REPRODUCTIVE COSTS OF SONS AND DAUGHTERS IN ROCKY-MOUNTAIN BIGHORN SHEEP

Differential maternal investment theory predicts that in sexually dimo rphic and polygynous species mothers should invest more in sons than i n daughters. We tested the hypothesis that bighorn ewes that raise son s incur greater reproductive costs than ewes that raise daughters. Alt hough ewe mass gain during lactation and subsequent winter body mass l oss were independent of lamb sex, lambs born tile year following the w eaning of a son had lower survival than lambs born after a daughter. T he effects of lamb sex on subsequent reproductive success of ewes beca me more evident at high population density. Lamb sex did not affect ma ternal survival. Population density, weather, and ewe age did not alte r. the relationship between lamb sex and subsequent reproductive succe ss 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 da ughter had similar numbers of sons and daughters. Our results show tha t for bighorn sheep ewes, sons have a greater life-history cost than d aughters, suggesting a differential maternal investment in the sexes.