Summary: While & Wapstra (2008)

Hatching asynchrony, the time span between hatching of the first and last egg, potentially confers a fitness benefit to offspring and / or parents and decoupling hatching asynchrony hypotheses from those related to onset of parental incubation is important. While & Wapstra (2008) experimentally tested the adaptive nature of hatching asynchrony in a non avian species, Egernia whitii and examined whether there are fitness advantages to offspring in asynchronous birthing. They used arginine vasopressin to manipulate birthing synchrony in female lizards and examined offspring growth and survival. Offspring from asynchronous treatments suffered from higher mortality, but had increased mass at 6 weeks. Offspring from asynchronous clutches also had a higher growth rate. Differences were driven by offspring mass at birth and the development of a greater mass hierarchy in asynchronous litters. It is thought that mortality in asyncrhonous litters was a function of increased aggression between conspecifics, resulting in decreased basking and feeding rates, loss of condition and eventual starvation by one individual. While & Wapstra (2008) found that birthing asynchrony in E. whitii appears removed from the constraints on the production of follicles (asynchronously), ovulation and embryonic development as asynchrony is observed at first at birth. Females of this lizard species exert considerable control over asyncrhonous birthing, suggesting that asynchrony in E. whitii is not simply the result of synchronous developmental constraints. Their results suggest that hatching asynchrony represents a trade-off between the associated costs and benefits.