Thermal ecology of the garter snakes Thamnophis sirtalis concinnus (Hallowell) and Thamnophis ordinoides (Baird & Girard)

Graduation date: 1964In recent years, studies dealing with temperature regulation,\ud temperature sensitivity, and physiological responses to temperature\ud in lizards and other reptiles have increased tremendously. It is\ud notable that snakes have been largely ignored in such studies. This\ud no doubt is due to their less direct relationship to ancestral endotherms.\ud However, problems in the ecology of snakes themselves\ud may be elucidated by studies of this kind.\ud The present study deals with two species of garter snakes\ud ( Thamnophis sirtalis concinnus and Thamnophis ordinoides) which are\ud abundant in Oregon's Willamette Valley. The species sirtalis is the\ud most wide ranging snake in the United States. It is commonly found\ud near permanent water, though occasionally it is encountered in rather\ud dry situations. In contrast to sirtalis, the monotypic species\ud ordinoides is a strictly terrestrial northern Pacific Coast form, which typically is associated with areas of dense vegetation.\ud T. s. concinnus is often seen basking on mild days of the coldest\ud winter months (November-February) while T. ordinoides rarely\ud emerges during these months.\ud The distinct differences in habitat preference and winter\ud behavior exhibited by these snakes suggest differences in thermal\ud preferences and critical levels. To examine this possibility, and the\ud responses of the snakes to thermal acclimation, comparative data on\ud the following variables have been sought: 1) Body temperature of\ud snakes in the field and its relationship to environmental temperatures;\ud 2) body temperature of snakes in a thermal gradient box; 3) critical\ud thermal maximum and minimum; 4) metabolic rate; 5) the effect of\ud thermal acclimation on items two, three, and four.\ud While more investigations are needed to confirm and clarify\ud the results of this study, some tentative conclusions may be stated\ud here: 1) T. s. concinnus and T. ordinoides are diurnal, heliothermic\ud snakes which consistently maintain body temperatures higher than\ud the air and substrate temperatures. 2) T. s. concinnus is more\ud tolerant of seasonally extreme temperatures than T. ordinoides. 3)\ud T. s, concinnus prefers slightly higher body temperatures than T.\ud ordinoides. 4) If temperature tolerances and preferred temperatures\ud are heritable characteristics developed in the phylogeny of the\ud species, these differences probably are primarily related to the difference in distribution of each species as a whole, and secondarily\ud to differences in habitat preference and habit which have resulted from\ud competetive interactions with other species. 5) There are sexual\ud differences in the preferred temperatures of both species, the most\ud significant of which is the relatively high preferred temperature of\ud pregnant females. 6) Initially, the preferred temperatures of both\ud species are inversely related to acclimation temperature. Later,\ud they may be directly related to acclimation temperature. 7) The\ud critical thermal maxima and minima are similar in both species and\ud are directly related to acclimation temperature. However, reacclimation\ud affects the original response differently in the two species.\ud 8) Both species exhibit typical metabolic rate responses to thermal\ud acclimation. 9) Since metabolic rate responses are the same in both\ud species, the greater cold tolerance of T. s. concinnus cannot be\ud attributed to a greater ability to compensate for low temperature