Feeding, Thermoregulation, and Offspring Viability in Gravid Garter Snakes (Thamnophis sirtalis): What Makes Laboratory Results Believable?

Abstract

Animals held in the unnatural surroundings of the laboratory sometimes may exhibit unusual behavior, making experimental results difficult to interpret unless the natural history of the species is well understood. Some well-studied species, such as the garter snake Thamnophis sirtalis, which usually seem to adapt well to captivity, are therefore commonly used in laboratory experiments and assumed to function normally. However, this assumption may sometimes be unfounded. In this study, captive gravid T. sirtalis showed reduced feeding behavior, similar to free-ranging gravid snakes from the same population but showed very different thermoregulatory behavior, choosing low temperatures instead of high. By contrast, gravid congeners, Thamnophis elegans, showed similar feeding and thermoregulatory behavior in captivity and in the field. Choice of lower temperature by gravid T. sirtalis apparently led to extended periods of parturition for individual females and a high incidence of dead young in litters. Other studies of T. sirtalis in captivity also have yielded fairly high proportions of dead offspring, suggesting that (1) this species has a relatively high background rate of natural stillbirth; and/or (2) it is not always as suitable a species for behavioral/reproductive work in the laboratory as it seems to be. Whether or not the first proposition is true, I conclude that T. sirtalis in this study behaved differently from snakes in the field, exacerbating the rate of stillbirth. Thus, laboratory studies of behavior will be most fruitful if informed by field studies.