Optic foramen morphology and activity pattern in birds

The optic nerve is the sole output of visual information from the ganglion cell layer of the retina to the brain in vertebrates. The size of the optic nerve is predicted to be closely associated with activity pattern, and, in many birds, the size of the optic foramen approximates the size of the optic nerve. Specifically, nocturnal species should have relatively smaller optic foramina than diurnal species because of differences in retinal pooling between activity patterns. If optic foramen morphology varies predictably with activity pattern in birds, this variable may be useful for interpreting activity pattern for birds that do not have soft tissue available for study, specifically for fossils. Across 177 families (from 27 orders), we describe four different optic foramen morphologies, only one of which corresponds well with the size of the optic nerve and is therefore appropriate for activity pattern analyses. Here, we test our hypothesis that nocturnal species will have relatively smaller optic foramina than diurnal species, across all species that we measured that have a discrete optic foramen. Regression analyses using species as independent data points and using comparative methods yielded significant differences in optic foramen size between nocturnal and diurnal species relative to three variables: head length, orbit depth, and sclerotic ring inner diameter. Nocturnal species consistently exhibit significantly smaller relative optic foramen diameters than diurnal species. Our results indicate that optic foramen diameter, in combination with either the sclerotic ring or the orbit diameter, can be used to predict activity pattern.