Female territorial aggression and steroid hormones in mountain spiny lizards

In certain species, females are highly aggressive. For example, female mountain spiny lizards, Sceloporus jarrovi, express aggressive behaviours when defending their territories. Despite the occurrence of female aggression, relatively little is known about its underlying hormonal mediation. As a first step in understanding neuroendocrine mechanisms of female aggression, this study documented seasonal changes in territorial aggression, plasma steroid hormone levels, and reproductive condition in free-living female mountain spiny lizards. We determined how seasonal patterns of testosterone (T), oestradiol (E), dihydrotestosterone (DHT) and corticosterone (B) corresponded to seasonal patterns of aggression. At times of the year when females were most aggressive, plasma levels of T and E were elevated. At times of the year when females were least aggressive, plasma B was elevated. Furthermore, when levels of T were elevated, females were more aggressive, and when levels of B were elevated, females were less aggressive. Plasma levels of DHT were not associated with aggression. These seasonal patterns are consistent with the hypotheses that T, and perhaps E, promote female aggression, while B suppresses female aggression. Despite these associations of hormones and aggression, no hormone was associated with aggression in a one-to-one fashion. This suggests that if steroid hormones contribute to the expression of female aggression, they must interact with each other or with other factors (for example, other hormones, receptor levels, environmental factors) to mediate fully the expression of aggression. Copyright 1999 The Association for the Study of Animal Behaviour.