Eye structure and foraging in King Penguins Aptenodytes patagonicus

Anterior eye structure and retinal visual fields were determined in King Pe nguins Aptenodytes patagonicus using keratometry and an ophthalmoscopic ref lex 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 nar row (maximum width 29 degrees), with the bill placed approximately centrall y - a topography found in a range of bird species which employ visual guida nce of bill position when foraging. Upon immersion in water, the optical po wer of the cornea is abolished, with the effect that the monocular fields d ecrease 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 c hange alters retinal illumination by 300-fold (2.5 log(10) units). When div ing, this permits the retina to be pre-adapted to the low ambient light lev els that the birds encounter upon reaching mesopelagic depths. These pengui ns also forage at depths where ambient light levels, even during the day, c an fall below the equivalent of terrestrial starlight. Under these conditio ns, 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 an d broad cyclopean visual field.