Growth and mortality in mountain hares: the effect of sex and date of birth.

Current theory suggests that high juvenile growth effort, may result in higher mortality. This prediction is tested in mountain hares (Lepus timidus), by examination of post-weaning growth and mortality of males and females. Dates of birth were estimated from weight at first capture and growth in body weight and hind foot length was described by the logistic growth equation. Although adult female hares are on average larger than males, this results from a longer period of growth and not from a faster growth rate. There was no clear sex bias in mortality, a slight but not significantly greater proportion of males suffered mortality during the growth period. Predictions as to sex difference in mortality should specify which aspects of growth (rate or duration) are the agents of mortality, since the two parameters are not necessarily positively associated. Overall asymptotic body size decreased and rate of growth increased from early through to late-born young. Rate of growth in hind foot length was greater in late-born males but not in late-born females as compared with those born earlier. It is hypothesized that late-born males are more tightly constrained to complete growth and subsequent sexual maturation earlier than late-born females.