Why do geckos, such as the crested gecko below, lick their eyeballs? The easy answer is: because they can. (We humans would lick a whole lot more than just our lips if our tongues were longer. I shudder to think what my toddler son would do with his.)
An equally easy answer is it's because some species of gecko (again, like the crested gecko) don't have eyelids. Therefore, they use their tongues and saliva to keep their eyes moist and clean. Probably the gecko equivalent of a seductive eye wink would be a seductive eyeball licking. Tongues can be just as expressive as eyelids, so it probably works out for them. Though, if you're a thirsty gecko, it's got to suck when you, out of habit, lick your eyeball with your dry tongue. Stay hydrated, my reptilian friends.
Photos 1 & 2 courtesy Erin