Macaque Fertility Signals

Abstract

Females of many animal species exhibit signals of fertility, but few animal groups do so with such prevalence, diversity of signal types, and variability between closely related species, as anthropoid primates. An excellent group for studying the evolution of such signals is the macaques, a group of some 22 species of Old World monkey. The macaques exhibit the same general mating system, but a great amount of inter‐specific variation in the type and number of different signals they display. In this article, we discuss the evolution of these traits, including the types of variation in fertility that they may indicate and the hormonal mechanisms that underlie them. We discuss the different signals that macaques exhibit, and review the distribution and abundance of these signals across species. Even closely related species differ in the type and number of signals displayed, as well as in their apparent accuracy in reflecting the timing of female ovulation. We offer some potential explanations for this variation, though much is still poorly understood. We finish by discussing future directions for research that are likely to prove fruitful. Key Concepts Females of many anthropoid primates, including macaques, exhibit signals of fertility. Macaque signals of fertility include swellings of the hindquarters, changes in facial colour, olfactory signals, and auditory oestrus and copulation calls. Three types of variation in fertility might potentially be communicated to others: intra‐cycle variation in the probability of ovulation; inter‐cycle, intra‐female variation in the probability of conception in that cycle, and inter‐female differences in overall fecundity. The expression of intra‐cycle signals of the probability of ovulation is linked to the hormones oestrogen (positively) and progesterone (negatively). There is a great deal of inter‐specific variation in the presence and type of fertility signals shown, as well as in their apparent accuracy in reflecting ovulatory function. Though we understand some of the evolutionary pressures that may have led to inter‐specific variation, the sources of much of the variation remain unclear.