Does co-occurrence drive vertical niche partitioning in parapatric tamarins (Saguinus spp.)?

Stable species coexistence is a result of a balance between niche and fitness differences. One of the most common competitive patterns that arise from interactions between closely related species is character displacement, which may lead to divergence of certain traits and niche partitioning in areas where species co-occur. In this study, we used data on 35 groups of two parapatric tamarins (Saguinus midas and S. bicolor) to test for asymmetries in group size and competitively driven divergence in forest strata use, under the influence of resource availability fluctuation across the year. We hypothesised that S. midas is competitively superior to S. bicolor, and this would result from differences in fitness-related traits (group size). If so, species patterns of vertical habitat across the year should shift in sympatry relative to allopatry. We found that species had similar group sizes, suggesting they have similar competitive abilities. Further, both pied and red-handed tamarins used lower heights whenever in larger groups, which may be related to a diet shift in response to increases in intragroup competition. In addition, we refuted our prediction of vertical niche partitioning in sympatry. Instead, S. midas moved upwards into the canopy with increasing rainfall over the year (a proxy for resource availability) both in sympatry and allopatry, whereas S. bicolor was unresponsive to both rainfall and geographic contact with S. midas groups. Therefore, the difference in vertical habitat use between these species over part of the year is probably more related to the degree of resource seasonality experienced by each species within their ranges, than to competition.