Body mass and individual fitness in female ungulates: bigger is not alwaysbetter

In female vertebrates, differences in fatness often correspond to differenc es in phenotypic quality, suggesting that larger females have greater fitne ss. Variation in individual fitness can result from variation in life span and/or variation in yearly reproductive success, but no study has yet asses sed the relationships between the components of fitness and phenotypic qual ity while controlling for life span. We tried to fill this gap using data f rom long-term monitoring (23 years) of marked roe deer and bighorn sheep, t wo ungulates with very different life histories. In both species, we found a strong positive relationship between an adult female's mass and her proba bility of reaching old age: over the long term, bigger is indeed better for ungulate females. On the other hand, we found no evidence in either specie s that heavier females had higher fitness when differences in life span wer e accounted for: over the short term, bigger is not necessarily better. Our results indicate that, while broad differences in phenotypic quality affec t individual fitness, when differences, in life span are accounted for phen otypic quality has no residual effect on fitness. Therefore, within a given range of phenotypic quality, bigger is not always better, for reasons whic h may differ between species.