EvoDevo in owl ear asymmetry—The little owl (Athene noctua)

Abstract

Owls are well adapted to nocturnal hunting. This includes vision tuned to low level light conditions, silent flight, and asymmetrical ears. Asymmetrical ears facilitate sound localization and evolved up to seven times independently in the evolutionary history of owls. However, there are also owl species with a crepuscular or diurnal lifestyle, like the little owl (Athene noctua), that have symmetrical ears as adults. Here we show that a small, but significant ear asymmetry occurs in the embryonic development of little owls, despite the presence of symmetrical ears in adults. In the asymmetric period, the left ear opening is bigger in area than its counterpart on the right. The asymmetry in the little owl occurs in the same stages at which the asymmetry in the barn owl (Tyto furcata) develops, but in the little owl the asymmetry vanishes shortly before hatching. Asymmetries in the size of the ear openings are also found in the adults of other owl species, most of them belonging to the genus Strix. We interpret our finding as an indication of a secondarily evolved diurnal activity in little owls. Further, ear asymmetry might be more deeply rooted in the evolution of owls than previously assumed.