Reproductive costs of sons and daughters in Rocky Mountain bighorn sheep

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Abstract
Differential maternal investment theory predicts that in sexually dimorphic and polygynous species mothers should invest more in sons than in daughters. We tested the hypothesis that bighorn ewes that raise sons incur greater reproductive costs than ewes that raise daughters. Although ewe mass gain during lactation and subsequent winter body mass loss were independent of lamb sex, lambs born the year following the weaning of a son had lower survival than lambs born after a daughter. The effects of lamb sex on subsequent reproductive success of ewes became more evident at high population density. Lamb sex did not affect maternal survival. Population density, weather, and ewe age did not alter the relationship between lamb sex and subsequent reproductive success of the ewe. The year after weaning a son, ewes were more likely to have a daughter than a son, while ewes that had previously weaned a daughter had similar numbers of sons and daughters. Our results show that for bighorn sheep ewes, sons have a greater life-history cost than daughters, suggesting a differential maternal investment in the sexes.

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