Eye structure and foraging in King Penguins Aptenodytes patagonicus

Anterior eye structure and retinal visual fields were determined in King Penguins Aptenodytes patagonicus using keratometry and an ophthalmoscopic reflex technique. The cornea is relatively flat (radius 32.9 mm) and hence of low refractive power (10.2 dioptres in air) and this may be correlated with the amphibious nature of penguin vision. The large size of the eye and of the fully dilated pupil may be correlated with activity at low light levels. In air, the binocular field is long (vertical extent 180 degrees) and narrow (maximum width 29 degrees), with the bill placed approximately centrally - a topography found in a range of bird species which employ visual guidance of bill position when foraging. Upon immersion in water, the optical power of the cornea is abolished, with the effect that the monocular fields decrease and binocularity is lost. King Penguins have a pupil type which has not hitherto been recorded in birds. In daylight it contracts to a square-shaped pinhole but dilates to a large circular aperture in darkness. This change alters retinal illumination by 300-fold (2.5 log(10) units). When diving, this permits the retina to be pre-adapted to the low ambient light levels that the birds encounter upon reaching mesopelagic depths. These penguins also forage at depths where ambient light levels, even during the day, can fall below the equivalent of terrestrial starlight. Under these conditions, the birds must rely upon the detection of light from the photophores of their prey. In this they are aided by their absolutely large pupil size and broad cyclopean visual field.