Meeting Abstract

15.5  Saturday, Jan. 4 11:30  The effect of evolutionary transitions to diurnality on scleral ring and orbit morphology in birds NIEDERMEYER, P/M*; SHIH, C; HALLEY, M/A; OSBORN, J/L; SCHMITZ, L; Pitzer College niedermeyer1023@gmail.com

Diel activity pattern is considered one of the major axes of the morphological and functional diversity of vertebrate eyes. When examined in a broad comparative framework, the eye shape of diurnal, cathemeral, and nocturnal vertebrates corresponds well to predictions from physiological optics, with nocturnal species having the best ability to see in dim-light conditions. However, it has been suggested that evolutionary transitions to diurnality do not prompt a morphological response that correlates with lower light sensitivity, possibly reflecting decreased strength of selection. We investigated the effect of transitions to diurnality on skeletal visual morphology in birds, focusing on Strigiformes (48 species) and Caprimulgiformes (59 species, including Apodiformes). We collected linear measurements of scleral ring and orbit morphology that are closely correlated with visual light sensitivity: orbit length, a proxy for eye size; the optical ratio, an indicator of ocular image formation; and the optic nerve foramen index, considered a measure for retinal summation. We performed all analyses in a detailed phylogenetic framework, accounting for uncertainty in reconstructions by repeating the analyses over a sample of 1000 trees. While orbit length and the optic nerve foramen index show only little variation, the optical ratio is decreased in diurnal species, indicating lower light sensitivity. Our results show that evolutionary transitions to diurnality in birds are associated with changes in skeletal visual morphology, at least in parts of this functional system. It is possible that uncoupled evolution of visual performance features is a potential source of morphological diversity.