HALL, MI; Stony Brook University, New York: Anatomical Correlates of Activity Pattern in Birds

Activity pattern often involves extremes of light availability to which visually dependent animals must adapt. There is evidence that dependence on photopic (plentiful light) and scotopic (low light) conditions markedly impact the gross anatomy of the eye and orbit. However, it is unclear whether this trend is more dependent on phylogeny or on the objective conditions set by the physics of light. To test this, I analyzed measurements of eye and orbit size and shape in birds over a range of activity patterns. This sample includes taxa that are closely related phylogenetically as well as those that are more distantly related but share similar ecologies and/or activity patterns. Study groups include representative species of Strigiformes, Caprimulgiformes, Apodiformes, and Falconiformes. The results show consistent differences between photopic and scotopic eyes in birds. Photopic eyes have enlarged posterior chambers that allow larger retinal area, maximizing the number of photoreceptor cells and thereby increasing visual acuity. Scotopic eyes tend to have enlarged anterior chambers to maximize light particle collection, thereby increasing sensitivity. The needs of acuity and sensitivity can not be met well simultaneously. However, scotopic eyes are often absolutely larger than photopic eyes regardless of body size, possibly in order to heighten sensitivity while maintaining as much acuity as possible. Preliminary orbit analyses show similar patterns to the eyeball data, but with lower correlation coefficients. The current data set suggests that activity pattern is more important than phylogenetic affiliation in determining eyeball and orbit size and shape.