Facultative sex allocation in snow skink lizards (Niveoscincus microlepidotus)

Mathematical models suggest that reproducing females may benefit by faculta tively adjusting their relative investment into sons vs, daughters, in resp onse to population-wide shifts in operational sex ratio (OSR). Our field st udies on viviparous alpine skinks (Niveoscincus microlepidotus) document su ch a ease, whereby among- and within-year shifts in OSR mere followed by sh ifts in sex allocation. When adult males were relatively scarce, females pr oduced male-biased litters and larger sons than daughters. The reverse was true when adult males were relatively more common. That is, females that we re courted and mated by few males produced mainly sons (and these were larg er than daughters), whereas females that were courted and mated by many mal es produced mainly daughters (and these were larger than sons). Maternal bo dy size and condition also covaried with sex allocation, and the shifting p attern of sexual size dimorphism at birth may reflect these correlated effe cts rather than a discrete component of an evolved sex-allocation strategy.