Sex-ratio variation and reproductive costs in relation to density in a forest-dwelling population of red deer (Cervus elaphus)

For dimorphic species in which the variance in reproductive success of males is more pronounced than that of females, theories of adaptive variation in sex ratio predict that mothers should invest more heavily in sons than in daughters. By using harvest data from a forest-dwelling red deer population that experienced a marked reduction in population density we tested the hypothesis that adaptive sex-ratio variation should occur only when populations are much below carrying capacity. More specifically, we tested whether at low density, females in better than average condition were more likely to produce male offspring and to invest in individual sons rather than were females in poorer than average condition. We also investigated female reproductive costs arising from a decrease either in body mass or in reproduction. We did not find any support for a biased sex ratio or investment toward male calves by high-quality mothers at any population density. Costs of reproduction in terms of body mass and pregnancy rates were only detectable for females that reproduced as yearlings and not for those that reproduced as adults. Our results therefore do not support the hypothesis of adaptive sex-ratio variation in a population living below carrying capacity. The four-fold difference in party sizes (defined as the number of deer aggregate in the same party in which no individual was more than 50 m from any other) observed in our population living in a closed forest habitat compared with populations living in more open habitats previously studied might account for such a discrepancy. We suggest that a smaller party size may decrease the intensity of sexual selection and could be the proximal cause for the lack of adaptive sex-ratio variation we report for the population studied here.