Are there benefits to being born asynchronously: an experimental test in a social lizard

Hatching asynchrony, or processes analogous to hatching asynchrony (i.e., birthing asynchrony), has now been identified in a number of nonavian taxa. These systems are of importance because, unlike birds, they allow us to decouple hatching asynchrony hypotheses related to adaptive hatching patterns, which suggest that it confers a fitness benefit to either the parent or offspring, from those that focus on the onset of parental incubation. However, to date research on these systems has remained descriptive, solely documenting the extent of asynchronous hatching/birthing. Here we provide the first experimental test of the potential adaptive nature of hatching/birthing asynchrony outside avian systems. Using the birth hormone arginine vasotocin, we manipulated the birthing (a)synchrony of females in a population of the lizard Egernia whitii and examined the effect on offspring growth and survival. We show offspring from asynchronous treatments suffered increased mortality but benefited from increased mass at 6 weeks compared with offspring in synchronous treatments. Differences in mortality and size between treatments were driven by offspring mass at birth, and the development of a greater mass hierarchy within asynchronous compared with synchronous litters. This resulted in the smaller offspring suffering both an increased risk of mortality and decreased growth. Despite this, the mechanism by which these patterns are produced remains unclear, as we found no link between birth order and mortality, size, or any factors that affected either of these. © The Author 2007. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved.