The relationship between concealed ovulation and mating systems in anthropoid primates: a phylogenetic analysis.

Under the assumption that the absence of visual signs of ovulation (usually termed "concealed ovulation") is an adaptation in primates, there are two main hypotheses concerning the evolution of this trait: it is a means to promote paternal care and increase paternity confidence, and it is a means to confuse paternity issues and thereby improve male behavior toward (potential) offspring. The first hypothesis assumes that ovulatory signs have been lost under monogamy, and the second assumes a one- or multimale mating system. Data on mating systems and signs of ovulation, respectively, for extant taxa were parsimoniously mapped into phylogenetic trees that had been independently derived from systematic studies. The positions of change in ovulatory signs were then related to mating system in the phylogenies. Slight signs of ovulation and a multimale mating system, respectively, were inferred to be ancestral for anthropoid primates. Ovulatory signs were estimated to have disappeared 0-1 times under monogamy and 8-11 times in a nonmonogamous context. Thus, the second hypothesis receives the most support concerning the ultimate reason for the loss of ovulatory signs. On the other hand, monogamy was inferred to have evolved independently seven times in the anthropoid primates, four to six times in the absence of ovulatory signs, and one to three times in the presence of ovulatory signs. Thus, there is a linkage between absence of ovulatory signs and monogamy, but the temporal relationship is generally such that the lack of ovulatory signs is more likely to promote monogamy than monogamy is to promote a lack of ovulatory signs. These results imply that the function of the absence of ovulatory signs may have changed in lineages in which monogamy has evolved.