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