Exaggerated sexual swellings in female nonhuman primates are reliable signals of female fertility and body condition

In some species of Old World monkeys and apes, females exhibit exaggerated swellings of the anogenital region that vary in size across the ovarian cycle. Exaggerated swellings are typically largest around the time of ovulation, and swelling size has been reported to correlate positively with female quality, supporting the hypothesis that exaggerated swellings are honest signals of both female fecundity and quality. However, the relationship between swelling size and timing of ovulation is weak in some studies, and the relationship between swelling size and female quality has also not been consistently reported. Here, we collated empirical studies that have reported either swelling size and estimated timing of ovulation (N = 26) or swelling size and measures of individual quality (N = 7), to assess the strength of these relationships using meta-analytical methods. Our analyses confirmed that the period of maximal swelling size is closely associated with the most fertile period of the ovarian cycle and that a large proportion of ovulations occur during the maximal swelling period. A small, positive effect size was also found for the relationship between swelling size and body condition. In contrast, the relationships with age and social rank were not significant. Swelling size, therefore, potentially signals both female condition and timing of the fertile phase. Males are likely to benefit from allocating mating effort according to swelling size, while females with large swellings potentially benefit from exerting control over matings in species in which female control is compromised by male mating strategies.