Egg mass determines hatchling size, and incubation temperature influences post‐hatching growth, of tuatara Sphenodon punctatus

Abstract

The size of reptile hatchlings can be phenotypically plastic in response to incubation temperature, and size is a trait likely to influence fitness -i.e. hatchling size is proposed as an indicator of quality. The parental and incubation temperature effects on the size of one of New Zealand's most biologically significant reptile species, the tuatara Sphenodon punctatus are investigated. Artificial incubation at constant temperatures is used to produce founders for new captive and wild populations of tuatara and to augment existing rare populations. We compare size of hatchling tuatara from artificial and natural incubation treatments. The relationship of hatchling size with incubation temperature and sex is examined, and we investigate whether our results support differential fitness models for the evolution of temperature-dependent sex determination in tuatara. Initial egg mass is the most important factor affecting size of hatchling tuatara and is still an important influence at 10 months of age. Incubation temperature does not greatly influence size of hatchlings, but significantly influences size by 10 months of age. Constant artificial incubation conditions result in larger, but possibly less aggressive, juveniles than those from more variable natural incubation conditions by 10 months of age. Evidence from size patterns of tuatara incubated in natural nests supports differential fitness models for the adaptive significance of temperature-dependent sex determination. Thermal variation has little effect on size of male hatchlings, but female embryos that develop in more stable thermal conditions, in more reliable sites for hatching, are bigger and have longer jaws.

Predicting the fate of a living fossil: how will global warming affect sex determination and hatching phenology in tuatara?

“…Incubation temperature has little effect on body size at hatching in tuatara (Nelson et al 2004b), but the energetic consequences of early maturity will depend on whether hatchlings immediately emerge from the nest or overwinter in the nest cavity. Evidence from a review of emergence patterns in turtles that nest in spring and reach hatchling stage in autumn suggests that the phenomenon of overwintering in the nest may be adaptive in ensuring that hatchlings emerge in favourable spring conditions (Gibbons & Nelson 1978).…”

“…Because parameter b 3 , the temperature at which the development rate peaks, is unknown, we systematically fixed b 3 at 18C intervals between 25 and 358C to produce 11 nonlinear development rate functions (see figure S2 in the electronic supplementary material). The 12 development rate functions (one linear-with-threshold and 11 nonlinear) were then applied to predict development rates from temperatures recorded in 14 natural S. punctatus nests where hatch dates were known ( Nelson et al 2004b), with the aim of selecting the function that was the best predictor of total incubation time.…”

“…(g) Testing sex ratio predictions Using temperatures from 16 S. punctatus nests where hatchling sex had been determined by laparoscopy ( Nelson et al 2004b), we compared the sex ratio predicted by the biophysical model with the sex ratio of offspring from each nest. We applied the sex ratio function for S. punctatus that has a pivotal temperature of 228C and a TRT 0.05 of 1.18C , and predicted mixed-sex nests for sites with CTEs between 21.2 and 22.258C, all-female nests for CTEs below 21.28C and all-male nests for CTEs greater than 22.258C.…”

Bite‐force performance of the last rhynchocephalian (Lepidosauria:Sphenodon)

“…Aggressive competition occurs among female Sphenodon for nesting sites on Stephens Island (Nelson et al 2004). This is similar to aggressive interactions among female Galapagos land iguanas (Conolophus subcristatus), that compete intensely for limited nesting sites in their rocky volcanic habitat (Pough et al 2003).…”

“…Sphenodon is sexually dimorphic with males attaining larger size (Buller 1877;Robb 1977;Crée et al 1995Crée et al , 1999Tracy 1997;Nelson et al 2004). This is associated with territoriality and fighting among males (Gans et al 1984;Gillingham étal.…”