Primate sexual swellings as coevolved signal systems

Many female catarrhine primates possess visually conspicuous organs that ap parently function to increase the sexual interest of adult male conspecific s around the time the female is ovulating - i.e. sexual swellings. The hypo thesized functional benefits for both sexes of these sexual swellings are r eviewed (honest signaling; paternity confusion; paternity confidence and pa ternal investment; protection; incitement of precopulatory male-male compet ition; and postcopulatory sexual selection), as well as an additional hypot hesis that has not yet been applied to this problem (sensory exploitation). Currently available evidence is presented that supports or fails to suppor t each of these hypotheses. Predictions associated with broad groupings of these hypotheses, which could be tested in noninvasive field studies, are t hen presented. Ecological circumstances are discussed that could have led t o differential mating success among female primates, and hence to sexual se lection on females and directional evolution of sexual swellings. It is con cluded that the available evidence does not support the paternity confidenc e-paternal investment hypothesis; that the paternity confusion hypothesis l acks empirical support, but could still be viable; and that insufficient da ta exists at present to rigorously test the other hypotheses. The ecologica l factors that may have led to differential reproductive success among fema les as a function of mating frequency or mate choice likewise require furth er empirical investigation.