Terrestrial ecology of semi-aquatic giant gartersnakes (Thamnophis gigas)

Wetlands are a vital component of habitat for semiaquatic herpetofauna, but for most species adjacent terrestrial habitats are also essential. We examined the use of terrestrial environments by Giant Gartersnakes (Thamnophis gigas) to provide behavioral information relevant to conservation of this state and federally listed threatened species. We used radio telemetry data collected 1995–2011 from adults at several sites throughout the Sacramento Valley, California, USA, to examine Giant Gartersnake use of the terrestrial environment. We found Giant Gartersnakes in terrestrial environments more than half the time during the summer, with the use of terrestrial habitats increasing to nearly 100% during brumation. While in terrestrial habitats, we found Giant Gartersnakes underground more than half the time in the early afternoon during summer, and the probability of being underground increased to nearly 100% of the time at all hours during brumation. Extreme temperatures also increased the probability that we would find Giant Gartersnakes underground. Under most conditions, we found Giant Gartersnakes to be within 10 m of water at 95% of observations. For females during brumation and individuals that we found underground, however, the average individual had a 10% probability of being located > 20 m from water. Individual variation in each of the response variables was extensive; therefore, predicting the behavior of an individual was fraught with uncertainty. Nonetheless, our estimates provide resource managers with valuable information about the importance of protecting and carefully managing terrestrial habitats for conserving a rare semiaquatic snake.