Facultative sex allocation in snow skink lizards (Niveoscincus microlepidotus)

Mathematical models suggest that reproducing females may benefit by facultatively adjusting their relative investment into sons vs. daughters, in response to population-wide shifts in operational sex ratio (OSR). Our field studies on viviparous alpine skinks (Niveoscincus microlepidotus) document such a case, whereby among- and within-year shifts in OSR were followed by shifts in sex allocation. When adult males were relatively scarce, females produced male-biased litters and larger sons than daughters. The reverse was true when adult males were relatively more common. That is, females that were courted and mated by few males produced mainly sons (and these were larger than daughters), whereas females that were courted and mated by many males produced mainly daughters (and these were larger than sons). Maternal body size and condition also covaried with sex allocation, and the shifting pattern of sexual size dimorphism at birth may reflect these correlated effects rather than a discrete component of an evolved sex-allocation strategy.