Family, sex and testosterone effects on garter snake behaviour

To better understand how genes and hormones interact to affect behaviour in nature, I used a factorial design to test for effects of family membership, sex and testosterone level on activity and defensive behaviour of the common garter snake, Thamnophis sirtalis. Behaviours (latency to move, defensive strikes, response distance) were scored prior to hormone manipulation (when snakes were 39 days of age), while sham or testosterone-containing implants were in place (190 days), following implant removal and simulated hibernation (284 days), and when snakes were 428 days of age. Family membership had pervasive effects on all three behaviours and on their ontogenetic trajectories, suggesting strong genetic or maternal components to behavioural variation. Sex had a significant effect on the number of defensive strikes; females struck more frequently than males, but ontogenetic trajectories were similar between the sexes. Testosterone manipulation also had an effect on strikes: snakes in the elevated-testosterone treatment group struck less frequently than shams while implants were in place. Sex and treatment effects on latency to move and response distance were lacking. Family*treatment interaction effects were lacking for latency to move and number of defensive strikes but were present for response distance. Possibly, genetic or maternally induced variation in strikes is mediated through variation in circulating hormone levels, whereas variation in response distance is mediated through receptor-level phenomena.