Genital swellings, or sexual swellings, are prominent reproductive characteristics observed in many female nonhuman primates, particularly in species where females mate with multiple partners. These swellings, which signal female fertility, are hormonally driven and occur during the early part of the female menstrual cycle. Their large size and bright coloration make them highly visible.
Genital swellings have developed separately in at least three primate groups, showing how similar traits can evolve independently in response to similar needs. In chimpanzees, bonobos, macaques, and baboons, for example, such swellings significantly influence male mating behavior.
Several hypotheses explain the evolutionary role of genital swellings. They may promote male competition, encouraging females to mate with the fittest males. They may create uncertainty around paternity, prompting greater investment from multiple males. Or they may accurately signal a female’s reproductive health and genetic quality.
