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 perhaps against a stationary membrane, kept moist by sitting in a pool of water, as occurs in mudskippers. Later, while animals still spent a lot of time half submerged in water, a mobile lower lid evolved which perhaps prevented floating debris from impacting on the cornea. But how to keep the cornea moist (allowing atmospheric oxygen to diffuse into it) once these early terrestrial pioneers started to venture further away from water? This was dealt with by evolving a fold in the conjunctiva, the nictitating membrane, which could move freely over the cornea. Tear glands evolved to provide a watery solution which could be spread over the cornea. Water evaporates rapidly and in time they began to also secrete an oil . 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 needs further investigation.