Birthing asynchrony is not a consequence of asynchronous offspring development in a non-avian vertebrate, the Australian skink Egernia whitii

1. One of the major impediments to our understanding of avian hatching asynchrony is the difficulty in separating hypotheses that concentrate on the onset of incubation from those that explain hatching asynchrony adaptively. To address this, ideally we need a system(s) in which we can uncouple these two sets of hypotheses. Such a system exists in the skink genus Egernia. 2. We documented birthing asynchrony (analogous to hatching asynchrony) and the mechanisms underlying birthing asynchrony in the White's skink, Egernia whitii (Lacépède, 1804), a medium-sized, viviparous skink species that displays complex sociality. 3. Egernia whitii females gave birth to offspring asynchronously (100% of litters), with an average of >2 days between births. We show that embryo gestation is synchronous with asynchrony first observed at birth. Furthermore, when birth of the entire litter was induced at the birth of the first offspring, there was no difference in development, mass or weight between first (naturally) born offspring and second and third (induced) offspring. 4. These results suggest that birthing asynchrony in this species is not a consequence of the constraint of asynchronous offspring development, but rather, females retain offspring despite all offspring being fully developed. This suggests that birthing asynchrony in this genus may have an adaptive explanation. 5. We discuss the potential adaptive explanations of birthing asynchrony in this genus, taking into account the potential links with indirect parental care and social organization, specifically the advantages that birthing asynchrony may offer to offspring in a highly competitive social environment. © 2007 The Authors.