Female territorial aggression and steroid hormones in mountain spiny lizards

In certain species, females are highly aggressive. For example, female moun tain spiny lizards, Sceloporus jarrovi, express aggressive behaviours when defending their territories. Despite the occurrence of female aggression, r elatively little is known about its underlying hormonal mediation. As a fir st step in understanding neuroendocrine mechanisms of female aggression, th is study documented seasonal changes in territorial aggression, plasma ster oid hormone levels, and reproductive condition in free-living female mounta in spiny lizards. We determined how seasonal patterns of testosterone (T), oestradiol (E), dihydrotestosterone (DHT) and corticosterone (B) correspond ed to seasonal patterns of aggression. At times of the year when females we re most aggressive, plasma levels of T and E were elevated. At times of the year when females were least aggressive, plasma B was elevated. Furthermor e, when levels of T were elevated, females were more aggressive, and when l evels of B were elevated, females were less aggressive. Plasma levels of DH T were not associated with aggression. These seasonal patterns are consiste nt with the hypotheses that T, and perhaps E, promote female aggression, wh ile B suppresses female aggression. Despite these associations of hormones and aggression, no hormone was associated with aggression in a one-to-one f ashion. This suggests that if steroid hormones contribute to the expression of female aggression, they must interact with each other or with other fac tors (for example, other hormones, receptor levels, environmental factors) to mediate fully the expression of aggression. (C) 1999 The Association for the Study of Animal Behaviour.