Why do many of the larger mammals (humans, elephants, ungulates) and some birds (ostriches, ground hornbills) have eyelashes, but reptiles and most birds don't? I don't know if how significant a developmental change it would take to modify a reptiles scale into a lash-like structure, but the few examples of eyelashed birds prove that eyelashes can arise from other forms of integument besides hair. Also, it is my understanding that one of the purposes of eyelashes is to help protect the eye from dust and other debris. Do eyelash-less animals have some other adaptation(s) that serves or negates this function?

Hi Bryan,

I don't personally know of any reptile species with 'eyelashes', but there certainly is no reason why fine hair-like structures can't arise from reptile integument: just look at the tiny setae on the soles of gecko's feet, they are even finer than eyelashes (you have to look at tree-living geckos, the leopard gecko that is a popular pet does not have them).

The answer, as with most biology questions, must lie in natural selection: if there is an advantage for a reptile to have eyelashes, natural selection would favour those that do.

There are other ways to protect eyes, most notably the nictitating membrane, a sort of third eyelid that travels sideways across the eye, rather than up and down. You can see this translucent pink membrane, if you look closely, when a bird blinks.

Hope this helps,