Conclusions

 

So what do we learn from this account of the diversification of blinking across the species? It seems likely that blinking in the first creatures to adopt a terrestrial lifestyle involved globe retraction and lower lid elevation. Why the lower lid? It is hard to say. Perhaps it arose from the time when animals still spent a lot of time half submerged in water. Elevation of the lower lid might prevent floating debris from impacting on the cornea. While globe retraction and lower lid elevation have a protective function, an even more important need was to keep the cornea moist. This was dealt with by evolving a fold in the conjunctiva, the nictitating membrane, which could move freely over the cornea. Lacrimal glands evolved to provide a watery solution which could be spread over the cornea. Water evaporates rapidly and in time the Harderian gland evolved. This secreted an oily solution under the surface of the nictitating membrane. Meibomian glands took over this function in some mammals. Now the tear film, with an oily surface layer, could remain unbroken for many minutes at a time. So much so that in many species, blinking occurs very infrequently. In birds, by contrast, blinking occurs every few seconds. Why this is so remains a mystery. Whatever the cause, the fact that blinking in birds is closely linked to head turns has to be taken into account. Some birds and many mammals developed upper eyelid blinks. The ecological pressures which led to this are not easy to fathom.

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Diversification of blinking across the species is extraordinary, largely unexplained and yet remains a topic which is sadly neglected.