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 estradiol (E) are positively associated with levels of female aggression. Here, we manipulated hormone levels in free-living females and examined aggression expressed by females returned to their natural territories. Females received one of the following: (1) ovariectomy + empty implant (OVEX), (2) ovariectomy + 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 females. 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 females had restored levels of display behavior although, unlike SHAM, no T-IMP females expressed the overt aggressive behavior of charging. These data are most consistent with the hypothesis that an ovarian factor such as E promotes female aggression, since ovariectomy reduced both plasma E and aggression 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 role for T in promoting female aggression since T-implants resulted in elevated plasma T and restored display behaviors. This study represents one of the first studies examining the hormonal basis of female aggression in free-living females.