Morphological variation within a macaque hybrid zone

A hybrid zone exists between Macaca tonkeana and Macaca hecki (Primates: Cercopithecidae), centered along the Tawaeli-Toboli road in the narrow isthmus that connects North and Central Sulawesi, Indonesia. The current study demonstrates morphological substructure from north to south across the hybrid zone. Macaques to the northwest of the Tawaeli-Toboli road more closely resemble M. hecki, and macaques to the southeast of the road resemble M. tonkeana. While morphology shifted for both males and females over a distance of 1,500-2,000 m, adult males were significantly more M. tonkeana-like across the morphological gradient. This suggests that in the study area, males of M. tonkeana-like morphology are dispersing into hybrid groups at the expense of M. hecki-like males. A permutation analysis of diagnostic character states indicated that associations existed among several morphological traits. This could be due to the operation of one or several nonexclusive evolutionary processes, including recent secondary contact, pleiotropic effects, physical linkage of loci, natural selection against hybrids, the influx of parental types, or assortative mating. Continued environmental perturbation associated with the Tawaeli-Toboli road is likely to be a significant factor in the future of the M. tonkeana/M. hecki hybrid interaction.