Variance in the reproductive success of dominant male mountain gorillas

Using 30 years of demographic data from 15 groups, this study estimates how haremsize, female fertility, and offspringsurvival may contribute to variance in the siringrates of dominant male mountain gorillasthroughout the Virunga Volcano Region. As predicted for polygynous species, differences in haremsize were the greatest source of variance in the siringrate, whereas differences in female fertility and offspringsurvival were relatively minor. Haremsize was positively correlated with offspringsurvival, even after removing all known and suspected cases of infanticide, so the correlation does not seem to reflect differences in the ability of males to protect their offspring. Haremsize was not significantly correlated with female fertility, which is consistent with the hypothesis that mountain gorillashave minimal feeding competition. Haremsize, offspringsurvival, and siringrates were not significantly correlated with the proportion of dominant tenures that occurred in multimale groups versus one-male groups; even though infanticide is less likely when those tenures end in multimale groups than one-male groups. In contrast with the relatively small contribution of offspringsurvival to variance in the siringrates of this study, offspringsurvival is a major source of variance in the male reproductive successof western gorillas, which have greater predation risks and significantly higher rates of infanticide. If differences in offspringprotection are less important among male mountain gorillasthan western gorillas, then the relative importance of other factors may be greater for mountain gorillas. Thus, our study illustrates how variance in male reproductive successand its components can differ between closely related species.