Impact of intrasexual selection on sexual dimorphism and testes size in the Mexican howler monkeys Alouatta palliata and A. pigra.

One of the goals of physical anthropology and primatology is to understand how primate social systems influence the evolution of sexually selected traits. Howler monkeys provide a good model for studying sexual selection due to differences in social systems between related species. Here, we examine data from the sister howler monkey species Alouatta palliata and A. pigra inhabiting southeastern Mexico and northern Guatemala. We use a resampling approach to analyze differences in sexual dimorphism of body and canine size. In addition, we compare testes size as a way of gauging the intensity of sperm competition in both species. Morphometric data were collected from wild-caught individuals, including body mass and length, and dental data were obtained from casts from wild individuals and from museum specimens. Although A. pigra individuals are larger than their A. palliata counterparts, we find that both species exhibit similar levels of sexual dimorphism for all of the variables considered. Testicular volume results indicate that A. palliata male testes are on average twice as large as those of A. pigra males, suggesting more intense sperm competition in the former species. Our study shows that A. pigra is not highly sexually dimorphic as was once thought, and testes size differences suggest the need for a clearer understanding of howler monkey social systems.