Abstract:
Thermal tolerances are affected by the range of temperatures that species encounter in
their habitat. Daniel Janzen hypothesized in his ªWhy mountain passes are higher in the
tropicsº that temperature gradients were effective barriers to animal movements where climatic
uniformity was high. Sea turtles bury their eggs providing some thermal stability that
varies with depth. We assessed the relationship between thermal uniformity and thermal
tolerance in nests of three species of sea turtles. We considered that barriers were ªhighº
when small thermal changes had comparatively large effects and ªlowº when the effects
were small. Mean temperature was lower and fluctuated less in species that dig deeper
nests. Thermal barriers were comparatively ªhigherº in leatherback turtle (Dermochelys coriacea)
nests, which were the deepest, as embryo mortality increased at lower ªhighº temperatures
than in olive ridley (Lepidochelys olivacea) and green turtle (Chelonia mydas) nests.
Sea turtles have temperature-dependent sex determination (TSD) and embryo mortality
increased as temperature approached the upper end of the transitional range of temperatures
(TRT) that produces both sexes (temperature producing 100% female offspring) in
leatherback and olive ridley turtles. As thermal barriers are ªhigherº in some species than in
others, the effects of climate warming on embryo mortality is likely to vary among sea turtles.
Population resilience to climate warming may also depend on the balance between temperatures
that produce female offspring and those that reduce embryo survival.
