Nonlinear and population-specific offspring sex ratios in relation to high variation in prey abundance
Résumé
Montagu's harrier (Circus pygargus ), a specialist raptor in western France, faces huge variations in the abundance of its main prey, the common vole (Microtus arvalis ). This simple predator/prey system provides fine-tuned data for investigating patterns of sex allocation under environmental variability. We analysed variations in brood sex ratio at fledging (1364 chicks from 451 broods) in two adjacent harrier populations for respectively 16- and 8-year surveys of both the predator and its prey. Overall sex ratio (number of males/total number of offspring) was close to parity (0.513) but this apparent equilibrium resulted in fact from opposite skews in the two populations which differed by almost 10% (0.561 vs 0.464). Brood reduction only is unlikely to produce such a difference in average brood sex ratios. Brood sex ratio of the predator was affected by prey abundance. Contrary to expectations and whatever the population, relatively more offspring of the smaller sex were produced during peak years of the vole cycle but also during poor years, thus providing the first evidence for a nonlinear influence of environmental quality (prey abundance) on sex ratio. Assuming that observed sex ratios in both populations are the result of an adaptive strategy that maximizes harrier reproductive output, we discuss possible origins of the bias and why nonlinearity may be involved in sex ratio adjustment. These results further point out that sex ratio analyses should take into account population characteristics, and more generally, environmental variations both in space and time.