Cryptic sexual dimorphism reveals differing selection pressures on continental islands - Sulawesi Babbler (Pellorneum celebense) Morphometric Dataset

Birds 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 aren’t 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 Southeast 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.

Methods
Birds were sampled by mist netting during expeditions to southeast Sulawesi between 1999 and 2017. We captured babblers from the P. c. rufofuscum populations on the southeast peninsula of Sulawesi, and on the smaller islands of Kabaena, Muna, and Buton. Captured birds were measured using the methods recommended by the Ringer's Manual (Redfern and Clark 2001) to obtain morphometric data including wing length (maximum chord), bill length (tip of bill to the base), skull length (from back of skull to base of bill), weight, and tarsus length (minimum). Only adult birds measured by NMM were included in morphometric analyses.