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What is the difference between a circular and a cat's-eye (slit) pupil?

What is the difference between a circular and a cat's-eye (slit) pupil?



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I've been to local zoo the other day and one lizard caught my attention: its pupils are circular, which, I thought, is not usual for reptiles. Turns out it is, but now I can't find any explanation on why some animals have one kind of pupil and others have the other. Lizards can have both, and so can snakes. The only difference I have found is that circular pupil can't shrink quite as much as a cat's-eye pupil, but that hardly explains why circular pupil even evolved in the first place as I don't see any advantage to it.

Ideas?

P. S. Fish only have circular pupils so that shape is older, right?…


Circular pupils are always functionally superior to vertical pupils; a slit does not correctly focus light from all directions whereas a circular pupil does. If you observe cats when they're hunting at dawn and dusk*, they have big, circular pupils; it's only when they're in bright light that the pupil shrinks to a slit. So why have vertical pupils at all? Because, as you mention they're capable of letting a more controlled range of light into the eye.

Thus vertical slits let you be active in a wider range of light conditions at the cost of poorer vision in bright light; whilst circular pupils continue to function well in brighter light but at the cost of not allowing such a large range of control over the amount of light entering the eye.

It is then easy to see why you get both kinds of pupil: circular pupils are favoured by animals that are typically active in bright light and need good vision under these circumstances; vertical pupils are favoured by animals that are primarily active in low light but need some ability to see in bright light.

*- it's often stated that cats are nocturnal, this is untrue; cats are crepuscular - that is they are most active at dawn and dusk.


Your mention of cats hinted that vertical pupils have to do with night vision, and indeed they do.

The retinas of cats and other nocturnal animals are very sensitive to even the tiniest amount of light. This can make their eyes hurt when exposed to bright sunlight. So their pupils have to shrink as much as possible in the sunlight, and, as you pointed out, vertical pupils can shrink more than circular ones.

EDIT: People pointed out in the comments that cats have vertical eye pupils only in bright sunlight. In low-light conditions, the pupils expand to a circle.


In bright light, a cat can also squint, thus approximating a circle with a square-like aperture. So a vertical pupil is not that much of a liability to focusing. The same cannot be said of a horizontal pupil (unless the eyelids are vertical). Additionally, a vertical pupil might optimize detection of horizontal movement, which is likely an advantage in hunting prey on the savannah.


There was a study published this last year (2015) that suggests that the pupil style of an animal is related to its strategy for predation. This means that land animal pupils evolved as adaptations to the niche that they filled.

The study distinguishes between herbivorous, active, and ambush foraging behaviors. Ambush predators tend towards vertical pupils, where as active predators (those that chase down and kill their prey) tend more towards circular. The pupils of herbivorous foragers tended to be mostly horizontally-elongated as is the case with goats. Eyes also tend to be forward set in predatory animals and more at the side of the head in prey animals.

In Why do animal eyes have pupils of different shapes? Banks, et. al. Science Advances
07 Aug 2015: Vol. 1, no. 7, e1500391 DOI: 10.1126/sciadv.1500391 It is suggested that

Abstract
There is a striking correlation between terrestrial species' pupil shape and ecological niche (that is, foraging mode and time of day they are active). Species with vertically elongated pupils are very likely to be ambush predators and active day and night. Species with horizontally elongated pupils are very likely to be prey and to have laterally placed eyes. Vertically elongated pupils create astigmatic depth of field such that images of vertical contours nearer or farther than the distance to which the eye is focused are sharp, whereas images of horizontal contours at different distances are blurred. This is advantageous for ambush predators to use stereopsis to estimate distances of vertical contours and defocus blur to estimate distances of horizontal contours. Horizontally elongated pupils create sharp images of horizontal contours ahead and behind, creating a horizontally panoramic view that facilitates detection of predators from various directions and forward locomotion across uneven terrain.


Figure 1. Why do animal eyes have pupils of different shapes? Banks, et. al. Science Advances
07 Aug 2015: Vol. 1, no. 7, e1500391 DOI: 10.1126/sciadv.1500391


I see one possible advantage to round pupils. A slit pupil gives higher visual acuity vertically than horizontally because of diffraction and when a slit pupil is very nearly shut, there's high diffraction in the horizontal direction giving poor horizontal visual acuity. A round pupil on the other hand gives a creature the highest visual acuity in all directions for a given area of pupil. A smaller pupil is better for reducing chromatic aberration but chromatic aberration can be pretty much neglected because for any creature with evolutionary pressure for sharp vision in all directions, good accomodation will ensure at least one wavelength of light focuses onto the retina forming a sharp image because the sun emits a continuous range of wavelengths.


Human eyes are able to absorb blue, green, and red wavelengths of light, allowing us to visualize the different combinations of these colors. Cats’ eyes have similar light wavelength absorption, although the colors are likely not as rich or vibrant as what we can see. This may be because they have better night vision than we do. Dogs’ eyes can only absorb blue-violet and red wavelengths of light, so their color vision is more limited. Certain birds and fish can actually see wavelengths of light within the ultraviolet spectrum of light in addition to reds, greens, and blues - this means their color vision is richer and more vibrant than even humans!

Cats, and to a lesser degree, dogs, can see better in the dark than humans. Why? For starters, their pupils and corneas (the clear layer that covers the pupil and colored part of the eye, called the iris) are larger than humans’. This allows more light to reach the back of the eye where the retina (the inner layer at the back of the eye) can process it. They also have an increased proportion of cells (“photoreceptors”) that process information in low light, called rods, compared to the human eye.


Why Do Cats Have Slit Pupils And Humans Have Round Pupils?

Human beings have round pupils cats have pupils shaped like vertical slits.  What's the advantage for cats and people, respectively, in those different pupil shapes?

Different pupil shapes seem to be an adaptation to different activity patterns during a 24-hour day.

Nocturnal Animals And Slit Pupils

Cats belong to the category of nocturnal animals, who do most of their foraging for food at night.  Other nocturnal animals include rats, bats, mice, flying squirrels, and owls.

These animals rely heavily on hearing and touch to get around, but they also have the ability to see in very dim light.  Their eyes have big lenses and sensitive retinas.

Daytime Animals

But many nocturnal animals, such as cats, are not strictly nocturnal they move around during the day as well as at night. They need some way to protect their sensitive eyes in daylight, and slit pupils provide that protection.

Like a theater curtain, a slit pupil can close as much as necessary to prevent too much light from entering the eye.

Humans And Round Pupils

For humans and other animals active mainly in the daytime, visual sensitivity is not as important as the ability to see small details in bright light.

So the eyes of humans, lizards, ground squirrels, and most birds have a design suited for operation in bright light without the special protection of slit pupils.  In dim light, our eyes are not as sensitive as those of nocturnal animals.

What Is The Most Common Pupil Shape?

Of all pupil shapes found in vertebrates animals with backbones round pupils are the most common.  Non-circular pupil shapes found in vertebrates include not only the vertical slits familiar to us from cats' eyes, but also horizontal slits, crescents, heart shapes, and keyhole shapes.


Why do cats have slit-shaped eyes?

Ever wondered why cat eyes have vertical slit-shaped pupils, while sheep have horizontal bars? A recent study suggests it has to do with their place in the food chain.

Martin Banks from the University of California in Berkeley and colleagues examined the eyes of more than 200 land animals and found their status as predator or prey was correlated to the shape of their pupils. Their findings were published in Science Advances in August.

A cat’s pupil

Depending on the light, the shape of a domestic cat’s pupil changes from vertical slit to alluring almond to almost fully round. Like opening or closing theatre curtains, muscles on either side of the cat’s pupil open the slit wide or cause it to narrow. Overall a cat’s pupils can expand by 135-fold and can perform like built-in night vision goggles. By contrast human pupils expand by a factor of 15.

The slit-shaped pupil, with its remarkable light control, is how a cat can hunt in near-darkness, and also in bright daylight, says Ron Douglas, an animal vision biologist at City University London. Banks’ study shows the slit-shaped pupil is most often found in animals that hunt by day and night, and especially among predators that ambush their prey – including cats, snakes and crocodiles. Those that chase down their prey, such as cheetahs and wolves, tend to have circular pupils.

Like most predators, a cat’s eyes face forward. Their brain compares the slightly different images relayed from the left and right eye to help estimate distance – a process called stereopsis. We do this too – try walking down stairs with one eye shut. Why might being an ambush predator dictate the shape of your pupil? Because to pounce on a mouse, a cat must be superb at judging distance – and that’s where a slit-shaped pupil can help, says Banks.

Tiny pupils deliver the sharpest image and perform best for stereopsis.

Paradoxically, an alternative way to judge distance is to blur parts of the image. And for that, a wide open pupil is best – as any photographer could tell you. Set a wide aperture on your camera and while your subject will remain in focus, the foreground and background will blur. Predators also use this blurry, shallow depth of field to estimate distance.

But to use both measuring tricks at once, a cat would need a small and a large pupil at the same time. Impossible, right? Not if it is slit-shaped, Banks says: “We think the vertical slit is a really clever adaptation … It makes the pupil small horizontally and tall vertically. It’s really pretty cool.”

The effect works best in daylight, when the cat can contract its pupil to its narrowest.

A sheep’s pupil

With its horizontal bar-shape, a sheep’s pupil could not be more different from a cat’s. Banks found that most land animals with bar-shaped pupils were herbivores that had to keep a constant eye out for predators. Their horizontal pupils, in eyes placed on either side of their head, allow them to scan the horizon all around them for possible attackers.

Sheep spend much of the day with their head tilted downward as they graze. As their head pitches towards the ground their eyeballs roll like spirit levels keeping their pupils parallel to the horizon.

“That their eyeballs swivel like this means it must matter what their orientation is,” Douglas says. Banks suspects another benefit of horizontal pupils is to reduce glare form the overhead sun.

The self-levelling horizontal eye would also give sweeping views of the ground ahead – part of the reason, perhaps, why goats, sheep and horses are so surefooted across uneven terrain.

The round human eye allows us to hunt, gather and observe fine details. Credit: Mitchell Smith /gettyimages

A human pupil

Round pupils like our own are less well understood. Humans are consummate generalists. Besides hunting and gathering, we require our eyes to detect intricate details such as facial expressions. “There’s more demand on our eyes,” Banks says. Perhaps there’s a trade-off – we forgo some of the light-controlling mechanisms of the cat, say, for eyes better suited to picking up colour and other details in the scene around us.

Banks also thinks our height might partly explain our pupil shape. Even among ambush predators, where slit-shaped pupils are most common, taller species such as lions tend to have round pupils. Banks thinks that may be because the blurry, shallow depth of field trick domestic cats use to judge distance is most effective at close range. The eye-to-prey distance of a cat poised to pounce on a mouse is much shorter than that of a lion poised to pounce on an antelope.

The w-shaped cuttlefish pupil is a distinctive feature of the most bizarre eye in the animal world. Credit: Fotosearch/gettyimages

A cuttlefish pupil

The winner of the most bizarre pupils, though, must be cuttlefish. Their bulbous eyes and distinct W-shaped pupils let them look forwards and backwards at the same time. The pupils can also dilate to an almost-perfect circle. Douglas suspects this geometry may explain why cuttlefish have one of the fastest pupil reflexes of any animal – about twice as fast as humans.

Belinda Smith

Belinda Smith is a science and technology journalist in Melbourne, Australia.

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We put Peter's question to zoologist Max Gray. Max - So this is all to do with the fact that cats are very active at night and they need good night vision as well as having day vision - so they're not fully nocturnal and not fully diurnal. They're active at both times of day. But they've got very sensitive eyes so they can see at night but, as a result, during they day they can get very over-sensitive so they need to be able to control the amount of light that gets into their eyes really, really precisely - much more precisely than we do. And so, by having a vertical slit as a pupil they can narrow that down to a vertical slit and that controls the light reasonably well but they may need more control than that. By having a vertical slit they can then use their eyelids to close that slit further down to a point. So they have two ways of controlling the amount of light getting into their eyes.

Chris - Ingenious. What about animals like cows and horses - do they not have the slit? Certainly rabbits have slit running front to back, not up and down, don't they?

Max - Often it's not perfectly front to back. It's kind of at an angle and their eyes will move around as they move their heads because they're prey animals.

Chris - They're more interested in what's happening on the horizon?

Max - Yes. And when they're down eating it helps them see around them when they're vulnerable.

Chris - And I suppose if you're a predating animal like a cat, then you're going to be fixating on something and pouncing so you need to have very good depth perception and having a very small pupil is going to give you that, isn't it?

Max - Yes. That's mostly why you have eyes on predators at the front like ours and looking forwards because that's how you hunt. Whereas animals like rabbits and sheep will have eyes on the side of their heads so they can have a better field of vision for things that might pounce on them.

Chris - And we have a round one because that's the best compromise between distance, day, night?

Max - Yes. It's also easiest as well. Interestingly, if you look at cat species that are active during the day, lions are a good example. They don't have slit pupils, they have round, circular and you can go away and google this. If you google lions eyes, you look and they have round pupils like humans.


Eye shape reveals whether an animal is predator or prey, new study shows

The eyes say it all. They answer questions about a creature’s social scale, and its place in the pecking order. The geometry of the eye indicates whether an animal is the hunter, or the hunted – and how tall it walks.
Scientists from the Universities of California Berkeley and Durham in Britain have
discovered just how much they can learn from pupils. As every householder knows, when the domestic cat narrows its eyes to slits, it does so vertically. Sheep, deer and horses however have eyes with horizontally elongated pupils.

Martin Banks, professor of optometry at Berkeley and Gordon Love, director of the Centre for Advanced Instrumentation at Durham, have learned something else. So important is it for a grazing animal to keep an eye on the ground that when it drops its head, the pupil rotates by up to 50 degrees to stay horizontal.
“The first key visual requirement for these animals is to detect approaching predators, which usually come from the ground, so they need to see panoramically on the ground with minimal blind spots,” said Professor Banks. “The second critical requirement is that once they do detect a predator, they need to see where they are running. They have to see well enough out of the corner of their eye to run quickly and jump over things.” The two scientists and their colleagues report in the journal Science Advances that they looked at the eyes of 214 closely-studied animals, all terrestrial vertebrates. These included Australian snakes, every species from the cat and dog families as well as hyenas and mongooses, and domestic grazing animals as well as tapirs and rhinoceroses. The challenge was to see if they could predict a relationship between an animal’s ecological niche and the shape formed by the pupil in its eye.
They found a pattern. The smaller ambush predators – those little creatures that lie in wait for their lunch – are more likely to have pupils that narrow vertically. Hunters that prowl by day or night need to make the most use of available evening light yet exclude the glare of the sun, which is why the eyes must narrow dramatically. The mouse-hunting domestic cat can change the area of its pupil gaze 135-fold and the insectivorous gecko 300-fold. Round-eyed humans – that is, with circular pupils – can reduce them 15-fold. But humans walk tall. So do lions and tigers, and they too have round eyes and circular pupils. The big cats are “active foragers”: they hunt down their prey. The researchers included 65 ambush predators with eyes in the fronts of their heads for this study. Of these, 44 had vertical pupils and 82% had shoulder heights less than 42 cms or 16.5 inches. So the reasoning is that binocular vision and vertical slit pupils together make it easier for small animals to pounce, by using the difference between close focus on the innocent dinner and the out-of-focus or blur beyond and before it, to judge the distance precisely. The team started with a classic 1942 text on the physiology of the eye that proposed that slit-shaped pupils allowed for different musculature and a greater range of light entering the eye. But the theory did not explain why the eye slits could be sometimes vertical, sometimes horizontal. When a grazing animal lifts its head, its eyes are elongated horizontally. But surely, when it drops its head to crop grass, the eyes would appear near vertical to the ground? The scientists set out to observe the eyes swivel to stay parallel with the ground. Professor Love began research in astronomical technology but joined the eye project years ago. “The physics of huge telescopes, microscopes, and eyes is all rather similar so it wasn’t such a big jump,” he said. Professor Banks went to Oakland Zoo in California to observe at first hand, and Professor Love took his camera to the Yorkshire Dales to record changing pupil shapes in the field. “The photography part was new and fun and took more time than I care to remember,” he said. “You might think that sheep would be easy to photograph. I now have eternal respect for David Attenborough and his colleagues.”


How Can I Help My Cat Without Going to the Vet?

If your cat has a sore, weeping eye then you can gently bathe it to remove any built-up discharge from around the eye. To do this, boil some water, then pour it into a dish and let it cool. Then, dip some clean cotton wool in this water and use it to gently wipe around your cat’s eye.

This will help prevent the discharge from building up and may make the eye temporarily more comfortable, but is unlikely to fix the underlying issue. You should still see a veterinarian after cleaning the eye.

If you think there may be something trapped in your cat’s eye, DO NOT attempt to bathe it or remove the object as you may make things worse. You should arrange an emergency appointment with your veterinarian.


RESULTS

Optical properties

Both monofocal and multifocal optical systems were found in birds(Table 1). Multifocality was more common and was detected in 29 species in 10 out of 12 examined orders. Only five species in five orders had monofocal optical systems(Table 1, Fig. 4C,E).

Multifocality, cone pigment and pupil size

Order . Species . English common name . Pupil diameter (mm) . Multifocal optics . Cone pigment . Reference (pigment) .
Struthioniformes Struthio camelus Linnaeus 1758 Ostrich 11 Yes U, S, M, L (Wright and Bowmaker, 2001)
Dromaius novaehollandiae Latham 1790 Emu 10 No L (Sillman et al., 1981)
Rhea americana Linnaeus 1758 Rhea 10 ? S, M, L (Wright and Bowmaker, 2001)
Sphenisciformes Spheniscus humboldti Meyen 1834 Humboldt penguin 5 Yes U, S, L (Bowmaker and Martin, 1985)
Spheniscus magellanicus Forster 1781 Magellanic penguin 4.5 ?
Pygoscelis adeliae Hombron and Jacquinot 1841 Adelie penguin 4.5 ?
Pygoscelis papua Forster 1781 Gentoo penguin 5 ?
Pygoscelis antarcticus Forster 1781 Chinstrap penguin 5 ?
Aptenodytes patagonicus Miller 1778 King penguin 7 ?
Aptenodytes forsteri Gray 1844 Emperor penguin 7 No
Eudyptes chrysolophus Brandt 1837 Macaroni penguin 5 ?
Strigiformes Athene cunicularia Molina 1782 Burrowing owl 13 Yes
Bubo virginianus Gmelin 1788 Great horned owl 15 Yes
Bubo scandiacus Linnaeus 1758 Snowy owl 14 Yes
Asio flammeus Pontoppidan 1763 Short-eared owl 13 Yes
Strix nebulosa Forster 1772 Great grey owl 14 Yes
Strix uralensis Pallas 1771 Ural owl 12.5 Yes
Aegolius funereus Linnaeus 1758 Boreal owl 12 Yes
Falconiformes Buteo lineatus Gmelin 1788 Red-shouldered hawk 8 Yes
Haliaeetus leucocephalus Linnaeus 1766 Bald eagle 9 Yes
Anseriformes Lophodytes cucullatus Linnaeus 1758 Hooded merganser 5 Yes
Anser anser Linnaeus 1758 Domestic goose 7 No
Passeriformes Phylloscopus trochilus Linnaeus 1758 Willow warbler 2 ?
Phylloscopus collybita Vieillot 1817 Chiffchaff 2 Yes
Fringilla montifringilla Linnaeus 1758 Brambling 3 Yes
Fringilla coelebs Linnaeus 1758 Chaffinch 3 Yes
Erithacus rubecula Linnaeus 1758 European robin 2.5 Yes
Emberiza schoeniclus Linnaeus 1758 Reed bunting 2.5 ?
Troglodytes troglodytes Linnaeus 1758 Winter wren 2 ?
Cyanistes caeruleus Linnaeus 1758 Blue tit 2 Yes U, S, M, L (Hart et al., 2000)
Prunella modularis Linnaeus 1758 Hedge accentor 1.5 Yes
Phoenicurus ochruros Gmelin 1774 Black redstart 2.5 Yes
Coraciiformes Coracias caudatus Linnaeus 1766 Lilac-breasted roller 5 Yes
Psittaciformes Cacatua goffiniana Finsch 1863 Tanimbar cockatoo 6 Yes
Cacatua sulphurea Gmelin 1788 Yellow-crested cockatoo 6 Yes
Melopsittacus undulatus Shaw 1805 Budgerigar 3 Yes U, S, M, L (Bowmaker et al., 1997)
Poicephalus senegalus Linnaeus 1766 Senegal parrot 6 Yes
Amazona aestiva Linnaeus 1758 Blue-fronted parrot 6 Yes
Neopsephotus bourkii Gould 1841 Bourke's parrot 3 Yes
Psittacus erithacus Linnaeus 1758 Grey parrot 7 Yes
Galliformes Tetrao urogallus Linnaeus 1758 Western capercaillie 6 Yes
Gallus gallus Linnaeus 1758 Domestic chicken 6 No U, S, M, L (Bowmaker et al., 1997)
Ciconiiformes Ciconia ciconia Linnaeus 1758 White stork 7 No
Pelecaniformes Phalacrocorax carbo Linneaus 1758 Great cormorant 7 ?
Columbiformes Columba livia Gmelin 1789 Homing pigeon 5.5 Yes U, S, M, L (Bowmaker et al., 1997)
Order . Species . English common name . Pupil diameter (mm) . Multifocal optics . Cone pigment . Reference (pigment) .
Struthioniformes Struthio camelus Linnaeus 1758 Ostrich 11 Yes U, S, M, L (Wright and Bowmaker, 2001)
Dromaius novaehollandiae Latham 1790 Emu 10 No L (Sillman et al., 1981)
Rhea americana Linnaeus 1758 Rhea 10 ? S, M, L (Wright and Bowmaker, 2001)
Sphenisciformes Spheniscus humboldti Meyen 1834 Humboldt penguin 5 Yes U, S, L (Bowmaker and Martin, 1985)
Spheniscus magellanicus Forster 1781 Magellanic penguin 4.5 ?
Pygoscelis adeliae Hombron and Jacquinot 1841 Adelie penguin 4.5 ?
Pygoscelis papua Forster 1781 Gentoo penguin 5 ?
Pygoscelis antarcticus Forster 1781 Chinstrap penguin 5 ?
Aptenodytes patagonicus Miller 1778 King penguin 7 ?
Aptenodytes forsteri Gray 1844 Emperor penguin 7 No
Eudyptes chrysolophus Brandt 1837 Macaroni penguin 5 ?
Strigiformes Athene cunicularia Molina 1782 Burrowing owl 13 Yes
Bubo virginianus Gmelin 1788 Great horned owl 15 Yes
Bubo scandiacus Linnaeus 1758 Snowy owl 14 Yes
Asio flammeus Pontoppidan 1763 Short-eared owl 13 Yes
Strix nebulosa Forster 1772 Great grey owl 14 Yes
Strix uralensis Pallas 1771 Ural owl 12.5 Yes
Aegolius funereus Linnaeus 1758 Boreal owl 12 Yes
Falconiformes Buteo lineatus Gmelin 1788 Red-shouldered hawk 8 Yes
Haliaeetus leucocephalus Linnaeus 1766 Bald eagle 9 Yes
Anseriformes Lophodytes cucullatus Linnaeus 1758 Hooded merganser 5 Yes
Anser anser Linnaeus 1758 Domestic goose 7 No
Passeriformes Phylloscopus trochilus Linnaeus 1758 Willow warbler 2 ?
Phylloscopus collybita Vieillot 1817 Chiffchaff 2 Yes
Fringilla montifringilla Linnaeus 1758 Brambling 3 Yes
Fringilla coelebs Linnaeus 1758 Chaffinch 3 Yes
Erithacus rubecula Linnaeus 1758 European robin 2.5 Yes
Emberiza schoeniclus Linnaeus 1758 Reed bunting 2.5 ?
Troglodytes troglodytes Linnaeus 1758 Winter wren 2 ?
Cyanistes caeruleus Linnaeus 1758 Blue tit 2 Yes U, S, M, L (Hart et al., 2000)
Prunella modularis Linnaeus 1758 Hedge accentor 1.5 Yes
Phoenicurus ochruros Gmelin 1774 Black redstart 2.5 Yes
Coraciiformes Coracias caudatus Linnaeus 1766 Lilac-breasted roller 5 Yes
Psittaciformes Cacatua goffiniana Finsch 1863 Tanimbar cockatoo 6 Yes
Cacatua sulphurea Gmelin 1788 Yellow-crested cockatoo 6 Yes
Melopsittacus undulatus Shaw 1805 Budgerigar 3 Yes U, S, M, L (Bowmaker et al., 1997)
Poicephalus senegalus Linnaeus 1766 Senegal parrot 6 Yes
Amazona aestiva Linnaeus 1758 Blue-fronted parrot 6 Yes
Neopsephotus bourkii Gould 1841 Bourke's parrot 3 Yes
Psittacus erithacus Linnaeus 1758 Grey parrot 7 Yes
Galliformes Tetrao urogallus Linnaeus 1758 Western capercaillie 6 Yes
Gallus gallus Linnaeus 1758 Domestic chicken 6 No U, S, M, L (Bowmaker et al., 1997)
Ciconiiformes Ciconia ciconia Linnaeus 1758 White stork 7 No
Pelecaniformes Phalacrocorax carbo Linneaus 1758 Great cormorant 7 ?
Columbiformes Columba livia Gmelin 1789 Homing pigeon 5.5 Yes U, S, M, L (Bowmaker et al., 1997)

Visual pigments are classified after their spectral absorbance maximums: U,ultraviolet/violet sensitive S, short wavelength sensitive M, medium wavelength sensitive L, long wavelength sensitive. Question marks indicate ambiguous optical characters. Pupil diameters are rough estimations

Species in the orders of Strigiformes, Falconiformes, Passeriformes,Coraciiformes, Columbiformes and Psittaciformes had reflexes with clear multifocal characteristics (Fig. 4D,F–H). The rings that indicate multifocality were less obvious, but present, in birds from the orders of Struthioniformes,Sphenisciformes, Anseriformes and Galliformes. Furthermore, both monofocal and multifocal optical systems were found in these orders(Table 1, Fig. 4C,E). Among the domesticated birds, monofocal optical systems were present in domestic goose(A. anser) and domestic chicken (G. gallus). By contrast,the homing pigeons (C. livia) had multifocal systems. In several species, the reflexes had intermediate characteristics with indistinct rings.

Pupil dynamics in (A) humans [Homo sapiens sapiens, data from De Groot and Gebhard (De Groot and Gebhard,1952)], (B) cats [Felis sylvestris broken line data from Wilcox and Barlow (Wilcox and Barlow,1975)] and mice [Mus musculus solid line data from Grozdanic et al. (Grozdanic et al.,2003)], (C) snowy owls (Bubo scandiacus, N=2),(D) Ural owls (Strix uralensis, N=2), (E) blue-fronted parrots (Amazona aestiva, N=2), and (F) grey parrots(Psittacus erithacus, N=2). Pupil size is given as percentage area of the fully opened pupil. No systematic differences between individuals of the same species of bird were observed, and pupil sizes were averaged over both individuals, 8–10 samples/intensity level. The gradient bar in A illustrates rod (scotopic)-, rod and cone (mesopic)- and cone (photopic)-based vision in humans. The steepest portions of the curves were compared by their first derivatives [f′(x)]. The responsiveness of the pupillary light reflex is very high in mice and similar tendencies are present in parrots. Humans, cats and owls have pupil dynamics of lower gain. Furthermore, the parrot pupils open fully at illumination levels comparable to human mesopic conditions while the owl pupils reach this state in dimmer, human scotopic illumination. The horizontal broken line marks the relative size of the innermost zone of the multifocal optics (the line in B applies to the mice eyes only). The lens system can be regarded as multifocal for pupil sizes that exceed this level. Error bars are standard deviations.

Pupil dynamics in (A) humans [Homo sapiens sapiens, data from De Groot and Gebhard (De Groot and Gebhard,1952)], (B) cats [Felis sylvestris broken line data from Wilcox and Barlow (Wilcox and Barlow,1975)] and mice [Mus musculus solid line data from Grozdanic et al. (Grozdanic et al.,2003)], (C) snowy owls (Bubo scandiacus, N=2),(D) Ural owls (Strix uralensis, N=2), (E) blue-fronted parrots (Amazona aestiva, N=2), and (F) grey parrots(Psittacus erithacus, N=2). Pupil size is given as percentage area of the fully opened pupil. No systematic differences between individuals of the same species of bird were observed, and pupil sizes were averaged over both individuals, 8–10 samples/intensity level. The gradient bar in A illustrates rod (scotopic)-, rod and cone (mesopic)- and cone (photopic)-based vision in humans. The steepest portions of the curves were compared by their first derivatives [f′(x)]. The responsiveness of the pupillary light reflex is very high in mice and similar tendencies are present in parrots. Humans, cats and owls have pupil dynamics of lower gain. Furthermore, the parrot pupils open fully at illumination levels comparable to human mesopic conditions while the owl pupils reach this state in dimmer, human scotopic illumination. The horizontal broken line marks the relative size of the innermost zone of the multifocal optics (the line in B applies to the mice eyes only). The lens system can be regarded as multifocal for pupil sizes that exceed this level. Error bars are standard deviations.

Most of the observed multifocal systems were bifocal, i.e. there were only two zones of different refractive powers(Fig. 4D,E). In all of these eyes, the outer zone of the bifocal system had bright upper sides, indicating hyperopic refractive state relative to the central zone(Fig. 4D,E). Most of the parrots – the grey parrot (P. erithacus), Tanimbar cockatoo(Cacatua goffiniana), yellow-crested cockatoo (Cacatua sulphurea) and blue-fronted parrot (A. aestiva) had more complex multifocal systems with several zones of different refractive powers(Fig. 4G,H).

Pupil shapes and dynamics

All of the birds studied had circular pupils, except for the emperor penguins (Aptenodytes forsteri), which had diamond-shaped pupils when they were strongly constricted, and some of the homing pigeons (C. livia), which had slightly oval pupils.

The parrots reached maximum pupil sizes at higher intensities than the owls(Fig. 5C–F). The parrots also had a more active pupillary light reflex (higher gain), thus opening their pupils within narrower ranges of intensities than the owls (blue-fronted parrot, gain=25.1 grey parrot, gain=30.4 snowy owl, gain=16.2 Ural owl,gain=10.5). The results from the birds were compared with data from humans(diurnal, circular pupil, monofocal gain=14.6), cats (nocturnal, slit pupil,multifocal gain=18.6) and mice (nocturnal, switching circular pupil,multifocal gain=52.5) (Fig. 5A,B). The owls had gains in a similar range to those in humans and cats. The parrots had higher gains, but not as high as mice(Fig. 5A–F).

In all studied birds, the border between the inner and outer refractive zones of the optical system was at about 50% of the maximum pupil area(Fig. 5B–F). The Ural owls did not close their pupils to fully block the outer refractive zone of the optical system. Within the illumination ranges used in the study, no bird closed the pupils to less than 30% of maximum pupil size and neither did the mouse (Fig. 5B–F).


The Pupil Chronicles

At the center of your eye is a colored globe called the iris. The black hole right at the center of the iris is called the pupil. The pupil takes in light, which helps to form an image on the retina at the back of the eye. The pupil and iris contract and expand in order to control the amount of light that falls on the retina. Under different lighting conditions, the pupil adjusts itself to maintain visibility. For example, in low lighting, the pupil expands to let in more light. Some animals are nocturnal, like owls, others are awake only during the day, and some are active both day and night, like cats. One reason that animals have different shaped pupils is to be able to see at their preferred time of day. However, it seems that controlling illumination levels is not the only reason for the difference in shape.

Another reason for modified shapes of pupils in different species is pure survival advantage. According to whether an animal is a predator or a prey, they need a visual system that aids them in their foraging activities. Each pupil shape has different attributes related to these two domains: illumination and ecological niche.


Predator or prey? The shape of animals' eyes holds the key, study reveals

It is often said that the eyes are a window to the soul and now research suggests that their shape can be used to distinguish between predator and prey in the animal kingdom.

A study by the University of California and Durham University found that animals with pupils shaped like vertical slits are more likely to be ambush-predator species such as cats and crocodiles.

Meanwhile, plant-eating “prey” species such as sheep and goats tend to have horizontal, elongated “letterbox” pupils. And circular pupils are linked to “active foragers” – animals that chase down their prey rather than creeping up and ambushing them.

The analysis of 214 species, which appears in the journal Science Advances, suggests that there are good evolutionary reasons for these differing optical designs.

Tests showed that eyes with horizontal-slit pupils offered an expanded field of view. Located on each side of the prey animal’s head, they provide a panoramic visual display that improves its chance of spotting approaching danger.

The slits also have the added advantage of limiting the amount of dazzling light from the sun, making it easier to see the ground.

“The first key visual requirement for these animals is to detect approaching predators, which usually come on the ground,” said the report’s lead scientist, Professor Martin Banks of the University of California at Berkeley. “They need to see panoramically on the ground with minimal blind spots. Once they do detect a predator, they need to see where they are running. They have to jump over things.”

The research found that vertical slits, meanwhile, give the predator the improved depth of field and the ability to judge distances that helps them secure their prey.

But the study also foundresearchers discovered that vertical slits only came into their own at ground level. For that reason, while shorter animals such as domestic cats have vertical-slit pupils, larger lions and tigers do not. Like dogs, their pupils are round.

Being tall, humans also had round pupils, as did most birds with an aerial viewpoint.

“A surprising thing we noticed from this study is that the slit pupils were linked to predators that were close to the ground,” said Dr William Sprague, a researcher on the Berkeley team. Among 65 frontal-eyed ambush-predators in the study, 44 had vertical pupils and 82 per cent of them had shoulder heights of less than 16.5 inches42cm, the research found.

The report also looked at what happens to the orientation of the horizontal pupil when the animal lowers its head to graze.

“If the pupil follows the pitch of the head down, they would become more vertical and the theory falters,” said Professor Banks about putting his ideas to one final test.

“To check this out, I spent hours at Oakland Zoo , often surrounded by school kids on field trips, to observe the different animals. Sure enough, when goats, antelope and other grazing prey animals put their heads down to eat, their eyes rotated to maintain the pupils’ horizontal alignment with the ground,” he said.

The research builds on earlier work by the late Gordon Walls, a Berkeley professor of optometry, who published The Vertebrate Eye and It’s Adaptive Radiation in 1942. This put forward the theory that slit-shaped pupils allow for a greater range in the amount of light entering the eye. But this is the first study to examine why the orientation of the slit – horizontal or vertical – matters.

Gaze of killer

The crocodile’s vertically slitted eyes help it judge distance at low levels, allowing it to move snappily to take its prey at speed.

Taking a wider view

The letterbox shape of the sheep’s pupils has the same effect as on a camera, giving it a panoramic view of its perilous environment.


Watch the video: Why Are Cats Pupils Slit Shaped? (August 2022).