Title: Vision in Animals
Author: Melvyn Greenberg
Questions often asked of veterinarians are:
- Why do cats see better at night?
- How sharp is my dog’s eyesight?
- Can my cat watch television?
- Is it true that dogs are colour blind?
- Or does my pet have colour vision?
Vision in animals is a complex sense that is affected by many factors.
In the retina, at the back of the eye, are specialized cells called rods and cones, also known as photoreceptors. Rods are better for night vision and cones utilized for day vision.
Cats and dogs, through evolution, have developed more rods for sensitive night vision, being mostly a nocturnal species, and had to sacrifice the number of cones thus rendering their colour vision inferior. It is postulated that these animals see six times better at night than humans do.
There are a fair number of reasons for better night vision:
- Amount of light entering the eye is increased over 5-fold in cats due to the size of the dilated feline pupil and the diameter of the cornea with respect to the animal’s size
- The tapetum lucidum is a disc structure at the back of the eye causing the eyes to reflect in the dark. It gives the eye its rich colour and acts as a mirror that reflects light back to the retina. The size of the eye and the function of the tapetum is of little importance during the day; it may, in fact, cause some scattering of light and blurring of vision.
- The increased numbers of rods are very sensitive to low light levels and can function in intensities thousands of times more than those required of cones. The rods undergo an adaptation to darkness through nerve and biochemical reactions
Colour vision is the speciality of cones in the eye’s retina. Four types of cones have been identified, with mammals having anywhere from one to all four cell populations. Rats have just one cone population limited to perceiving different shades of yellow light.
In species with more than one type of cone photoreceptors, richer colour vision is possible.
Dogs do not see in black and white. Dogs have two populations of cones; one population absorbs light in the blue-violet spectrum while the second absorbs light in the yellow spectrum. Dogs can, therefore, be likened to colour-blindness in people, who lack the green cone population; they can see colours but are unable to distinguish between red and green shades. This means that guide-dogs do not differentiate between red and green traffic lights and rather rely on changes of illumination to cross streets.
Similarly, horses and cattle have cones absorbing in blue and yellow wavelengths which means that bulls cannot perceive the red cloth colour used by bullfighters – they are stimulated by the movement.
Responses to rapidly flickering lights are generated by cones. Pets can perceive individual flickering images on television screens, which have a dramatic effect on their interest. They are thus attracted by the rapid flickering movements, thus making them excellent predators, and do not see what humans see.
Similarly dogs and cats can detect the flickering of fluorescent lights, a fact that must be taken in to account when designing the lighting of veterinary practices, animal welfare facilities, grooming parlours and boarding kennels.
Sharpness of vision is determined by a number of factors.
- Due to differences in anatomy and physiology of the lens, cats and dogs are incapable of changing the shape of their lens. Instead, they change its position in the eye by retracting it, to view distant objects, and moving it forward for viewing nearby objects. This reduces the accommodative capability by 75% in comparison to a human.
- Habitat and breed also influences the visual acuity e.g. outdoor cats and small breed dogs tend to be near-sighted while indoor cats and large breed dogs tend to be far-sighted.
It is estimated that a horse needs to be 7.5 metres from an object in order to see it as well as a person standing 11 metres away while the dog and cat are worse. A cat has to be more than 7 times closer to an object to see it as sharply as humans do. Dogs are halfway between horses and cats. This means that horses have better eyesight focus than dogs!
Compared to humans, animals have inferior colour vision, binocular vision, accommodative capabilities and visual acuity.
In cats, binocular and colour vision may be in a closer spectrum to humans than to dogs.
Animals have superior night vision and flicker detection. They are also likely to have better motion detection and low contract vision.
These properties enable dogs and cats to see well at night while people are left groping in the dark.