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Ageing, infecundity and reproductive senescence in free-ranging female rhesus monkeys

The reproductive performance of 760 free-ranging female rhesus monkeys (Macaca mulatta), 168 of whom were 20 years of age and older at the time observations were begun, was assessed. The monkeys were resident on Raccoon Key or Key Lois, two islands located in the Florida Keys, USA. During 1992 and 1993, live birth rates generally declined with age among the Raccoon and Lois females aged eight years and older. This age-related deterioration of female fertility was the result of proportionately more younger females bearing live young during successive birth seasons, and proportionately more older females experiencing an inability to bear live offspring even after a barren year. It is suggested that (1) older females may be more strongly inhibited by the suckling stimulus than are their younger peers, and (2) the risk of a permanent loss of fecundity increases with each additional year of life or parturition. The live birth rates of females aged 16–24 years were greater on Raccoon Key than they were on Key Lois, because the Raccoon females within this age range were more successful at bearing live offspring during successive birth seasons; the reason for this difference could not be determined. Inter-population differences in both the body condition of the females and the severity of female–female competition for access to males were not considered to be plausible explanations. It is possible that the difference in female fertility between the islands is the result of the greater age of the adult males on Key Lois, or the phytochemicals eaten by the females on Raccoon Key.