The evolution of exaggerated sexual swellings in primates and the graded-signal hypothesis

Females of some Old World primate taxa advertise their sexual receptivity with exaggerated sexual swellings. Although a number of hypotheses have been proposed, the function of this conspicuous trait remains unsolved. This review updates information on the phylogenetic distribution of exaggerated swellings and identifies aspects of the morphology, physiology and behaviour of species with this conspicuous trait. Some of these patterns represent new information, while other patterns have been previously identified, but not in ways that control for phylogeny. This review shows that exaggerated swellings are correlated with some features that serve to confuse paternity certainty among males, while other features tend to bias paternity towards more dominant males. Hypotheses for the evolution of exaggerated swellings are then, reviewed and critically evaluated. Individually, no single hypothesis can account for all the patterns associated with exaggerated swellings; however, a combination of different hypotheses may explain the contradiction between confusing and biasing paternity. I suggest that exaggerated swellings can be viewed as distributions representing the probability of ovulation (the graded-signal hypothesis). In the context of this probabilistic model, exaggerated swellings enable females to manipulate male behaviour by altering the costs and benefits of mate guarding, so that dominant males tend to guard only at peak swelling, but females can mate with multiple males outside peak swelling to confuse paternity. This hypothesis makes testable predictions for future comparative and observational research. (C) 1999 The Association for the Study of Animal Behaviour.