Adaptive Significance Of Temperature-dependent Sex Determination Research Articles

Climate change overruns resilience conferred by temperature-dependent sex determination in sea turtles and threatens their survival.

Temperature-dependent sex determination (TSD) is the predominant form of environmental sex determination (ESD) in reptiles, but the adaptive significance of TSD in this group remains unclear. Additionally, the viability of species with TSD may be compromised as climate gets warmer. We simulated population responses in a turtle with TSD to increasing nest temperatures and compared the results to those of a virtual population with genotypic sex determination (GSD) and fixed sex ratios. Then, we assessed the effectiveness of TSD as a mechanism to maintain populations under climate change scenarios. TSD populations were more resilient to increased nest temperatures and mitigated the negative effects of high temperatures by increasing production of female offspring and therefore, future fecundity. That buffered the negative effect of temperature on the population growth. TSD provides an evolutionary advantage to sea turtles. However, this mechanism was only effective over a range of temperatures and will become inefficient as temperatures rise to levels projected by current climate change models. Projected global warming threatens survival of sea turtles, and the IPCC high gas concentration scenario may result in extirpation of the studied population in 50 years.

The significance of predation in nest site selection of turtles: an experimental consideration of macro‐ and microhabitat preferences

Nest selection has important consequences for fitness in freshwater turtles, as nest predation is extremely high in most populations. However, nest predation has rarely been considered a major factor affecting nest site selection. Thermal qualities of a nest are often suggested as a factor influencing maternal choice of nest site because incubation temperature affects offspring performance and growth, as well as the sex ratio of the clutch in species with temperature dependent sex determination (TSD). Yet experimental manipulations of the nesting environment (habitat and threat) to test factors influencing maternal nest site selection in turtles are uncommon. In this paper, we show that nest site selection in an Australian turtle (Emydura macquarii) with genetically determined sex determination (GSD) exists at two spatial scales, with predation as the major factor affecting the location of a nest. Females prefer to nest in areas where nest predation is minimal but when the risk of direct predation is increased, they trade‐off minimising nest predation with survival by locating their nests closer to shore. However, experimentally reducing cover demonstrated that females prefer to nest in open areas, which are more common closer to shore. Hence females are forgoing preferred microhabitat to locate their nests away from shore to minimise nest predation. Despite a preference for open nesting areas, females exhibit similar repeatability of microenvironment selection to a North American species with TSD. Repeatability of nesting overstory vegetation in the North American turtle, Chrysemys picta, suggests that females may exert some control over the thermal qualities of the nests; a key assumption in theoretical models of the microevolution and adaptive significance of TSD in reptiles. Incubation temperature is one factor that may affect microhabitat preferences in both turtles but other factors (e.g. predation and soil moisture) affected by microhabitat may be critical in the evolution of repeatability of overstory cover.

Repeatability of microenvironment-specific nesting behaviour in a turtle with environmental sex determination

Evolutionary thermal adaptation is increasingly being elucidated in a variety of systems. However, one of the most striking examples, temperature-dependent sex determination (TSD) in reptiles, has proven stubbornly difficult to decipher. Theoretical models suggest that selection on and heritable variation in thermal sensitivity of embryonic sex determination and maternal behavioural choice of thermal qualities of nest sites control the microevolutionary potential of TSD. To begin addressing this important issue, we conducted a multiyear field study of nesting behaviour in painted turtles (Chrysemys picta), which display TSD. We detected a significant field repeatability for overstory vegetation cover around nests at oviposition, a trait that is correlated inversely with nest temperature during embryonic sex determination and positively with offspring sex ratio (percentage of male). Neither clustered nesting nor individual spatial repeatability in nest site choice caused this striking pattern. Instead, females consistently preferred nest environments with particular quantities of overstory vegetation cover. These findings confirm a crucial assumption of models concerning the microevolution and adaptive significance of TSD in reptiles. The results also indicate that this system may be able to evolve via long-term maternal nesting behaviour in response to skewed sex ratios caused by environmental perturbations, such as gradual long-term climate change.

Sex Determination and Sex Ratios inCrocodylus palustris

Incubation temperature determines sex in the mugger crocodile, Crocodylus palustris . Exclusively females are produced at constant temperatures of 28.0°C through 31°C. At 32.5°C, only males are produced. Both sexes are produced in varying proportions at 31.5, 32.0, and 33.0°C. Embryo survival is not affected within this range, but developmental rate and total incubation time are strongly temperature dependent. In natural nests laid in breeding enclosures, cool incubation temperatures produced only females whereas males were produced only in warm nests. Clutch sex ratios were female or male biased. Yearly sex ratios (=percent male) varied from 0.05 to 0.58; overall sex ratio during six nesting seasons was 0.24 (1 male: 3 females). Sex ratio and incubation time vary with nest location and temperature in a manner consistent with the constant temperature results. Incubation time decreases with increasing incubation temperature, and is an accurate predictor of sex ratio in the field and laboratory. To date, temperature-dependent sex determination (TSD) has been reported in five species of Crocodylus and in three species of Alligatorinae; but the TSD patterns in these groups differ. The TSD pattern of C. palustris is similar to that of C. porosus . Nesting in C. palustris is synchronized with the seasonal availability of thermal regimes suitable for incubation. Resultant sex ratios are a consequence of when and where eggs are laid. Early nests are located in warm, sunny sites; in contrast, late season nests are located in the shade. An egg transplant experiment demonstrated that sex ratios could be altered by simple manipulations of nest temperatures in the field. The adaptive significance of TSD in crocodilians may relate to the influence of incubation temperature on various hatchling attributes, particularly growth.