Recent evolution of extreme sexual dimorphism in the huia (Heteralocha acutirostris; Callaeidae)

The New Zealand wattlebirds (Callaeidae) are an endemic New Zealand passerine family whose species show extreme variation in bill morphology. In particular, the extinct huia (Heteralocha acutirostris) has attracted considerable attention because it exhibited extreme sexual dimorphism in bill morphology. However, the phylogenetic relationships within the Callaeidae, crucial for understanding bill evolution in the family, have not been resolved to date. Here we present phylogenies based on complete mitochondrial genome sequences and nuclear ultraconserved elements. Ko over bar kako (Callaeas spp.) is strongly supported as sister taxon to saddleback/ti over bar eke (Philesturnus spp.) and huia, diverging around 6.8 Ma. Saddleback and huia are estimated to have split from each other 5 Ma, indicating that the extreme sexual bill dimorphism in huia has evolved within this time frame. Our estimates for the divergences within the Callaetidae are similar to, or younger than, those of most other endemic New Zealand avian families, therefore the observed bill variation is not a consequence of a longer divergence time. Instead, the expansion of the huia into the wood-foraging niche, combined with the sexual dimorphism it evolved in order to optimise feeding on this resource, has been the main contributor to the large variation of bill morphologies within this family.

Automated summary

The New Zealand wattlebirds, especially the extinct huia, exhibit extreme variation in bill morphology. Phylogenetic analyses based on genetic information reveal that saddleback and huia diverged around 5 million years ago, and the extreme sexual dimorphism in huia evolved within this time frame. This suggests that the expansion of huia in the wood-foraging niche and the resulting sexual dimorphism were the main factors contributing to the large variation in bill morphologies within this family.