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

David Crews, Rebecca Robker, Mary Mendonça

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25 Citations (Scopus)


In many vertebrates, breeding seasons are protracted and mating behavior is temporally associated with gonadal growth and increased sex steroid hormone secretion. In the redsided 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. In males, there was no statistical difference in the size of the brain areas when compared either by season or by hormonal manipulation. In females, on the other hand, there were significant seasonal fluctuations in both the POA and the VMH. Estrogen manipulation induced significant changes in a manner that paralleled the seasonal fluctuations in size. This sex difference in the seasonal variation in brain area size may be due to the fact that in the male, courtship behavior is androgen independent, whereas in the female, sexual receptivity is estrogen dependent.

Original languageEnglish
Pages (from-to)5356-5364
Number of pages9
JournalThe Journal of Neuroscience
Issue number12
Publication statusPublished - 1 Dec 1993
Externally publishedYes


  • Amygdala
  • Brain nuclei
  • Medial forebrain bundle
  • Nucleus sphericus
  • Preoptic area
  • Reptile
  • Seasonal differences
  • Sex differences
  • Snake
  • Steroid hormones
  • Ventromedial hypothalamus

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