The effect of initial weight of the ewe on later reproductive effort in domestic sheep (Ovis aries

In ungulates, females typically need to reach a critical threshold body weight in order to reproduce. Females born in `poor ' years may lose 1 year of reproduction, as they reach the critical body mass 1 year later. Some studies report also a lasting effect of low initial body weight into prime age, while several other studies have shown that nearly all prime-age females ovulate or are pregnant. However, the quality of the offspring (as measured by offspring body weight) is often not considered. We tested whether female ungulates with an initially low body weight (at around weaning) reach the same reproductive output as females with higher initial weights, and thus whether or not they are able to compensate for their initial low weight. Data on body weight of 123 533 lambs of domestic sheep Ovis aries from 73 299 litters that in turn derived from 32 359 different ewes were used. All ewes and lambs had been free-ranging on outlying pastures in Norway during the entire summer season. Ewes with an initially low body weight produced smaller offspring throughout their lifespan. Ewes with an initially low body weight also produced fewer lambs at ®rst and second parturition, but lifetime number of offspring was not related to ewes ' initial weight. We concluded that bigger was better: the smaller offspring of ewes with a low initial weight demonstrate a lasting cost on reproductive effort, with consequences for animal production systems and management of wild ungulates. Key words: cohort effects, life history, litter size, Ovis arie