Ontogeny of Fish Capture and Ingestion in Four Species of Garter Snakes (Thamnophis)

Abstract

AbstractThe feeding behaviour of four species of garter snakes (Thamnophis butleri, T melanogaster, T. radix, and T sirtalis) was observed in an attempt to determine differences across species and among age groups with respect to capturing, handling and swallowing large and small fish. Although belonging to the same genus, the four species differ widely in habitat and prey preferences (T. butieri being recognized as an earthworm specialist, T melanogaster as an aquatic specialist, and the other two species as generalists). Overall, feeding patterns were quite similar across species following a general sequence of capturing fish, maneuvering the jaws to either the head or tail of the fish (the head being "chosen" more often), and swallowing it. Head-first ingestion was related to prey size: the larger the fish the greater the probability the snake would ingest it head first. When this behaviour was analysed more closely, species and age differences emerged. Adults were more efficient at capturing and ingesting fish head-first and took less time to handle and swallow their prey than did juveniles. This difference was not as marked in the two specialist species as in the two generalist species. Considering differences within species and age classes with respect to their handling behaviour of either a large or a small fish, T melanogaster handled both sizes of prey in a time period that was not significantly different, T. butleri took significantly more time to handle a large fish whereas T radix and T. sirtalis took considerably more time to handle a large fish as newborns but not as adults. These results support field observations that T. melanogaster is an aquatic specialist, T. butleri, an earthworm specialist, and the other two species, generalists. In spite of the similarities between the two generalist species, similarities were also found between T. butleri and T. radix which are believed to the taxonomically related.