RUBENSTEIN, D.R.; WIKELSKI, M.; Cornell University; Princeton University: Steroid hormone patterns and aggression during breeding and nesting in female Gal�pagos marine iguanas.
We studied steroid hormone patterns and aggression during breeding and nesting in female Gal�pagos marine iguanas (Amblyrhynchus cristatus). Following mating, females display aggressively, signaling to males that they are no longer receptive. After a few weeks, they move to the nesting area where they fight for and defend nest sites. We sampled females at three stages during breeding and four stages during nesting to measure seasonal changes in testosterone (T), estradiol (E), corticosterone (B), and progesterone (P). We also examined hormone levels during aggressive interactions while nesting. We found that T and E increased throughout breeding and dropped off before nesting. B increased throughout breeding and nesting, but decreased significantly after females laid eggs. P was elevated throughout breeding, but declined steadily during nesting. Although T was up to five times lower during nesting than during breeding, T levels increased significantly from the time females that arrived on the nesting area until after they laid eggs; E was at baseline levels throughout nesting. However, females captured while fighting to defend or take over nests had significantly lower T, but higher E, than females digging or covering nests. Since fights typically only lasted seconds to minutes, the differences in T and E levels suggest that female iguanas can rapidly aromatize T in the body to elevate circulating levels of E during aggressive interactions. Moreover, fighting females also had P levels that were higher than those at any point during breeding or nesting, suggesting that P may also play a role in female aggression during nesting. These results suggest that the rapid (i.e., seconds to minutes) production and conversion of steroid hormones in the body may be an important mechanism underlying female aggression in reptiles and other vertebrate taxa.
