Seasonal fluctuations in brain nuclei in the red-sided garter snake and their hormonal control.

In many vertebrates, breeding seasons are protracted and mating behavior is temporally associated with gonadal growth and increased sex steroid hormone secretion. In the red-sided garter snake (Thamnophis sirtalis parietalis), mating behavior is restricted to the 2-4 weeks immediately following emergence from winter dormancy. During this period mating behavior is sex-specific: chin-rubbing is exhibited only by males and receptivity to chin-rubbing is exhibited only by females. It is remarkable that mating occurs when the gonads are small and circulating concentrations of gonadal hormones are low. As in other vertebrate species, limbic nuclei are involved in the mediation of mating behaviors. To determine if limbic nuclei are sexually dimorphic and, further, whether they fluctuate in size with the seasons, the volume of brain areas was measured in both sexes at different times of the year (spring, fall, hibernation) and after hormone manipulation (gonadectomy and gonadectomy plus hormone treatment). The areas of interest were the preoptic area (POA), ventromedial hypothalamus (VMH), and the nucleus sphericus (NS or amygdala); control areas included the external nucleus of the optic tract (a cell-rich area) and medial forebrain bundle (a fiber-rich area). Comparisons according to season and manipulation revealed only two instances of sexual dimorphism: the POA of females was significantly smaller than that of males during hibernation and the NS of females was significantly smaller than that of males in those animals not subjected to hibernation. This general lack of sexual dimorphisms in the brain despite the sex-specificity of the behavior patterns and their marked seasonality may reflect the dissociated reproductive pattern characteristic of this species.(ABSTRACT TRUNCATED AT 250 WORDS)