Embryo Development and Sex Ratios in the Red-Footed Tortoise (Chelonoidis carbonarius) at Masculinizing Temperatures

Abstract

Incubation temperature is a factor that can affect several traits in turtles such as body size, growth, shape, and sex in species with temperature-dependent sex determination. A clear understanding of these effects is particularly important in threatened species such as the red-footed tortoise (Chelonoidis carbonarius), classified in Colombia as Vulnerable mainly due to the capture of wild individuals to be kept as pets. A previous study on the effect of incubation temperature on sex determination concluded that constant temperatures of 31°C and 33°C are lethal to C. carbonarius embryos and that a temperature of 29°C produces 100% females. Although this showed that C. carbonarius has a temperature-based sex determination mechanism, its full reaction norm is still unknown. To fill in the gaps, we incubated 160 eggs from 47 nests at constant temperatures of 24°C, 26°C, and 28°C. The hatching success rates were 55%, 53%, and 60%, respectively, with extremely long incubation periods of 213, 164, and 138 d. Of the 75 neonates obtained, 58 reached 8 mo of age, at which time 45% (n = 26) were sexed. The sexual proportions (% males) obtained were 88.9%, 60%, and 0% from the 24°C, 26°C, and 28°C treatments. From the calculated thermal reaction norm, we estimated that the pivotal temperature is 26.05°C (95% CI = 25.31–26.69) and the transitional temperature range is from 24.34°C to 27.77°C. Incubation temperature also affected incubation period and hatchling body size and growth. Additionally, we found a significant effect of female body size and mass on egg size and mass, supporting a fecundity selection hypothesis of body size. Finally, we inspected hatchlings for sexual shape dimorphism using geometric morphometrics. We found significant differences in anal notch and plastron shape at 7 d of age, but only differences for anal notch at 1 and 2 mo of age. However, the statistical signal was weak and the results varied with age; thus this sexing technique was unreliable. Given the low critical thermal maxima for embryo survivorship (31°C or less), together with a 100% feminizing temperature of 27.9°C, global climate change coupled with deforestation constitute imminent threats to this species due to demographic impacts such as low recruitment rates and skewed primary sex ratios.