Seasonal and sexual dimorphisms in the green anole forebrain.

During the breeding season, male green anole lizards extend a throat fan (dewlap) in courtship. This behavior is facilitated by testosterone (T). Females extend a much smaller dewlap less often, even with the same dose of T. During the nonbreeding season when T is low, dewlap extension is reduced. To determine if parallels exist between structure and function, we investigated neuron soma size and density in the preoptic area (POA) and ventromedial nucleus of the amygdala (AMY), which are involved in the display behavior, in breeding and nonbreeding males and females. Cells from breeding animals were larger than cells from nonbreeding animals, but they were not sexually dimorphic. No significant effects existed in neuron density. This experiment indicates that portions of the anole forebrain important for sexual behavior are plastic and might be influenced by seasonal changes in steroid hormones. To investigate whether T can reverse the seasonal difference in soma size in both sexes, gonadectomized nonbreeding anoles were implanted with an empty or T propionate-filled capsule; animals were also tested for male-typical courtship behavior. Males and females treated with T had higher rates of dewlap extension, but across treatment groups these rates were greater in males. Neuron soma size in the POA and AMY was larger in males than females, but no effects of treatment were detected. Taken together, the results indicate that T can stimulate behavior in the nonbreeding season and suggest that a dissociation exists between the regulation of the courtship display and soma size of relevant brain regions.