Phenotypic quality influences fertility in Gombe chimpanzees

Fertility is an important fitness component, but is difficult to measure in slowly reproducing, long-lived animals such as chimpanzees (Pan troglodytes). We measured fertility and the effect of measured covariates on fertility in a 43-year sample of birth intervals of chimpanzees from the Gombe National Park, Tanzania using Cox proportional hazards regression with individual-level random effects. The birth hazard declined with mothers' age at a rate of 0.84 per year following age at first reproduction. This value is somewhat stronger than previous estimates. Loss of the infant that opened the birth interval increased the birth hazard 134-fold. Birth intervals following the first complete birth interval were shorter than this first interval, while sex of the previous infant had no significant effect. Maternal dominance rank was significant at the P < 0.1 level when coded as high/middle/low but was highly significant when we simply considered high rank vs. others. Individual heterogeneity had a substantial impact on birth interval duration. We interpret this individual effect as a measure of phenotypic quality, controlling for the measured covariates such as dominance rank. This interpretation is supported by the correlation of individual heterogeneity scores with similar independent measures of body mass.