Transitions between life-history stages are often accompanied by dramatic behavioral switches that result from a shift in motivation to pursue one resource over another. While the neuroendocrine mechanisms that regulate such behavioral transitions are poorly understood, arginine vasotocin (AVT) and neuropeptide Y (NPY) are excellent candidates because they modulate reproductive and feeding behavior, respectively. We asked if seasonal changes in AVT and NPY are concomitant with the seasonal migration to and from the feeding grounds in red-sided garter snakes (Thamnophis sirtalis parietalis). Male and female snakes were collected in different migratory states during both the spring and fall. The total number of AVT- and NPY-immunoreactive (ir) cells was then quantified in each brain region of interest. To correct for potential variation in region volume related to sexually dimorphic body size in this species, we first determined that snout-vent length is an accurate proxy for regional brain volume. We then corrected each individual's ir cell number by its SVL to directly compare seasonal changes in AVT and NPY between males and females. Within the supraoptic nucleus, both males and females had more AVT-ir cells during the fall compared with spring. As predicted, males had significantly more AVT-ir cells during the spring mating season in the hypothalamus (HYP) and bed nucleus of the stria terminalis, brain regions important in regulating reproductive behavior. Females also had significantly more AVT-ir cells in the HYP during the spring, as well as a significantly higher number of hypothalamic AVT cells than males. During the fall, males had significantly more NPY-ir cells in the cortex and posterior HYP compared with spring, possibly reflecting increased feeding behavior during summer foraging. Females did not exhibit significant main effects of season on NPY-ir cell number in any region. Neither AVT- nor NPY-ir cell number varied significantly with migratory status, but we did observe significant changes related to seasonal transitions in reproductive state. Our results indicate that changes in brain AVT and NPY are associated with seasonal transitions in reproductive and foraging behaviors, and may be involved in mediating sex differences in the timing of life-history events.

All animals use external cues from the environment to accurately time life‐history events. How the brain decodes environmental stimuli to effect changes in physiology and behavior, however, is poorly understood, particularly with regard to supplementary environmental cues such as temperature. We asked if low‐temperature dormancy alters the synthesis and/or release of gonadotropin‐releasing hormone (GnRH). We used the well‐studied red‐sided garter snake (Thamnophis sirtalis) for this study, as low‐temperature exposure is both necessary and sufficient to induce reproduction in northern populations of this species. Snakes were collected from the field and hibernated at 4°C or 10°C in complete darkness for up to 16 weeks. In males, increasing duration of low‐temperature dormancy significantly increased GnRH‐immunoreactive cell number and GnRH soma size (a proxy for relative cell activity) in the forebrain. These changes mirrored those in male reproductive behavior (reported previously) and plasma androgen concentrations. The changes in GnRH cell area observed in males were specific to the neuroendocrine population of cells in the medial preoptic area; soma size in the rostral GnRH cells did not change. Finally, temperature‐induced changes in GnRH were sexually dimorphic: neither hibernation temperature nor the duration of winter dormancy significantly modulated GnRH cell number or soma size in females, despite the fact that plasma estradiol and corticosterone increased significantly in response to both. These data demonstrate that the neuroendocrine GnRH system is sensitive to environmental temperature and suggest that GnRH neurons play a conserved but trans‐seasonal role in mediating changes in reproductive physiology and behavior in dissociated breeders.

Seasonal rhythms in physiology and behavior are widespread across diverse taxonomic groups and may be mediated by seasonal changes in neurogenesis, including cell proliferation, migration, and differentiation. We examined if cell proliferation in the brain is associated with the seasonal life-history transition from spring breeding to migration and summer foraging in a free-ranging population of red-sided garter snakes (Thamnophis sirtalis) in Manitoba, Canada. We used the thymidine analog 5-bromo-2′-deoxyuridine (BrdU) to label newly proliferated cells within the brain of adult snakes collected from the den during the mating season or from a road located along their migratory route. To assess rates of cell migration, we further categorized BrdU-labeled cells according to their location within the ventricular zone or parenchymal region of the nucleus sphericus (homolog of the amygdala), preoptic area/hypothalamus, septal nucleus, and cortex (homolog of the hippocampus). We found that cell proliferation and cell migration varied significantly with sex, the migratory status of snakes, and reproductive behavior in males. In most regions of interest, patterns of cell proliferation were sexually dimorphic, with males having significantly more BrdU-labeled cells than females prior to migration. However, during the initial stages of migration, females exhibited a significant increase in cell proliferation within the nucleus sphericus, hypothalamus, and septal nucleus, but not in any subregion of the cortex. In contrast, migrating males exhibited a significant increase in cell proliferation within the medial cortex but no other brain region. Because it is unlikely that the medial cortex plays a sexually dimorphic role in spatial memory during spring migration, we speculate that cell proliferation within the male medial cortex is associated with regulation of the hypothalamus-pituitary-adrenal axis. Finally, the only brain region where cell migration into the parenchymal region varied significantly with sex or migratory status was the hypothalamus. These results suggest that the migration of newly proliferated cells and/or the continued division of undifferentiated cells are activated earlier or to a greater extent in the hypothalamus. Our data suggest that sexually dimorphic changes in cell proliferation and cell migration in the adult brain may mediate sex differences in the timing of seasonal life-history transitions.

Many animals exhibit seasonal changes in life-history stages, and these seasonal iv