Sexual selection, seminal coagulation and copulatory plug formation in primates

This study examines the question of whether multipartner matings by female primates, with resulting sperm competition among males, may have favored the evolution of biochemical mechanisms to enhance seminal coagulationand copulatory plug formation. Comparative ratings of seminal coagulation (using a four-point scale where 1 = no coagulation and 4 = copulatory plug formation) were obtained for 40 species representing 26 primate genera. Coagulation ratings were highest (mean = 3.64) in those genera where females commonly mate with multiple partners, and lowest (mean = 2.09) in genera where females are primarily monogamous or belong to polygynous (one male) units(p < 0.0001). This result remained significant (p < 0.001) after the use of comparative analysis of independent contrasts (CAIC) to control for possible phylogenetic biases in the data set. Results indicate that sexual selection has played an important role in the evolution of seminal coagulation, and copulatory plug function, in primates.