Morphological Variation of Two Howler Monkey Species and their Genetically- Confirmed Hybrids.

Abstract

Hybridization challenges traditional species definitions, the most common being that a species comprises reproductively isolated individuals (Mayr, 1963). Although hybridization has been reported for several primate species, this dissertation is the first to investigate morphological variation in a Neotropical primate hybrid system. Two related howler monkey species, A. palliata and A. pigra, are known to hybridize in an area within Tabasco, Mexico. Using mitochondrial DNA, the SRY gene, and microsatellites, I identify hybrid individuals of different generations of crossbreeding and backcrossing to answer questions about hybrid morphology. What do hybrids look like when compared to purebred individuals? Is there a sex bias in the expression of hybrid morphology? I begin by comparing and contrasting the morphology of the two parent species and reporting how differences between them are shaped by differences in the extent of sexual selection. This will not only provide a basis for understanding the morphological variation present in hybrid individuals but also lay the groundwork for future research on the selective forces that hybrids are subject to. Therefore, the dissertation is comprised of three parts: 1) A review of the contributions of and the approaches used in the study of primate hybridization, 2) the impact of intra-sexual selection on sexual dimorphism and testes size in A. palliata and A. pigra, and 3) the morphology of hybrid versus purebred howler monkeys. My work sheds light on the range of variability in morphological expression when genetically distinctive populations crossbreed. It will also serve as a model for evaluating the issue of hybridization in the primate fossil record.