Sexual swellings are unreliable signals of fertility in female bonobos

In several species of primates, males often discern when to mate with a female based on cyclical changes in the size and firmness of her sexual swelling – a visual signal of a female’s probability to conceive. In a study of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, primatologist Pamela Heidi Douglas and colleagues investigated for the first time the relationship between ovarian hormones and sexual swellings in wild female bonobos. The likelihood that a female bonobo ovulates during her maximum swelling phase is much lower than in the closely related chimpanzees. Swellings are thus no reliable fertility signal for males and allow females to follow their own agenda when choosing a mate.