When to be born? Prolonged pregnancy or incubation enhances locomotor performance in neonatal lizards (Scincidae)

The degree of offspring development at hatching (or birth) varies among species within most major vertebrate lineages; altricial vs. precocial birds offer the clearest example of a trade‐off between early hatching and the degree of locomotor development of the hatchling. No such diversity has been reported for reptiles, but we suggest that natural selection may fine‐tune the time of hatching (in oviparous species) or birth (in viviparous species) to optimize offspring phenotypes and hence, maximize fitness. This hypothesis predicts enhanced neonatal performance after more prolonged incubation or gestation, within as well as among populations. Both published and original data on Australian scincid lizards support this prediction. In a field study, viviparous alpine skinks ( Niveoscincus microlepidotus ) that gave birth later in the season had faster‐running offspring, that had a higher probability of surviving through the first year of life. The enhanced performance and survival were not secondary results of larger offspring size. After controlling for effects of mean incubation temperature, prolonged development also correlated with enhanced locomotor performance in hatchlings from eggs of an oviparous skink ( Bassiana duperreyi ) incubated at warm temperatures (>20 °C) but not at cooler temperatures (<20 °C). We suggest that embryonic reptiles control their date of hatching or birth and thus, their stage of development at this critical life‐history transition.