Landscape Attributes Affecting the Natural Hybridization of Mexican Howler Monkeys

Abstract

Natural hybridization can be affected by genetic and environmental factors. For instance, genetic incompatibilities may impede the formation of hybrids, or cause infertility of hybrid offspring, whereas anthropogenic habitat disturbance can promote the contact between formerly isolated species, and therefore, increase the probabilities of hybrid formation. Although there are a number of studies addressing endogenous factors affecting hybridization, little is known about the effects of environmental factors, such as habitat fragmentation, on hybridization in animals. Here we evaluate whether habitat configuration in a fragmented landscape affects the process of natural hybridization between Mexican howler monkeys (Alouatta palliata and Alouatta pigra), and which attributes of the fragmented landscape, if any, are likely responsible for this effect. Based on the genetic and morphological identification of purebred and hybrid individuals, we assessed the relationship between several metrics of habitat configuration (number of habitat fragments, fragment size, fragment isolation, and fragment shape) and the demographic characteristics and occupancy patterns of groups in areas where exclusively purebred individuals occur and in areas of hybridization in Macuspana, Tabasco. Our results indicate that forest fragmentation is more severe where hybridization occurs, where there is a larger number of small, though less isolated, fragments. Additionally, there are differences in group size and composition between purebred groups in areas with just purebred animals and in areas where hybridization is occurring. In areas of hybridization, purebred groups tended occupy the largest fragments, whereas in the only area where groups of the two parental species and hybrids co-exist, groups that included hybrid individuals tended to occupy more isolated fragments than groups of purebreds. These results are congruent with the hypothesis that hybridization between Mexican howlers is facilitated in fragmented landscapes. Although we could not discover the mechanisms that underlie this hypothesis, it is possible that in a landscape with more fragments, which are also smaller but rather connected, individuals move more frequently between forest remnants, increasing the probabilities of interspecific encounters.