A heterochronic shift in skeletal development in the barn owl (Tyto furcata): A description of the ocular skeleton and tubular eye shape formation

Background The eyes of some birds of prey (Strigiformes and some eagles) are tubular in shape, which contrasts strongly with those in others, which are more globose (e.g., Galliformes) or flat (most diurnal birds). Regardless, all birds have an ocular skeleton composed of a ring of ossicles (annulus ossicularis sclerae) and a cartilage cup within the sclera. Results We show that the tubular eye of the barn owl, Tyto furcata, grows substantially in length to achieve its long axial length several weeks after hatching, well after the period when the visual input adjusts the optical system and when the scleral ossicles mineralize. This is in contrast to the chicken. The conjunctival papillae are morphologically different in each species, however, they are present for about 3 days in both birds before they degenerate. Conclusions Our data shows a heterochronic shift in the timing of scleral cartilage development and ossicle mineralization (but not induction) to later in development compared to in the chicken. These shifts likely relate to the altricial vs precocial nature of these birds and suggests that the scleral ossicles are likely functionally important bones for vision in owls and possibly other altricial species.