Koalas have beautiful eyes.
Their irises are chestnut brown*, with a vertical pupil, like a cat. Like a cat, and like us, koala eyes face forwards. (Kempster 2002)
(note * – rarely a koala is seen with blue eyes)
It is unusual for herbivores (plant-eaters) to have vertical pupils and forward-facing eyes. Most animals with this setup are small hunters that are active day and night. (Banks 2015)
Read more here: https://www.sciencefocus.com/nature/why-do-animal-pupils-come-in-different-shapes
Vertical slit pupils are an advantage to animals that are active day and night, compared to circular pupils (like ours). Pupils dilate and constrict to manage the amount of light entering the eye. A circular pupil can only change its area a small amount (~15 times area), while the vertical slit pupils of a cat and a gecko can change 135 and 300 times area respectively. I can’t find any scientific papers showing that the koala eye has been assessed for how much the pupil can dilate and constrict, but it is fair to assume that the vertical pupil gives the koala benefits both in the strong light of day and at night.
But why a vertical slit and not a horizontal one? The research done by Banks et al in 2015 also found that vertical slit pupils convey a detailed distance perception, especially in the short range. Verticals (tree trunks?) are in sharp focus even when they are in front or behind the object in focus, but horizontals (branches?) blur more when they are not the object in focus. The combination of these two gives excellent depth perception.
It would make sense that good short distance perception would be essential for a koala negotiating a network of branches high in the canopy of a tree. Koalas often jump at height too. A fall from the top of a big gum tree could cause injury or death. Watch:
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Koalas are unusual in many ways – they have fingerprints (Henneberg 1998), they are arboreal yet lack a tail, they have very deep voices for their size – and they will sometimes make full eye contact with a person. I think this is why many people feel a strong connection with koalas.
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What are a koala’s eyes saying?
We don’t really know what koalas are thinking or feeling. I’ve worked with koalas, in the wild, my whole life, and I think I understand a little of their body language – particularly for strong emotions like fear. But everything I know is really an educated guess.
For me, koala eyes seem quite expressive – they can appear disturbed or annoyed, fearful, wary or calm. Lluvia (above) just looks happy and content.
Maybe we feel that we can read a koala’s expression because we are accustomed to reading the expressions of humans. Koalas and humans share many facial features: a flat, round face, forward-facing almond-shaped eyes, a long nose between the eyes. Koalas even have white sclera (the whites of the eyes), which when seen gives an impression of an irritated side-eye. Koala sclera is not seen from most front-on views though. [see eye anatomy diagram below].
Here’s a few pictures of the same koala, female Pat, all taken when she was quite elderly. See what you think.
In this photo below, to my interpretation, her eyes are nervous. Her eyes are stretched wide open. Her ears are also pricked high up, and her posture is upright and leaning back.
But maybe its her body language and posture? Let’s see – in the pic below I’ve zoomed in so that you can’t see her ears or her posture.
Even without the body language, I still think her eyes look anxious.
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In this next photo, her eyes look alert, wary but calm. They are facing at the observer, open but not wide. Her ears are pricked forward and she is upright, but not leaning back.
In this photo below, koala Pat’s eyes look annoyed. I can’t really say why, but it might be the bit of white sclera visible, giving a side-eye impression. You can’t see her ears, but they are a bit pricked up, and one seems to be slightly back. She is not upright, and is leaning a bit forward. Her eyes are very wide open, like pic 1 – nervous – but she doesn’t look frightened at all, she just looks irritated.
Koala Eye Anatomy
Koalas have an upper and a lower eyelid, a row of dark eyelashes (hard to see unless up close), and a nictitating membrane/third eyelid – but that is rarely seen. Their cornea (the iris + pupil, or the seeing part of the eye) fills most of the space between the eyelids (the palpebral fissure). Their sclera (the white of the eye) is white, but is hidden most of the time.
The inner corner of the eye is called the medial canthus, and the outer corner the lateral canthus. Within the inner corner there is a lacrimal caruncle – a fleshy area that is usually pink-brown. The nictitating membrane/third eyelid emerges from this inner corner and stretches across the lower eye – I’ve shown this as a dashed line on the diagram, based on figure 1 in Hirst 1992.
I have never seen a nictitating membrane across the eye of a living wild koala. However I have several photographs of koalas that have a white area across the caruncle, that I think is a partially-extended nictitating membrane.
Normally when a koala is looking straight at you, you can’t see any white either side of their iris. This is common in mammals – most have dark sclera, and their corneas fill their eye space.
Humans are the exception. We have large areas of white sclera both sides of our iris, that is visible at most angles. The whites of our eyes focus attention on our iris, show the direction of our gaze, and convey health. The angle and position of our sclera, the speed of eye movement, and the position of our head can suggest boredom, fear, distaste, disrespect, and anger. We even have sayings to describe this: “giving the side-eye”, “showing the whites of its eyes”, and “don’t shoot until you see the whites of their eyes”. A tiny bit of white sclera showing in a human eye can convey a lot of information.
Extra white sclera in a human is often considered a sign of beauty. Princess Diana, John F Kennedy, Marilyn Monroe and Billie Eilish all have a condition called scleral show (or Sanpaku Eyes) where the white shows below their iris.
Koalas have white sclera. Though you don’t see it often, some koalas show it a bit more than others.
Koala Clancy always seemed a bit more charismatic than other koalas. Its the reason we named our foundation after him. He had expressive eyes, and was as photogenic as anything.
Now I look closely, many photos of Clancy show a tiny bit of white sclera in his eyes. See below:
You might think such a small amount of white in the eye would make no difference, but it can. Have a look at the pictures of chimps in this article: https://news.asu.edu/20230313-new-research-indicates-whites-human-eyes-are-not-uniquely-human
Which of the chimps look most expressive/appealing?
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Finally, it is often said that koalas have poor vision. But I’ve found that their vision seems to be excellent at close range. To watch them leaping around the upper canopy of a huge gum tree, or looking up from the ground at another koala high in a tree, it is clear that they see quite well. Perhaps they don’t have great long distance vision, but maybe they don’t need it. Vision may not be their primary sense, but it has been shown that they need vision to navigate their environment (Schmid 1991).
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REFERENCES:
Banks, M.S., Sprague, W.W., Schmoll, J., Parnell, J.A. and Love, G.D., 2015. Why do animal eyes have pupils of different shapes?. Science advances, 1(7), p.e1500391. https://www.science.org/doi/pdf/10.1126/sciadv.1500391
Hirst, L.W. and Brown, A.S., 1992. OPHTHALMOLOGIC EXAMINATION OF THE NORMAL EYE OF THE KOALA. Journal of Wildlife Diseases, 28(3), pp.419-423. https://scholar.archive.org/work/qluifmrwlra75grvvmmdmu3g2i/access/wayback/https://bioone.org/journals/Journal-of-Wildlife-Diseases/volume-28/issue-3/0090-3558-28.3.419/OPHTHALMOLOGIC-EXAMINATION-OF-THE-NORMAL-EYE-OF-THE-KOALA/10.7589/0090-3558-28.3.419.pdf
Kempster, R.C. and Hirst, L.W., 2002. Bony orbital anatomy of the koala (Phascolarctos cinereus). The Anatomical Record: An Official Publication of the American Association of Anatomists, 267(4), pp.288-291. https://anatomypubs.onlinelibrary.wiley.com/doi/pdf/10.1002/ar.10117
Schmid, L.M., Schmid, K.L. and Brown, B., 1991. Behavioural determination of visual function in the Koala (Phascolarctos cinereus). Wildlife Research, 18(3), pp.367-374. https://www.publish.csiro.au/WR/WR9910367