Cryptic sexual dimorphism reveals differing selection pressures on continental islands

AbstractBirds are well known for their sexual dimorphism. But not all forms of dimorphism are the same, and differences in morphology can be so subtle that they are not detected by casual observation. We report that this is the case with the Sulawesi Babbler (Pellorneum celebense), the first reported instance of sexual dimorphism in this species or any of the ground babblers of the South‐East Asian islands. Our finding is based on a combination of morphometric analyses, genetic sexing, and observation of breeding condition. We highlight the utility of unsupervised clustering approaches, widely used in the biomedical literature, for the investigation of sexual dimorphism in ecological and evolutionary contexts. The sexual dimorphism was weaker on the mainland of Sulawesi and stronger on the continental islands of Kabaena, Muna, and Buton. This suggests that different evolutionary pressures have led the species to partition niches differently in these habitats, which separated only recently in geological history. This kind of intraspecific niche partitioning is an intrinsic part of the ecological niche of such species, one we must not miss if we are to fully understand these endlessly fascinating organisms and systems.Abstract in Bahasa Indonesian is available with online material