Climate and seasonal reproduction in the Cayo Santiago macaques.

This paper reports the results of an eight-year study of seasonal reproduction in the free-ranging colony of rhesus monkeys on Cayo Santiago, Puerto Rico. There was a significant correlation between the start of the annual spring rainy season and the estimated median conception date (r = .94, p < .001) and median birth date (r = .94, p < .001). Conceptions followed the commencement of spring rains by 70 days. Significant and negative correlations were found between the number of days from the start of the rains to the first conception (r = -.93, p < 0.01) and median conceptions (r = -.91, p < .01). When spring rains were delayed, the interval between the onset of rains and conceptions decreased linearly. The median birth date for the Cayo Santiago population was accurately predicted for 1984 using two methods. When results from Cayo Santiago were applied retrospectively to data from the colony of rhesus monkeys near La Parguera, Puerto Rico, it explained the three-month difference in seasonality of reproduction between the two populations located at the same latitude (18°N). Photoperiod, as a function of latitude, appears to set the temporal limits of seasonal reproduction in rhesus macaques, while the onset of spring rains regulates reproductive activity within that range, at least in the Cayo Santiago population. The biological channel through which the effect is mediated has not been identified.