Convergent character displacement in sympatric tamarin calls (Saguinus spp.)

Character displacement, or a shift in traits where species co-occur, is one of the most common ecological patterns to result from interactions between closely related species. Usually, character displacement is associated to divergence in traits, though, they might be convergent, especially when used for aggressive interference between species. In the context of animal communication, territorial calls are predicted to converge in order to increase context recognition and decrease the costs of ecological interference competition. However, such signals might also be adapted to characteristics of the shared environment. In this study, we used data from 15 groups of two parapatric tamarins, Saguinus midas and S. bicolor , to test for similarities in long calls among sympatric and allopatric groups. We hypothesized that calls would converge in sympatric areas, as it would be mutually beneficial if both species recognize territorial contexts, but that convergence would depend on forest type due to acoustic adaptation. As predicted, long calls converged in sympatry, with S. midas shifting its calls towards S. bicolor’ s acoustic pattern. However, this shift only occurred in primary forest. In sympatric areas, S. midas produced sounds with narrower bandwidths in primary than in secondary forest, consistent with optimization of sound propagation while both species produced longer calls in primary forests independently of geographic location (i.e. sympatry and allopatry). Our results suggest that both social and environmental pressures are important in shaping tamarin sounds. As their effects can interact, analyses, which assume that these ecological pressures act independently, are likely to miss important patterns. Significance statement Territorial signals between closely related sympatric species are expected to be convergent to increase context recognition and decrease the costs of interference competition. However, such signals might also be adapted to characteristics of the shared environment, such as forest structure characteristics. We analysed vocalisations from two parapatric tamarins and found that, though their long calls asymmetrically converged in sympatry, only red-handed tamarins shifted towards pied tamarin call type, and this only occurred in primary forest. Our results suggest that both social and environmental pressures are important in shaping primate calls. Because their effects can interact, analyses which assume that these pressures act inde