Last month, one of the cheetah in the zoo I work at was a bit quiet and had been seen sneezing. When I went to see her, both of her eyes were partially covered by a pale, vertical membrane. This membrane is the nictitating membrane, otherwise known as the third eyelid.

Having another eyelid might sound strange, but it turns out that us humans are the weird ones. Crocodiles, carnivores, horses, cows, birds, lizards, platypus, aardvark, sharks… most vertebrates have a well-developed third eyelid. Because the structure is so common across so many different groups, people have speculated that even dinosaurs had one.

Nictitating membrane in the domestic cat (she honestly doesn't seem to mind us doing this)
Nictitating membrane in the domestic cat (she honestly doesn’t seem to mind us doing this).

The first anatomical description  of the third eyelid that I can find is by Richard Owen in 1866. He describes the way these eyelids move horizontally across the eye, from the inner to the outer corner. Mammals tend to have a membrane stiffened by cartilage and covered in conjunctiva (like the inside of the other two eyelids). Although it’s usually rolled back out of sight, if you have a cat or dog you might have seen it when they’re very sleepy. The membrane of birds, sharks and reptiles is thinner and often transparent.

The plica semilunaris in the human eye.
The plica semilunaris in the human eye.

In humans and most other primates the nictitating membrane has dwindled away to a small structure just visible where our upper and lower eyelids join near the nose. This is called the plica semilunaris.

So what’s it for? One function is protection of the eye from physical damage. Slow-motion video shows crocodiles, hawks and sharks close their third eyelids as they strike at prey, and aardvarks use them to protect against bites while eating termites.

Apparently the word “squeegee” dates back to 1844.
Apparently the word “squeegee” dates back to at least 1867

They also form part of the eye’s defence against bacteria and viruses, and contain lymphoid tissue and white blood cells as part of the ocular immune system. Even the tiny human plica semilunaris has been shown to contain this specialised tissue. Land animals use the nictitating membrane to help keep the eye moist, and it often contains tear-producing glands.

Additionally, a nictitating membrane can manually remove dust and debris from the eye. An anatomical description by Stibbe in 1928 describes the mammalian third eyelid as “a sort of “squeegee” at the inner side of the orbit, under which the eyeball can roll.”

Some nictitating membranes have evolved special functions that reflect the lifestyles of their owners. Woodpeckers (like the one in this video) close their opaque, relatively strong third eyelids immediately before they peck at a tree trunk. Researchers have suggested that this acts as a “seatbelt” for the eye, helping to prevent injury to the retina and sensitive parts of the eye that might otherwise be at risk from the forces created as they repeatedly hammer their heads against a hard surface.

Close-up of the eye of a blue shark, showing the nictitating membrane. Credidt: Apex Predators Program
Close-up of the eye of a blue shark, showing the nictitating membrane. Credit: Apex Predators Program, NEFSC/NOAA

Polar bears have a transparent third eyelid that covers the entire cornea. This certainly helps to protect the eyes when catching seals underwater, and possibly acts as sunglasses to shield the eyes from UV light and snow glare. Transparent nictitating membranes are so useful underwater that they’ve evolved independently in other aquatic and diving animals such as crocodiles, beavers, sharks and manatee. Here’s a great video of a crocodile’s nictitating membrane.

In the 40s, there was a theory that diving ducks use a specialised transparent nictitating membrane to adjust their vision so they can focus underwater without distortion. Many textbooks and popular books repeat this, even today. Unfortunately, although built-in duck contact lenses is a nice idea, a 1978 study by Sivak et al. disproved the theory. The curve of the eyelid follows that of the eye’s surface, and there’s no appreciable difference in light refraction.

Even today, studies are still being published about the anatomy and function of the nictitating membrane.

You humans do talk a lot of rubbish sometimes. Photo by Mike Baird via Flickr CC
You humans do talk a lot of rubbish sometimes. Greater Scaup photo by Mike Baird via Flickr CC

So, why were the third eyelids of our cheetah protruding? The third eyelids of cat species usually remain out of sight, but in illness they can become visible. This can be for several reasons. Injury of the eye itself will cause the third eyelid to remain closed to protect the eye, much as we will squint or keep our eye closed if we hurt it. There are some rare neurological conditions that affect the eyelids and cause protrusion. But protrusion of both third eyelids can also be a non-specific sign of illness unrelated to the eye or eyelid – for instance if the cat is running a temperature, is dehydrated or is feeling systemically unwell.

There isn’t much in the literature about the exact mechanism behind this – one theory is that mild dehydration might cause the fat pads behind the eyes to shrink. The eyes then retract into the socket and allow the third eyelids to protrude. A simpler possibility is that cats who feel lethargic will just allow the eyelids to protrude in the same way that sleepy healthy cats will. Regardless of the reason, it’s a well-known marker for “generally unwell”.

Our cheetah likely had a mild infection that was making her feel unwell. We treated her with oral medications and by the next day you couldn’t see her third eyelids at all. She’s back to normal now and has been introduced to her new boyfriend.- pakasuchus

Cheetah photo by Rob Gordon
Cheetah photo by Rob Gordon (website here)

Nice things that wouldn’t fit in the post:

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