder. In response to intruders, both males and females displayed elevated levels of circulating 11-ketotestosterone, but only females exhibited increases in testosterone. Neither aggressing male nor female fish showed changes in estradiol levels compared to control (nonaggressing) fish. Residents were more aggressive than the intruders and won most of the interactions. However, residents (or winners) did not show higher hormone levels than intruders (or losers). We suggest that aggression commonly modulates androgen levels in both male and female teleost fish.