Heterogeneity of reproductive aging in free-ranging female rhesus macaques

We here examine interindividual variability in reproductive aging in free-ranging female rhesus macaques (Macaca mulatta). Our study site, Raccoon Key, is an island, about 81 hectares in area, situated in the Gulf of Mexico. Our data were collected from 1992 to 1998; the last 7 years a large population of provisioned macaques were resident on the island. We have previously demonstrated that the birth rate realized by the Raccoon Key rhesus in aggregate declined incrementally with female age beginning in the second half of the first decade of life. In this paper, we examine females aged 4-27 years that reproduce annually (annual producers) or, alternatively, fail to reproduce at all (nonproducers) over a period of 4 years. If all females within a given population experience an incremental loss of reproductive capacity with each year of age, the observed number of nonproducers would increase, and the number of producers would decrease with female age, but not above chance levels. However, within the Raccoon Key rhesus macaque population, both did occur at frequencies greater than expected by chance alone indicating there is strong heterogeneity in female fertility. We argue that a sudden catastrophic loss of fertility by a minority of females each year is most consistent with our data, and argue that this outcome is consistent with the evolutionary theory of aging.