Seasonality of Conceptions in Captive Rhesus Macaques (Macaca mulatta)

Environmental variables have an effect on patterns of seasonality in conceptions across many animal species. Rhesus macaques ( Macaca mulatta ) are well documented as seasonal breeders, and it has been proposed that rhesus macaques are “relaxed income breeders.” This means they should respond to changes in photoperiod but that endogenous cues can allow deviations from photoperiod-timed seasonality. Despite this, little is known about what factors cause seasonal reproduction in rhesus macaques. We present the results of a natural experiment on the influence of different housing conditions (featuring different levels of environmental exposure) on the seasonal pattern of reproduction in rhesus macaques. The data come from a captive colony of rhesus macaques, housed in a variety of conditions between 1970 and 1990: indoors with regulated temperature and photoperiod, in enclosures with an open window allowing for photoperiod and temperature variation, or in outdoor enclosures with even more available natural environmental exposure. Conceptions (measured by day-of-year) occurred throughout the year and in a seasonal pattern in all treatment conditions, but the seasonal pattern differed for each treatment. The variability in conception dates was highest for the outdoor groups. These results are consistent with rhesus macaques as “relaxed income breeders.” However, comparison with other macaque species for which similar data are available casts doubt on previous applications of the income and capital breeder model to the macaques.