Strange Tunes—Acoustic Variation and Character Displacement in a Tarsier Hybrid Zone.

Natural hybridization provides valuable insights into evolutionary processes, such as speciation and the forces driving or hindering it. Sulawesi tarsiers Tarsius dentatus and T. lariang hybridize within a limited area, suggesting selection against hybrids. Their species- and sex-specific duet songs might serve as a premating barrier in sympatry, especially if differences are strengthened by character displacement. Individuals of mixed origin might face disadvantages if they inherit intermediate song traits. To shed light on the processes shaping this hybrid zone, we analysed 55 duet songs from within and outside the zone. For females and males, we identified temporal and frequency-related parameters that differ between species. We inspected hybrid songs for intermediate characteristics and analysed purebred songs for character displacement in sympatry. Female hybrid songs (N = 2) were intermediate in four to five of six parameters; interpretation of male hybrid songs (N = 2) was inconclusive, because only two parameters were reliably quantifiable. There was no character displacement in female songs in sympatry (N = 11) compared with monospecific areas (N = 17). In male songs, interspecific differences in note rate were significantly larger within the hybrid zone (N = 8) compared with outside (N = 13). Intermediate song traits indicate inheritance and may disadvantage hybrids during mate choice. Character displacement in male songs is consistent with female mate choice, because females should opt for unmistakable signals to avoid costly hybridization. Our findings thus suggest that duet songs of T. lariang and T. dentatus play an important role in limiting this hybrid zone. [ABSTRACT FROM AUTHOR]