Effects of basking opportunity on birthing asynchrony in a viviparous lizard

The optimal degree of hatching asynchrony may depend on environmental conditions, with synchrony favoured in some environments and asynchrony in others. Therefore, in such a context there should be strong selection on the ability of females to adjust their degree of asynchrony facultatively to match local conditions. While this scenario has traditionally only been of interest to avian biologists, hatching asynchrony, and processes analogous to hatching asynchrony (i.e. birthing asynchrony), have now been identified in a number of nonavian taxa. For example, in an Australian lizard genus, Egernia, birth of offspring within a litter occurs asynchronously, with females having considerable control over the degree of asynchrony. As with birds, if the advantages of birthing asynchrony depend on the environment, females should facultatively adjust the degree of birthing asynchrony based on the prevailing conditions. We examined the birthing asynchrony patterns of female White's skink, Egernia whitii, placed under different thermal environments (basking conditions) during gestation. We found that the basking conditions females experienced during gestation influenced offspring phenotype, both directly, by affecting offspring size, and indirectly, by affecting birthing asynchrony patterns. Specifically, we showed that females held under reduced basking conditions increased the spread over which they gave birth compared to females held under extended basking conditions. As birth spread can influence both offspring growth and survival, our results suggest strong selection on female behaviour (e.g. basking and birthing) in order to maximize offspring and/or parental fitness.