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

The idea that territorial aggression is regulated by androgensand that aggression itself can modulate androgen levels is wellestablished in males. In many species, females also displayaggressive behavior, yet little work has been conducted on theeffects of female aggression on hormone levels. In this study,we compared the effects of a simulated territory intrusion (amethod for testing the Challenge Hypothesis) on males and femalesof the fish, Neolamprologus pulcher. This cichlid fish fromLake Tanganyika is a particularly useful species to examinesex differences in the behavioral mediation of hormones as breedingpairs remain in a territory year round and both sexes defendthis territory against conspecific and heterospecific intruders.In our study, both sexes indeed aggressively defended theirterritory against a simulated territory intruder. In responseto intruders, both males and females displayed elevated levelsof circulating 11-ketotestosterone, but only females exhibitedincreases in testosterone. Neither aggressing male nor femalefish showed changes in estradiol levels compared to control(nonaggressing) fish. Residents were more aggressive than theintruders 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 androgenlevels in both male and female teleost fish.