Abstract
Purpose:
Compare the anatomy of avian ciliary muscles with particular attention to the order Strigiformes and the Tawny Frogmouth of the order Podargiformes and speculate how the anatomy might reflect the environmental and behavioral niche.
Methods:
Cases were selected from the archive of the Comparative Ocular Pathology Laboratory of Wisconsin (COPLOW). Five-micron H & E sections were examined. Crampton’s muscle and Brücke's muscle were identified. Length and area of the muscles were measured. We could not reliably identify Müller’s muscle.
Results:
In the majority of birds examined both muscles were present and no clear pattern of relative size could be detected. We measured length and area of Crampton’s and Brücke's in a variety of species and discovered that even within the same order there was wide variation. In four species of owls (Strigiformes, family Strigidae) examined, a Crampton’s muscle is evident but no Brücke's muscle. In contrast, a fifth owl species examined was the Barn owl (Tyta alba), which has a vestigial Brücke's muscle in addition to Crampton’s. Barn owls are in the family Tytonidae, while all other “typical” owls are in the family Strigidae. We also examined the Tawny Frogmouth (Podargus strigioides), a nocturnal raptor, of the order Podargiformes. It has a Crampton’s muscle as well as a vestigial Brücke's.
Conclusions:
Owls do not have a Brücke's muscle and, therefore lack lens accommodation, relying on corneal accommodation. Corneal accommodation may be accomplished by contraction of Crampton’s muscle, resulting in an increased curvature of the axial cornea. We speculate that the lack of Brücke's might be a consequence of the relatively round lens, relatively large globes, or a relatively large scleral ossicle. The Barn Owl and Frogmouth have a vestigial Brücke's. These birds are closely related to the true owls, are also nocturnal predators and also have a relatively round lens and a robust ossicle.
Keywords: 404 accommodation •
456 ciliary muscle •
754 visual acuity