Are howler monkey species ecologically equivalent? Trophic niche overlap in syntopic Alouatta guariba clamitans and Alouatta caraya.

According to the principle of competitive exclusion, niche differentiation allows the stable coexistence of closely related species. We analyzed dietary profile and diversity, and dietary overlap between syntopic brown howlers (BR; Alouatta guariba clamitans) and black and gold howlers (BLG; A. caraya) in the Atlantic Forest of NE Argentina, with the objective of evaluating the degree of trophic niche overlap and potential interspecific competition for food. During 12 months, we collected data on feeding behavior of two groups of each howler species using the scan sampling method, together with data on food availability. Both at the group- and species-level, we analyzed feeding behavior in terms of monthly percentages of time spent feeding on each food type and specific food item, dietary diversity (Shannon index H'), and we estimated dietary overlap using the percentage index and the Morisita-Horn index (C(H)). Across months, both howlers showed species-specific preferences for certain food items, and BLG had a more diverse diet (mean+/-SE, H'=2.77+/-0.08) than BR (H'=2.39+/-0.09). However, diets of both species overlapped extensively (percentage index=45.64+/-2.97%; C(H)=0.6+/-0.05) and diets of conspecific groups did not overlap more than diets of groups of different species. Given their high degree of trophic overlap, syntopic BR and BLG meet one of the conditions necessary for interspecific food competition to occur. Although at present we lack direct evidence for interspecific competition in these howler species, we conclude that high levels of niche overlap may have an important role in maintaining the essentially parapatric distribution of howler species throughout the Neotropics.