Ovarian hormones influence territorial aggression in free-living female mountain spiny lizards

Females are aggressive in many species but relatively little is known about the hormonal basis of female aggression, especially in free-living animals . Female mountain spiny lizards aggressively defend territories from other females. Previously, we showed that plasma levels of testosterone (T) and e stradiol (E) are positively associated with levels of female aggression. He re, we manipulated hormone levels in free-living females and examined aggre ssion expressed by females returned to their natural territories. Females r eceived one of the following: (1) ovariectomy + empty implant (OVEX), (2) o variectomy + T implant (T-IMP), or (3) sham surgery + empty implant (SHAM). OVEX females had reduced plasma levels of E but not T relative to SHAM fem ales. T-IMP females had elevated plasma levels of T. Levels of display and aggression in OVEX females were reduced relative to SHAM females. T-IMP fem ales had restored levels of display behavior although, unlike SHAM, no T-IM P females expressed the overt aggressive behavior of charging. These data a re most consistent with the hypothesis that an ovarian factor such as E pro motes female aggression, since ovariectomy reduced both plasma E and aggres sion but had no effect on plasma T. The results from the T-IMP females are also consistent with this hypothesis if we assume that the effects of T are due to aromatization to E in target tissues. The data do not rule out a ro le for T in promoting female aggression since T-implants resulted in elevat ed plasma T and restored display behaviors. This study represents one of th e first studies examining the hormonal basis of female aggression in free-l iving females. (C) 1999 Academic Press.