Summary

July 2022

 

Blinking, which occurs in almost all terrestrial creatures, has received little or no attention in the scientific literature. Yet the requirement to keep the eye moist, allowing oxygen in the atmosphere to reach the corneal cells which have no blood supply, was as important in its way as developing lungs and limbs, to fishes adopting a terrestrial lifestyle. This website is an attempt to capture the diversification which has occurred across the species. Video recordings were made of blinking in 2 species of fishes, 5 species of frog, 15 species of turtle, 20 species of lizard, 15 species of crocodile, 545 species of birds and 89 species of mammals. Blinking in two species of mudskipper involved retraction of the globes through skin slits into the skull. In frogs, blinking involved retraction of the globes with elevation of the lower lids, a transparent part of which (the nictitating membrane) lay behind the lids, only appearing during a blink and rising vertically to cover the corneas. Two types of blink were observed in turtles. One involved unfurling and vertical movement of opaque nictitating membranes from the lower lids, raising of the lower lids and retraction of the globes. In the other, the nictitating membranes arose from the inner canthus of the eye, moved horizontally and operated independently from the lower lid blink. In both, there was retraction of the globes. Blinking in lizards involved retraction of the globes, horizontal movement of the nictitating membranes and independent elevation of the lower lids. Crocodiles had a similar blink with, in some species, heavily armoured upper lids sinking like lids over the orbits as the globes retracted. Nictitating membrane blinks with no globe retraction or lower lid elevation also occurred. The commonest blink in birds comprised horizontal movement of nictitating membranes arising from the inner canthus, on head turns. In some orders of birds, this was accompanied by an upper eyelid blink. Preening and drowsiness were associated with elevation of the lower eyelids. In some species blinks occurred with pecking. There was no globe retraction in birds and most had no visible eye movements. Blinking occurred with eye movement in those species where this was preserved. Globe retraction was a major feature in the blinking of most species of mammals. This was associated with horizontal movement of the nictitating membrane arising in the inner canthus and closure of the eyelids. The latter made it impossible to see the nictitating membrane without prising the eyes open. In the meerkat and okapi, blinking involved the nictitating membrane without eyelid closure. In hominids, both globe retraction and nictitating membrane blinks have been lost. The diversification of blinking in terrestrial animals lends itself to further studies on its origins.