SEASONAL FLUCTUATIONS IN BRAIN NUCLEI IN THE RED-SIDED GARTER SNAKE AND THEIR HORMONAL-CONTROL

In many vertebrates, breeding seasons are protracted and mating behavi or is temporally associated with gonadal growth and increased sex ster oid hormone secretion. In the red-sided garter snake (Thamnophis sirta lis parietalis), mating behavior is restricted to the 2-4 weeks immedi ately following emergence from winter dormancy. During this period mat ing behavior is sex-specific: chin-rubbing is exhibited only by males and receptivity to chin-rubbing is exhibited only by females. It is re markable that mating occurs when the gonads are small and circulating concentrations of gonadal hormones are low. As in other vertebrate spe cies, limbic nuclei are involved in the mediation of mating behaviors. To determine if limbic nuclei are sexually dimorphic and, further, wh ether they fluctuate in size with the seasons, the volume of brain are as was measured in both sexes at different times of the year (spring, fall, hibernation) and after hormone manipulation (gonadectomy and gon adectomy plus hormone treatment). The areas of interest were the preop tic area (POA), ventromedial hypothalamus (VMH), and the nucleus spher icus (NS or amygdala); control areas included the external nucleus of the optic tract (a cell-rich area) and medial forebrain bundle (a fibe r-rich area). Comparisons according to season and manipulation reveale d only two instances of sexual dimorphism: the POA of females was sign ificantly 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 patt ern characteristic of this species. In males, there was no statistical difference in the size of the brain areas when compared either by sea son or by hormonal manipulation. In females, on the other hand, there were significant seasonal fluctuations in both the POA and the VMH. Es trogen manipulation induced significant changes in a manner that paral leled the seasonal fluctuations in size. This sex difference in the se asonal variation in brain area size may be due to the fact that in the male, courtship behavior is androgen independent, whereas in the fema le, sexual receptivity is estrogen dependent.