What shape is the pupil of squirrels?

In the animal kingdom there is a striking variety of pupil shapes, with great variety among relatively close relatives. Cats have vertical slits. Siberian tigers have round pupils. Cuttlefish have a W shape. Goats have what I would call a horizontal bar. Many foxes have vertical slits, many dogs have round ones.

I just started paying attention to it and I was looking at some good pictures of both Japanese and European squirrels. I sometimes feel that I see a round irregularity at the centre, but can never be sure.

Here's a good photo of a squirrel eye showing the pupil being round.


Also, these acrylic squirrel eyes for taxidermy show a round pupil.


Weird pupils let octopuses see their colorful gardens

Octopuses, squid and other cephalopods are colorblind – their eyes see only black and white – but their weirdly shaped pupils may allow them to detect color and mimic the colors of their background, according to a father/son team of researchers from the University of California, Berkeley, and Harvard University.

For decades, biologists have puzzled over the paradox that, despite their brilliantly colored skin and ability to rapidly change color to blend into the background, cephalopods have eyes containing only one type of light receptor, which basically means they see only black and white.

Why would a male risk flashing its bright colors during a mating dance if the female couldn’t even see him but a nearby fish could – and quickly gulp him down? And how could these animals match the color of their skin with their surroundings as camouflage if they can’t actually see the colors?

According to UC Berkeley graduate student Alexander Stubbs, cephalopods may actually be able to see color – just differently from any other animal.

The key is an unusual pupil – U-shaped, W-shaped or dumbbell-shaped – that allows light to enter the eye through the lens from many directions, rather than just straight into the retina.

Human and other mammalian eyes have round pupils that contract to pinholes to give us sharp vision, with all colors focused on the same spot. But as anyone who’s been to the eye doctor knows, dilated pupils not only make everything blurry, but create colorful fringes around objects, what is known as chromatic aberration.

This is because the transparent lens of the eye – which in humans changes shape to focus light on the retina – acts like a prism and splits white light into its component colors. The larger the pupillary area through which light enters, the more the colors are spread out. The smaller our pupil, the less the chromatic aberration. Camera and telescope lenses similarly suffer from chromatic aberration, which is why photographers stop down their lenses to get the sharpest image with the least color blurring.

The unusual pupils of cephalopods (from the top, a cuttlefish, squid and octopus) allow light into the eye from many directions, which spreads out the colors and allows the creatures to determine color, even though they are technically colorblind. (Photos by Roy Caldwell, Klaus Stiefel, Alexander Stubbs)

Cephalopods, however, evolved wide pupils that accentuate the chromatic aberration, Stubbs said, and might have the ability to judge color by bringing specific wavelengths to a focus on the retina, much the way animals like chameleons judge distance by using relative focus. They focus these wavelengths by changing the depth of their eyeball, altering the distance between the lens and the retina, and moving the pupil around to changes its off-axis location and thus the amount of chromatic blur.

“We propose that these creatures might exploit a ubiquitous source of image degradation in animal eyes, turning a bug into a feature,” Stubbs said. “While most organisms evolve ways to minimize this effect, the U-shaped pupils of octopus and their squid and cuttlefish relatives actually maximize this imperfection in their visual system while minimizing other sources of image error, blurring their view of the world but in a color-dependent way and opening the possibility for them to obtain color information.”

U-shaped pupils

Stubbs has been fascinated by the color blind/camouflage paradox since he read about it in high school, and during diving excursions to Indonesia and elsewhere experienced firsthand how colorful cuttlefish, squid and octopus – and their surroundings – are.

He came up with the idea that cephalopods could use chromatic aberration to see color after photographing lizards that display with ultraviolet light, and noticing that UV cameras suffer from chromatic aberration. He teamed up with his father, Harvard astrophysicist Christopher Stubbs, to develop a computer simulation to model how cephalopod eyes might use this to sense color. The two will publish their hypothesis online this week in the journal Proceedings of the National Academy of Sciences.

They concluded that a U-shaped pupil like that of squid and cuttlefish would allow the animals to determine the color based on whether or not it was focused on its retina. The dumbbell-shaped pupils of many octopuses work similarly, since they’re wrapped around the eyeball in a U shape and produce a similar effect when looking down. This may even be the basis of color vision in dolphins, which have U-shaped pupils when contracted, and jumping spiders.

“Their vision is blurry, but the blurriness depends on the color,” Stubbs said. “They would be comparatively bad at resolving white objects, which reflect all wavelengths of light. But they could fairly precisely focus on objects that are purer colors, like yellow or blue, which are common on coral reefs and rocks and algae. It seems they pay a steep price for their pupil shape but may be willing to live with reduced visual acuity to maintain chromatically-dependent blurring, and this might allow color vision in these organisms.”

The big-fin reef squid Sepioteuthis lessoniana vividly changes color while signaling to members of its own species. (Photo courtesy of Gary Bell/

“We carried out extensive computer modeling of the optical system of these animals, and were surprised at how strongly image contrast depends on color,” said Harvard’s Stubbs, a professor of physics and of astronomy. “It would be a shame if nature didn’t take advantage of this.”

The younger Stubbs extensively surveyed 60 years of studies of color vision in cephalopods, and discovered that, while some biologists had reported an ability to distinguish colors, others reported the opposite. The negative studies, however, often tested the animal’s ability to see solid colors or edges between two colors of equal brightness, which is hard for this type of eye because, as with a camera, it’s hard to focus on a solid color with no contrast. Cephalopods are best at distinguishing the edges between dark and bright colors, and in fact, their display patterns are typically regions of color separated by black bars.

“We believe we have found an elegant mechanism that could allow these cephalopods to determine the color of their surroundings, despite having a single visual pigment in their retina,” he said. “This is an entirely different scheme than the multi-color visual pigments that are common in humans and many other animals. We hope this study will spur additional behavioral experiments by the cephalopod community.”

According to the new theory, the pupil of the cuttlefish Sepia bandensis maximizes chromatic blur, allowing the animal to detect color. (Photo by Roy Caldwell)

Stubbs noted that cephalopods may not be losing much color information by having only one type of photoreceptor, since red colors are blocked by water so that only a reduced range of optical light actually penetrates to the shallow depths where they live. Having one photoreceptor that responds to a broad range of colors at that depth would allow them to see in dim light with their pupil fully dilated, while the off-axis pupil maintains the potential for spectral discrimination in high-light conditions.

Intriguingly, using chromatic aberration to detect color is more computationally intensive than other types of color vision, such as our own, and likely requires a lot of brainpower, Stubbs said. This may explain, in part, why cephalopods are the most intelligent invertebrates on Earth.

The work was supported by UC Berkeley’s Museum of Vertebrate Zoology, a Graduate Research Fellow Program grant to Alexander Stubbs, and Harvard University.

Insights into the adaptive significance of vertical pupil shape in snakes

Present address: Department of Biology, University of Florida, 112 Bartram Hall, Gainesville, Florida, 32611-8525, USA.

School of Biological Sciences, The University of Sydney, NSW, Australia

School of Biological Sciences, The University of Sydney, NSW, Australia

Present address: Department of Biology, University of Florida, 112 Bartram Hall, Gainesville, Florida, 32611-8525, USA.

School of Biological Sciences, The University of Sydney, NSW, Australia

School of Biological Sciences, The University of Sydney, NSW, Australia


Pupil shape in vertebrates ranges from circular to vertical, with multiple phylogenetic shifts in this trait. Our analyses challenge the widely held view that the vertical pupil evolved as an adaptation to enhance night vision. On functional grounds, a variable-aperture vertical pupil (i) allows a nocturnal species to have a sensitive retina for night vision but avoid dazzle by day by adjusting pupil closure, and (ii) increases visual acuity by day, because a narrow vertical pupil can project a sharper image onto the retina in the horizontal plane. Detection of horizontal movement may be critical for predators that wait in ambush for moving prey, suggesting that foraging mode (ambush predation) as well as polyphasic activity may favour the evolution of vertical pupil shape. Camouflage (disruption of the circular outline of the eye) also may be beneficial for ambush predators. A comparative analysis in snakes reveals significant functional links between pupil shape and foraging mode, as well as between pupil shape and diel timing of activity. Similar associations between ambush predation and vertically slit pupils occur in lizards and mammals also, suggesting that foraging mode has exerted major selective forces on visual systems in vertebrates.

Table S1 Pupil shape, activity and foraging mode for the 127 snake species (5 families) used in this study.

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Pond Fisheries Management


Introduction for BugMobile vs. The Invasive Species


House Finch Conjunctivitis




Eye's Iris Directly Senses Light and Causes the Pupil to Constrict

In the eye, the pupil is the opening in the middle of the iris. It appears black because most of the light entering it is absorbed by the tissues inside the eye.

The shape of the pupil varies between species. Experimenting with mice, neuroscientists at Johns Hopkins Medicine report new evidence that the eye's iris in many lower mammals directly senses light and causes the pupil to constrict without involving the brain.

In a report in the June 19 issue of the journal Current Biology, the researchers detail how the pupils in a mouse's eyes get smaller when the animal is moved from a dark to a lit room even when the nerve connections between the animal's brain and eyes are severed. Their findings prove that mouse eyes have a photosensitive function built directly into the ring of sphincter muscle surrounding the pupil.

The mammalian eye adapts to changing light conditions by constricting or enlarging the pupil. That action, referred to as pupillary light reflex, is controlled by opposing dilator and sphincter muscles in the iris. "The traditional view of this reflex is that light triggers nerve signals traveling from the eye's retina to the brain, thereby activating returning nerve signals, relayed by the neurotransmitter acetylcholine, that make the sphincter muscle contract and constrict the pupil," says King-Wai Yau, Ph.D., a neuroscientist at the Johns Hopkins University School of Medicine and an author of the report.

But research by Yau and his team several years ago, building on earlier work by others, showed that nocturnal mammals and mammals active at dawn and dusk, such as mice, rabbits, cats and dogs, don't need the brain for this reflex to work. Instead, Yau's lab found that even when isolated, the sphincter muscle contracts in response to light, employing a light-sensitive pigment called melanopsin.

The simplest explanation for that observation would be that the muscle is itself sensitive to light. However, other researchers offered the alternative explanation that there might be light-sensitive nerve fibers containing melanopsin present on the sphincter muscle that make the muscle contract by piggybacking on the brain's pupillary light reflex circuitry involving acetylcholine.

This suggestion prompted Yau's team to ask what would happen if the action of acetylcholine were blocked. "Sure enough, even after blocking the action of acetylcholine pharmacologically, the isolated iris sphincter muscle still contracted in response to light, adding confidence to the notion that the muscle is itself light-sensitive because it contains melanopsin," Yau says of the present work.

To go further, they used genetics to selectively prevent muscle from making melanopsin. Indeed, this approach removed the effect of light on the sphincter muscle, even when acetylcholine's action was intact. "We thus have convincingly proven that the sphincter muscle is intrinsically light-sensitive, a very unusual property for muscle," Yau says.

It turns out lower mammals that are active during the day, such as ground squirrels, lack the local pupillary light reflex Yau's team found the same applies to primates, whether nocturnal or not. "The broad picture," Yau says, "is that the local pupillary light reflex appeared early in primitive vertebrates such as jawless fish, even before the brain got involved. In the course of evolution, the brain became involved in this reflex, but the local mechanism remained dominant in amphibians. By the time mammals appeared, the local reflex was progressively less important, becoming extinct altogether in subprimates that are active during the day and in primates. It's the local light reflex's absence in human beings that allows doctors to quickly evaluate whether a comatose patient is brain-dead by checking his or her pupillary light reflex."

Do squirrels ever lose their nuts?

We put Vinnie's question to zoologist Max Gray. Max - Yes is the short answer, but not as much as people seem to think they do. It's quite a common mistruth that squirrels forget about 50 per cent of their nuts which is not quite how it works. Squirrels are actually very good at remembering where they've left their nuts.

Kat - How do they remember? Do they mark it out?

Max - They remember. Exactly the mechanisms involved in this has been studied in a lot more detail in birds, in a bird called the Florida scrub jay by somebody called Nicky Clayton here in Cambridge actually. They used a combination of both relative and non-relative directions and cues and landmarks, and that kind of thing. But we also believe that squirrels use their sense of smell to assist them. They may be able to smell because they don't bury their nuts very deep as they may still be able to smell the acorns. But they inevitably don't retrieve some of them. But the important point is that if they don't retrieve the nut, that's not necessarily because they've forgotten where it is.

Kat - They're saving it for later.

Max - Well, you would imagine a squirrel going about preparing for winter is frantically running around in oak forests, stealing all the acorns and burying them all over the place. But you're going to prepare as a squirrel, you're going to want to prepare for an unusually long winter or a winter that starts earlier or in case some of your acorns get dug up by other squirrels which happens.

Kat - They get nicked. Do they nick each other's acorns?

Max - Yes. Actually, there's some evidence that squirrels will fake-hide their acorns. They'll kind of scurry about in the Earth and not put an acorn there if there's other squirrels watching them.

The eyes have it!

Deer have an arsenal of senses to outwit potential predators. Ears like radar dishes a nose that can sniff out a needle in a haystack and eyes that seemingly see the very air we breathe. All working in concert!

But today we focus on those big, brown, beautiful eyes. Let’s start with the nuts and bolts.

Deer have a much higher density of rods in the retina than cones. Rods are photoreceptors and therefore are more sensitive to light but are not sensitive to color. Cones provide color sensitivity and high resolution vision.

Rods are more than one thousand times as sensitive as cones to light. Rod sensitivity is shifted toward shorter wavelengths (green) of the color spectrum peaking sharply in the blue and respond very little to red. Rods are also better motion sensors than cones.

A deer’s eye is equipped with a membrane, the tapetum lucidum, which reflects light back through the receptor layer of the retina passing light through the receptor layer twice. A deer’s eye also lacks a UV filter, unlike humans.

Pupil shape also plays a role in visual acuity. There are 3 basic pupil shapes: vertical, round, and horizontal. A recent study looked at pupil shape compared to a terrestrial animal’s ecological niche. Generally speaking, prey species have horizontal pupils while ambush predators have vertical pupils.

Conforming to the norm, deer have horizontal pupils.

Lastly, there is the placement of the eyes on the head. A deer’s eyes are on the sides of the head giving it a wide field of view.

So what does this all mean?

Even though deer have less than half the number of cones in the eye as humans, deer can still distinguish among different colors. During low-light conditions, deer are likely more sensitive to the blue to blue-green portion of the spectrum (due to the high rod density). Studies indicate that deer are less sensitive to light of long wavelengths (orange and red) and rely upon their perception of only 2 colors – yellow and blue.

Deer are essentially red-green color blind (like some people). Deer can distinguish blue from red, but not green from red. Red, orange, or green all look the same to a deer. Because deer lack a UV filter in their eye, their sensitivity to short wavelength colors is enhanced in the UV spectrum.

When are these colors of light most abundant? When the sun is below the horizon at dawn and dusk. What a coincidence! When are deer most active? Deer can see “more” in these low light situations because of the color spectrum visible to them and because the light is passed through the receptor twice.

So if you want to be “invisible” to deer don’t wear blue! Wearing blue makes you stick out like a sore thumb. And don’t wash your clothes in laundry detergent with UV brighteners.

Want to know if you glow in the woods? Test your clothes with a UV or blue light. You may be surprised by what you find.

Ok, now you know what colors to wear (or not wear). Onto movement. Deer may not be able to focus sharply on fine detail but shift your weight wrong once in the stand and you are done.

That high density of rods makes for great motion detection. This coupled with the placement of the eyes on the side of the heads allow deer to distinguish distant objects across a 310 degree field of view without moving their head. But that means deer have poor depth perception.

But what happens when a predator is detected? While a wide field of view is great for predator detection, you need to be able to see where you are going…in the woods…with trees…and branches…and logs…and bushes…to avoid being eaten!

Ever misjudge the height of a curb you are stepping off or the closeness of the counter you are placing that full glass of adult beverage? Not good!

Enter pupil orientation! Horizontal pupils improve image quality for horizontal contours in front of and behind the animal. This solves the problem of rapid flight in a forward direction despite lateral eye placement.

Whew! Thank goodness for that. Can you imagine the number of deer running into trees or poking their eyes with branches?

There is one last cool thing about deer eyes. Well, it is cool if you don’t have an eye phobia like me. I can scoop brains, sift through rumen contents, and dissect rotting carcasses with barely a nose crinkle. But show me an eyeball and I am completely creeped out. And this aspect of deer eyes completely creeps me out.

It’s obvious that deer move their heads in many different directions. For the horizontal pupil to be most beneficial, it should maintain alignment with landscape horizontal. Landscape horizontal changes based on head pitch (nose up or nose down). To maintain this horizontal pupil orientation, the eyes should rotate about the optic axes in response to head pitch. Because the eyes are lateral on either side of the head, the rotation needs to be opposite in direction for each eye. This phenomenon actually has a name – cyclovergence.

Low and behold, compensatory cyclovergence with head pitch is observed in mammals with lateral eyes. Therefore a deer maintains all the benefits of a horizontal pupil whether it’s feeding or walking through the woods. Cool – Yes! Completely Freaky – YES!

So when you hit the woods this fall, I wish you luck. Because given all the tools a deer has to outsmart you, you’re going to need it.

-Jeannine Fleegle, biologist
Deer and Elk Section

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The shape of the pupil in the eye

Cats have vertical pupil, horses have horizontal pupil.
In the animal kindom, s-shaped, w-shaped and "beads on a string" shaped pupils also occur, why?

the pupil allows light into the eye, therefore the shape plays an important role in how images are received for processing by the brain - imagine them as you would a camera aperture. Pupils that can open very wide are able to let in lots of light (useful in the dark), small pupils let in much less light, but offer greater depth perception (useful for animals that need to pinpoint the location of a target - be it food or a position). Round pupils allow reconstruction of an image with minimum distortion (useful when navigating through a complex 3 dimensional space), slitted pupils impose a movement or pattern filter (useful for spotting predators or prey).

All of the various pupil patterns seen in animals are a response to the way in which they need to perceive the world around them. The pupil shapes have given them an advantage over other pupil shapes that are possible. If you combine factors you start to see why pupils are the shape they are. Apes are an example of animals with round pupils that aren't that big. This is so they can accurately see the 3 dimensional space around them and assess the distances needed to be travelled within it. Cat's vertical eyeslits can open very wide to allow activity at night, can close small for daylight activity and they probably filter signals to give importance to those objects passing across their field of view. Horse pupils are horizontally slitted, which may allow better recognition of obstacles as they move into sight at speed (so effectively in the vertical plane).

Those are some of the theories explaining pupil shape - they give us a useful mental tool box for addressing these questions, but it is very difficult to be certain that there are not other issues, such as limitations on developmental pathways in the evolution of eyes, so this is a far from exhaustive answer!

Structure of the Eye (With Diagram) | Receptors | Biology

In this article we will discuss about the structure of the eye, with the help of suitable diagrams.

The eye is one of the most important of the receptors. It provides us with information on dimensions, colours and the distance of objects in our environment.

How the Eye Produces a Focused Image:

1. Light rays from an object enter the transparent cornea.

2. The cornea ‘bends’ (refracts) the light rays in towards one another.

3. The light rays pass through the aqueous humour and pupil.

4. The transparent, elastic lens is altered in shape.

i. Fatter, to decrease its focal length, or

ii. Thinner, to increase its focal length.

This is called accommodation.

5. The relatively small amount of refraction now produced by the lens brings the rays to focus on the retina.

6. The retina contains light-sensitive cells:

(i) RODS which work well when light intensity is low, and

(ii) CONES which detect colour.

These cells are stimulated by the light of the image, and convert the light energy into electrical energy.

7. Electrical energy, in the form of an impulse, travels along the optic nerve to the brain.

8. The brain de-codes the impulse to produce the sensation of sight.

Other Important Facts:

(i) The image of objects that we are looking directly at (i.e. which are in the centre of our field of vision) falls on a very sensitive part of the retina called the fovea, or yellow spot. This region has far more cones than rods. Cones provide a picture with greater detail and in better colour.

(ii) There are no rods or cones at the point where the retina is joined to the optic nerve. Images formed on this part of the retina are not converted into impulses and relayed to the brain. This region is called the blind spot. We have blind spots in both of our eyes, but are not usually aware of them. Each eye records a different part of our field of view and covers the blind spot of the other.

The ability of the lens to change shape and focus on objects at different distances is called accommodation.

This ability depends on:

(i) The elasticity of the lens

(ii) The existence of ciliary muscles which are used to alter the shape of the lens

(iii) The suspensory ligaments which transfer the effect of the ciliary muscles to the lens.

The Value of having Two Eyes:

Apart from overcoming the effect of the blind spot, two eyes view the same picture from two slightly different positions. This provides vision in three dimensions, the ability to judge distance (and therefore speed), and offers animals a chance of survival even if one eye is damaged.

The ‘Pupil’ (or Iris) Reflex:

Bright light could seriously damage the delicate light-sensitive cells of the retina. The intensity of light falling on the retina is therefore controlled by the iris. It has an antagonistic arrangement of circular and radial muscles.

Materials and methods


Wild animals in captivity were used out of preference in order to avoid genetic problems that may be present in domesticated animals. We cooperated with a number of zoological gardens and animal parks in Sweden. Domestic animals were used if no wild animals were available for study in an interesting phylogenetic group. All animals were investigated unrestrained in their usual surroundings.

A total of 20 species from the following phylogenetic groups was investigated: amphibians (subgroup: anurans), reptiles (subgroups: geckos,snakes and crocodiles) and mammals (subgroups: rodents, artiodactyls,carnivores and primates). Except for the crocodiles, which both had slit pupils, at least one species in each subgroup had a circular pupil and at least one other had a different pupil shape. In some additional species, only pupil shapes were determined.


Eccentric slope-based infrared videorefractometry is a method to determine the refractive state of the eye in non-cooperative subjects such as human infants and animals (Schaeffel et al., 1987, 1993). If applied on human eyes, accuracy of measurement is ∼0.5 dioptres. Multifocal optical systems can be detected because different zones of the eye are focused at different distances if monochromatic light is used. Multiple focal lengths manifest themselves as ring-shaped structures in photorefractive images of the pupil(Kröger et al., 1999). We used a digital infrared-sensitive video camera (DCR-TRV 730E Sony, Tokyo,Japan) in combination with an infrared photoretinoscope consisting of four rows of infrared light-emitting diodes at eccentricities ranging from 5 to 23 mm (Kröger et al., 1999). The distance between the retinoscope and the studied subject was 2 m maximum. The experiments were performed in dim light, and infrared light was used to prevent pupil constriction. Acquired video sequences were loaded onto a computer and single frames grabbed as still images using Premiere 6.0 software(Adobe, San Jose, CA, USA).

Pupil shapes

Pictures of animal eyes were taken using a digital camera (DSC-F707 Sony)under lighting conditions eliciting pupil constriction. Where possible, a flashlight was used to induce eye shine. This was especially useful in animals with dark irises. In one case (Mus musculus), infrared illumination had to be used because of an almost perfectly black iris and small eye size(the camera's flashlight did not illuminate the eye at close distance).

Watch the video: Σκίουρος στο σπίτι. (January 2022).