How animals see the world?

Have you ever heard that dogs see in black and white? Or that cats can see in the dark? Why we have our eyes in front of the face? And why goats have an horizontal pupil? This article will answer these and other questions about the eyes and vision, focusing on mammals.

HOW IMAGES ARE FORMED?

The eyes are the receptors responsible for capturing light and sending the signal through the optic nerve to the brain, which make the interpretation. Light is an electromagnetic wave as infrared, ultraviolet, X rays, microwaves, etc. In this post we will refer to visible light, that is, the part of the spectrum that can perceive humans and most mammals.

Parts of the eye. Source

Basically, the light passes through the pupil. It can regulate the amount of light thanks to the muscles associated with iris (which gives color to the eye). The lens focuses the objects. The image is projected inverted in the retina, to be sent as an electrical signal to the brain.

WHY DO WE SEE IN COLOR ?

In the retina there are two main types of photoreceptor cells: cons and rods. The main differences are:

RODS

• More sensitive in a few light conditions
• No color vision
• Motion-sensitive
• Less image detail

CONES

• Activated under conditions of high light
• Color vision
• Contrast-sensitive
• High image detail

That’s why in low light, vertebrates see in black and white and the image is not clear, since the rods are activated at maximum but the cones are inactive. Some primates have three different kinds of cones (trichromatic vision), which correspond to the red, green and blue colour (RGB). Some primates and other animals have monochromatic vision (they only have one type of cone) or dichromatic (two). Some animals have tetrachromic vision, like birds.

The cones are sensitive to different wavelengths, different colors. Photo taken from Colombian Primatological Association

Generalizing a lot, diurnal vertebrates have more cones than rods and nocturnal ones have more rods than cones, allowing them to see better in the dark. But they can really see in the dark?

SEEING IN THE DARK

In total absence of light it is impossible to see, although some animals can detect other radiation such as infrared (snakes) or ultraviolet (bees). In addition to the relation between rods and cones, other factors that improve vision in low light conditions are:

THE CORNEA

The bigger the eye and the cornea, the better use of light. The mammal with the greatest cornea in relation to the eye is the Philippine tarsier (Carlito syrichta ) a nightlife primate.

Philippines’ tarsier (photo: Yeo Kok Leng)

THE PUPIL

Another way to take advantadge of few light conditions is increasing the size of the pupil. According to the shape of it, the control of incoming light is more precise: it is the case of many cats. Compared with a round pupil, the elongated one opens and closes sideways and according to the position of the eyelid, pupil surface exposed to light can be controlled better.

The felines with vertical pupil can open it horizontally and control better the entry light than with a circular pupil. Image of an unknown author, adapted from Aquàrium-Liège Museum

 

THE TAPETUM LUCIDUM

Cats, dogs, bats, horses, whales, crocodiles, cattle and some nocturnal primates have in the retina or behind it a bright layer called tapetum lucidum, which increases up to 6 times the light gathering ability compared to humans. As if it were a mirror, the tapetum lucidum reflects the light reaching the eye to return back to the retina and harness light to the maximum.

Reflection of light due to the tapetum lucidum. Image taken from Exclusively cats

The tapetum lucidum is responsible for cat’s eyes appearing to glow in the dark and cat and dog’s pupils shine in blue/green when light falls upon the eye.

Tapetum lucidum shining on a dog. Photo Mireia Querol

WHY SOME ANIMALS HAVE THE EYES IN FRONT OF THE FACE WHILE OTHERS HAVE THEM ON THE SIDES ?

The position of the eye in mammals can be frontal, like a cat, or in the side, like a rabbit. This means distinct advantages:

• Binocular vision (stereoscopic): allows a good estimation of distance, but the field of view is smaller. A 3D image is generated. It is typical of carnivores that should focus attention to their prey or primates that should calculate the distance between the branches.
  
• Side vision (peripheral): allows each eye to send a different signals to the brain, so it is easier to notice their surroundings having a field of view of about 360 degrees. It is typical of herbivores, which must pay attention to the presence of potential predators .

  

  Visual field of a cat and a horse. The blind area is smaller in hervibores. Source: Sjaastad, Sand and O. Hove K. Photo taken from Eye Opener

WHY GOATS HAVE AN HORIZONTAL PUPIL?

In addition to the position of the eyes, the shape of the pupil is also related if you are a predator or a prey. Goats or horses have horizontal pupils, while cats like the margay have it vertical.

Pupil of a goat (horizontal) and a cat (vertical) Photo: Wikimedia Commons

Banks  says that “to calculate distances predators basis on stereoscopic vision (works better with a small pupil) and sharpness (works best with a larger one). Vertical pupils are small horizontally and large vertically”.

In the case of terrestrial prey attacked by predators, the tendency of the pupil is being horizontally because “can gather more light and and also reduces the sunlight, which could dazzle “. Exceptions such as rabbits or mice with a circular pupil, are because they have to pay attention also to the sky, from where a bird of prey can attack.

WHAT IS THE THIRD EYELID?

Some animals have the nictitating membrane (“third eyelid”), a transparent or translucent membrane that is used to protect and moisten the eye without losing visibility. Camels, seals and polar bears have it complete, whereas in other mammals, such as dogs or humans remains only reduced.

Nictitating membrane in a feline. Photo by Editor B

IS IT TRUE THAT DOGS AND BULLS SEE IN BLACK AND WHITE ?

Actually dogs and cats are able to detect colors, particularly gray, yellow and blue in softer tones. Cats may be able to perceive more colours.

Visible spectrum by a dog and a human. Source

In the case of bulls, it is also spread the myth that rage against the red colour or see in black and white. Actually bulls have dichromatic vision, like most diurnal mammals, since they only have blue and green cones. Therefore, they can’t see red, but it does not mean they see in black and white.

AND OTHER MAMMALS?

Horses see in blue and red tones. Most rodents see in black and white. Most species of the family of goats, sheeps and bulls see from green to violet. In addition, recent studies indicate that many mammals (especially nocturnal ones), contrary to what was believed, also can perceive ultraviolet radiation: rats and mice, reindeer, possibly cats and dogs, cows, pigs, ferrets, okapi…

We finish with a BuzzFeed video with the simulation of vision of some animals. If you have more questions about animal’s vision leave it in the comments!

REFERENCES

• Partes del ojo
• Aprovechar la noche: se descubre el misterio de la visión nocturna de mamíferos
• Por qué los felinos tienen las pupilas rasgadas
• La guerra entre las pupilas verticales y horizontales
• Comparative morphology of the tapetum lucidum. American College of Veterinary Ophthalmologists. Veterinary Ophthalmology (2004) 7, 1, 11–22.
• La visión en primates: No todos vemos el mundo del mismo color
• Muchos mamíferos ven radiaciones ultravioletas
• Revista electrónica de veterinaria
• Cover image: unknown author