Pivotal Temperature is Not for Everyone: Evidence for Temperature-Dependent Sex Determination in Three Gecko Species.

The prevalence of environmental sex determination (ESD) in squamate reptiles is often overestimated in the literature. This is surprising because we have reliable data demonstrating ESD in only a few species. The documentation of ESD in three species of geckos presented here has significantly increased our knowledge, given that satisfactory evidence for ESD existed in only eight other gecko species. For the first time, we document the occurrence of ESD in the family Sphaerodactylidae. Our finding of unexpected variability in the shapes of reaction norms among geckos highlights that traditional descriptions using parameters such as pivotal temperature, that is, temperature producing a 50:50 sex ratio, are unsatisfactory. For example, the gecko Pachydactylus tigrinus lacks any pivotal temperature and its sex ratios are strongly female-biased across the entire range of viable temperatures. We argue for the effective capture of the relationship between temperature and sex ratio using specific nonlinear models rather than using classical simplistic descriptions and classifications of reaction norms. The study considerably expands evidence of ESD in geckos, which is newly documented in three species. It represents the first finding of ESD in the family Sphaerodactylidae. Interestingly, the dependence of sex ratio on incubation temperatures is more variable than assumed by traditional models. For example, the tiger gecko, Pachydactylus tigrinus, has strongly female-biased sex ratios across its entire range of viable temperatures, which should have significant consequences for its population growth. image Variability in norms of reaction of environmental sex determination (ESD) among three species of geckos is documented. In Pachydactylus tigrinus, highly female-biased sex ratios were found across all temperatures.