Jounior Observer | Sunday Observer.lk

Last week we enlightened you about the factors influencing the colour vision of animals, including humans, and how your beloved pets - cats and dogs may be seeing the world. Perhaps, you learnt many interesting facts you were not aware of through that article titled ‘Do all animals see in colour?’ which appeared under Nature trail.

Did you make any attempts to observe how your pet’s eyes change during daytime and night time? If you have not seen the difference in their eyes during these two time periods so far, try to observe them when you can. You’ll be fascinated by the way the pupils change.

As we promised, this week we bring you more information about the colour vision of some other animals.

You will be surprised to learn that our feathered friends - the birds can see a greater range of colours than humans, including ultraviolet. Colour is more vibrant in a typical bird vision than in ours. For example, pigeons are among the best at colour detection with the ability to see millions of hues. They have this ability because they have more cones than humans, and can see five spectral bands, including ultraviolet light.

Hunting birds such as the kestrels, eagles and vultures are believed to have outstanding binocular vision.

Like the birds, insects too have good colour vision. What they perceive (obtaining knowledge of things through the senses) is very different to what humans see because most have dot-like, compound eyes known as ommatidia (simple cone-shaped lenses).

If you are wondering what’s special about compound eyes, let us explain. They have thousands of tiny lenses placed in a honeycomb pattern. Each lens makes up a small part of the overall picture.

Some insects have 30,000 lenses per eyeball. For instance, honeybees have very complex eyes - with thousands of lenses. Within each ommatidium, the visual cells that detect colour are arranged like the segments of an orange around the core.

Colour perception among insects varies depending on the insect. Some, like butterflies see more colours while others see less.

Do you know that mosquitoes and flies may also be seeing the world in colour? It has been observed by scientists that they are attracted or repelled by specific surface colours and coloured sources of light. However, they claim that the surface colours they seem to prefer do

not necessarily co-relate with the coloured sources of light they are attracted to.

Among the marine friends, the shark is said to have eyes that are very similar to that of humans, comprising a retina, lens, iris, cornea and pupil each. But when it comes to colour detection, they cannot do so because they have no retinal cones.

Their eyes are more designed to see in murky (dark, unclear) waters and pick up light, than colours. They can use this extra light they gather to see objects as far as 60 feet away. But still, the vision is not as acute (sharp) as ours. Meanwhile, shrimps and crabs are said to have the least developed vision even though they too have compound eyes.

The mantis shrimp (a predatory crustacean) has complex eyes with more than double the number of ommatidia found in a bee. It has about a dozen photoreceptors.

When it comes to monkeys, many Old World monkeys and apes are able to see colours like humans, even though most New World monkeys can’t. Scientists say that many species of monkeys have six different types of colour blindness (seen mostly among males than females).

Horses, are another species of animals humans are close to, especially those who are into horse riding. It is known that horses have a good vision range even though they cannot see right in front of them.

Literally, they can’t spot what’s between the eyes and directly ahead. That’s why they generally look down as they walk.How do you think snakes see? They use two sets of eyes to see! One set is the normal pair of eyes that’s visible even to us, and the other, the pit organs or vision pits. They can detect colours very well with their normal eyes.

The pit organs are used to detect heat, like infra-red heat. It is in the night that the snake uses its pit organs to ‘see’ heat signals coming out from warm-blooded creatures. Once the snake detects infra-red signals, there is no getting away from it.

So, watch out if you are going about in the nights in areas where snakes are known to wander.Now that you know there are so many animals out there who can or cannot see colours as well as we do, try to find out more about their vision spectrum.

You’ll be amazed at what you discover - like for instance, that bulls are colour blind and they only charge at the red-cape dangled before them at bull fights only because of its movements and not because it’s red!

How we see colour

You may have noticed how when light passes through a prism (a clear glass object with many sides) it separates light passing through it into the colour of the rainbow. Every colour has a different wave length.
The human eye and brain work together to translate light into colour.
The whole process begins in the retina of the eye. The retina contains millions of cells called photoreceptors which are sensitive to light. The two types of photoreceptors we have-rods and cones process light into nerve impulses and pass them along to the cortex of the brain. This is done via the optic nerve.
As rods or rod-shaped cells transmit mostly black and white information to the brain, they are responsible for helping you adjust to the darkness of a room when you first enter it. The cones are responsible for the higher levels of light intensity that help create the sensation of colour.
Cone cells come in three types and are each sensitive to the long, medium or short wave length of light (red, blue and green).
These cells working with the connecting near cells, give the brain the necessary information to interpret and name the colours we see!

* The human eye has at most 200,000 cones per square millimetre, but a house sparrow

* Bees and birds who see in ultraviolet (UV) light would see red as black and solid as striped.

* Bees also have trichomatic vision as they too have three different types of cone cells. But, the difference is that they are most sensitive to yellow, blue and ultraviolet wavelength of light.

* Many plants have UV coloured lines known as nectar guides, visible only to creatures that can see UV radiation. However, these lines are not present in plants pollinated by the wind or animals who do not see in UV light.

* Like bees, butterflies too see UV light. Some species even have the ability to differentiate between genders since they have different markings visible only to UV light. For example, both genders of clouded yellow butterflies may look the same to us, but to each other, they’ll appear totally different.

* Bees see about the same colour range as we do, but it is shifted toward the shorter wavelength.

* Snakes have rods and cones in their eyes like humans do, but in different numbers.

* Lizard, (including geckoe and turtle) retinas contain multicoloured oil droplets in their photoreceptors. So, they can perceive colour.

* Reptiles usually have smaller eyes than diurnal ones, but they have relatively large pupillary and lens aperture and cornea.

This improves their light-gathering ability, but reduces visual acuity(ability to focus).

* Trichomatic colour vision depends on the presence of three types of cone photopigment.

* New World monkeys (Platyrrhine parvorder) lack the trichomatic vision of Old World monkeys.