Fitness and hormonal correlates of social and ecological stressors of female yellow-bellied marmots

The effects of social and ecological stressors on female reproductive success vary among species and, in mammals, previous reviews have identified no clear patterns. However, few studies have simultaneously examined the relation between social rank and stressors and the relationships among rank, stressors and reproductive success. We used a long-term data set to study free-living facultatively social yellow-bellied marmots, Marmota flaviventris, to isolate the relationship between female social dominance rank and faecal glucocorticoid metabolite (FGM) levels (our measure of basal stress) in adult females. In addition, we examined whether rank and FGM levels were associated with reproductive success by quantifying the probability of an individual successfully weaning a litter and, for those who did, litter size. High-ranking females had lower FGM levels and larger litters. However, females with the highest FGM levels were significantly more likely to wean a litter. Importantly, body condition (as measured by previous year's mass) was also positively associated with breeding and with weaning larger litters. Thus, although low-ranking females probably experienced more social stressors than high-ranking females and although adult females often delayed their first reproduction until they were of a sufficient mass, our results suggest that elevated baseline FGM levels failed to mediate reproductive suppression. Perhaps, in species like marmots that have only a single chance per year to reproduce, reproductive suppression should be rare. These results highlight the importance of social status, body condition and predator abundance on determining reproductive success in highly seasonal breeders.