Comparative Anatomy Supports the Evolution of Nocturnality in the Extinct Hawaiian Ibis Apteribis

Evolution on islands often generates specialized lifestyles that are rarely seen in continental species. The biota on oceanic islands are, however, prone to extinctions following human colonization, resulting in an incomplete understanding of the lifestyles of species that evolved prior to colonization. For example, the Hawaiian Islands hosted a unique and diverse assemblage of endemic taxa, most of which became extinct following human colonization. Among these is Apteribis (Threskiornitidae), an extinct genus of flightless ibises for which nothing is known of their behaviour and ecology. To gain insight into the foraging behaviour and activity pattern of this unusual genus, we quantified their olfactory, visual, and somatosensory systems from direct measurements of skulls, CT scans, and endocasts. We then compared Apteribis with extant ibises with phylogeny-informed statistics to determine if they differed significantly in any of our measured traits. Our analyses show that the olfactory and somatosensory systems of Apteribis are comparable in size and anatomy to those of extant ibises and it was likely flexible in terms of preferred foraging habitat. In contrast, the visual system of Apteribis is greatly reduced in size, suggesting a nocturnal lifestyle, which is an unprecedent trait among ibises. Our data therefore suggests that Apteribis occupied a niche similar to that of New Zealand kiwi (Apteryx): nocturnal, flightless species that rely on tactile cues from its beak to detect prey. This study provides the first quantitative evidence for the evolution of a kiwi-like niche for a bird outside New Zealand, and emphasizes the remarkable diversity of avian lifestyles lost due to anthropogenic impact.