Are thermal barriers "higher" in deep sea turtle nests?

Abstract

Thermal tolerances are affected by the range of temperatures that species encounter in their habitat. Daniel Janzen hypothesized in his “Why mountain passes are higher in the tropics” 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 “high” when small thermal changes had comparatively large effects and “low” when the effects were small. Mean temperature was lower and fluctuated less in species that dig deeper nests. Thermal barriers were comparatively “higher” in leatherback turtle (Dermochelys coriacea) nests, which were the deepest, as embryo mortality increased at lower “high” 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 “higher” 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.

A Systematic Review of Metabolic Heat in Sea Turtle Nests and Methods to Model Its Impact on Hatching Success

“…Sea turtles lay their nests 40-80 cm below the surface of nesting beaches, and average nest depth varies between species (Kaska et al, 1998b;Koch et al, 2007;Santidrián Tomillo et al, 2017). Sand temperatures at these depths are frequently used to estimate temperatures inside the sea turtle nest chamber (e.g., Bentley et al, 2020), and much work has gone into predicting how climate change will drive changes in sand temperature, and thereby alter hatching traits (e.g., Hawkes et al, 2007;Fuentes et al, 2009;Stubbs et al, 2014).…”

Temperature‐Dependent Sex Determination in Sea Turtles in the Context of Climate Change: Uncovering the Adaptive Significance

“…A relationship has been found between hatching success and TSD curves in leatherback and olive ridley turtles in Costa Rica, where hatching success starts declining at nest temperatures that coincide with the upper end of the TRT (corresponding to 100% female production). [ 57 ] The existing relationship between TSD curves and thermal tolerance of sea turtle clutches supports the hypothesis that TSD curves and thermal tolerances follow a model of coadaptation. However, little is known about how this mechanism works and at what pace sea turtles may be able to adapt.…”

“…As a result, leatherback clutches are more sensitive to changes in temperature than those of other species that are shallower nesters and experience large temperature fluctuations. [ 57 ] Sand temperature, often due to differences in sand color, can also differ among nesting beaches, affecting the thermal tolerance of the turtles that nest there. For example, differences in thermal maxima were found in green turtles among females that nested in nearby nesting beaches where thermal conditions varied by ≈2 °C.…”

A Review of the Effects of Climate Change on Chelonians

“…Of the existing research regarding the effects of climate change on individuals, the majority of effort has focused on nest-site selection and its effects on egg development. Nest-site selection influences the thermal environment for the developing embryos, which can affect embryo survivorship and the sex of the offspring (e.g., [36][37][38]). Chinese softshell turtles (Pelodiscus sinensis) exhibited only a 73.7% hatching rate when incubated at 34 • C, considerably lower than the 81.7-96.9% hatching rate when they were incubated at 24-32 • C [20].…”