This dissertation examines the behavioral ecology of hybrids at the species boundary of white-collared lemurs (Eulemur albocollaris) and rufous lemurs (E. fulvus rufus) at Andringitra National Park, Madagascar. In addition, I present data on an allopatric population of E. albocollaris at Vevembe Forest and comparisons to previous studies of allopatric E. f. rufus. In particular, I investigate morphometrics, population densities, habitat structure and food availability, feeding ecology, ranging, and social behavior (including social structure and interactions with conspecifics). The goal of these analyses is to evaluate the contribution of ecological mechanisms in determining the directions and limitations of gene flow, in delimiting species boundaries, and in incipient speciation. The hybrids at Andringitra, E. albocollaris, and E. f. rufus vary considerably in testes volume and sexual dimorphism in canine height and body size. Resource defense may favor relatively large females in some E. f. rufus populations, while sperm competition is most pronounced in fission-fusion E. albocollaris. There are also considerable differences in abundance across brown lemur sites in southeastern Madagascar. The contact zone at Andringitra has the highest densities, while populations are most sparse in E. albocollaris sites. Population differences are partly explained by habitat quality. Vevembe has the lowest food availability, while the hybrid zone and E. f. rufus sites are similarly productive. Variation in habitats may also be reflected in feeding ecology; hybrids consume fruit almost exclusively, while E. albocollaris are dietary generalists and may employ fission-fusion to reduce competition. Moreover, Ficus species may serve as a keystone resource at Andringitra, allowing for larger hybrid densities. The southeastern brown lemurs also vary in intergroup behavior, with aggression rates in E. f. rufus double those of the hybrids or E. albocollaris. The highly productive environment at Andringitra may permit overlap of the parental species and highly introgressive hybridization. Dispersal and gene flow out of the contact zone may be limited by poor overall habitat quality and low population density to the south; increased intergroup agonism or, more likely, environmental characteristics (e.g., the lack of keystone resources) may be responsible for lower brown lemur abundance and gene flow in the north. I propose that these behavioral and ecological differences may serve as important mechanisms encouraging speciation in the southeastern brown lemurs.