Natural nests incubated in two different soil types lead to an overall balanced sex ratio in Podocnemis unifilis hatchlings on the lower Purus river, Brazil

Abstract

Temperature-dependent sex determination occurs in many species of turtles. Because substrates differ in their heat retention capacities, a relatively small change in ambient temperature can exert great influence on sex ratios of hatchlings from clutches deposited in different substrates even within the same geographical area. Since Podocnemis unifilis is one of the Amazonian turtle species (Podocnemididae) with high behavioural plasticity in the use of nesting sites, we tested the influence of soil types on hatchling sex ratios in the Piagaçu Purus Sustainable Development Reserve along the lower Purus River, Brazil. During the 2013 and 2014 nesting seasons, we evaluated 26 nests on a sand bank (n = 14) and clay banks (n = 12). Approximately half of the hatchlings that emerged from each nest were sexed via gonad histological analysis (n = 341). We found a higher incidence of females in nests from the sandy substrate and more males in nests from the clayey substrate (χ² = 40.466, df = 1, P < 0.0001). Ratios of 1 male to 2.28 females were recorded from sand bank nests, and 1.9 males to 1 female from clay banks. Overall, however, the hatchling sex ratio approximated to 1:1. Due to the gradual increase in global temperatures, it has been suggested for species with Temperature Sex Determination (TSD) that some turtle populations become increasingly female-dominated. However, it is likely that the use of different nesting substrates may help to mitigate the effects of global warming on hatchling sex ratio, especially in species with greater behavioural plasticity, such as P. unifilis. © 2020 Deutsche Gesellschaft für Herpetologie und Terrarienkunde e.V. (DGHT), Mannheim, Germany.