 |
 |
 |
 |
 |
 |
COLOURS IN NATURE
Do animals see colours?
It is often said that animals are colourblind. However, this question
is still a big debate among veterinarians. It's quite certain that cows
and sheep are colour-blind, dogs and cats have some colour sense and birds
have excellent colour vision. Rods and cones are the components of an
animal's retina, just like in humans, that work at interpreting images
for our brain. Rods are dedicated to light reception and work very well
in low light: some nocturnal animals have only rods. Cones are responsible
for colour vision and require lots of light to function well. That's why
in darkness it's very hard to make out colours.
The role of colour vision in an animal's perception, behavior, and ecological
setting, and its underlying retina and neuronal mechanisms vary enormously
in different groups of animals. Although colour perception plays a great
role in human life, there is still little evidence about the roles it
in nonhuman animals. Current research finds that colour vision in non-primate
mammals is very limited, and probably bears little resemblance to humans'.
Nevertheless, it is proven that animals see some form of colour vision,
as in the following examples:
Experiments in 1969 trained dogs to choose between different
coloured dog dishes, and in more recent experiments, dogs were able to
differentiate coloured circles.
This Costa Rican red-eyed treefrog has a very limited
visual pathway; its neural behavior and jumping reflex suggest it only
"sees" its prey if it is moving.
Eyespots on the Tanzanian caterpillar. Head coloring and fake eyes resemble
those of a snake, warning off predators. Real eyes are
small, barely visible, located near the snout. Earthworms and caterpillars
have "eye spots," and see only light and dark, helping them
stay out of the hot sun.
Comparison
of wavelengths visible to humans and bees. The range of vision
for the bee and butterfly extends into the ultraviolet. The leaves of
the flowers they pollinate have special ultraviolet patterns which guide
the insects deep into the flower.
Birds have an elaborate variety of cones cells, with
more morphological complexity and diversity than those of mammals.
Some animals see many more colours than humans do, such as pigeons. Birds
have an excellent colour vision: the structure of their retina is also
more complex than humans’, as illustrated in the picture.
Butterflies can see many different colours in a wide
variety of shades. Butterflies can use their colour vision when searching
for food. Butterflies are considered to have the widest visual range of
any animal. To the human eye many butterflies appear the same, but the
butterflies themselves can often identify each other quite easily from
ultraviolet markings.
Butterflies tend to avoid the colour green in their feeding behavior,
but are attracted to it during egg laying. The next generation needs to
be placed near a good source of food as caterpillars have a voracious
appetite. The green photoreceptors are instead used for the detection
of movement.
A
butterfly can be trained to land on a yellow portion of the coloured grid.
Moreover, butterflies have colour constancy (like humans) and will still
recognize yellow even if the illuminating light changes considerably (colour
constancy).
In nature colours are used for different purposes. For instance the colour
of animal fur or skin is usually adapted to the environment in which the
animals live. They need a discrete colour in order to be invisible for
enemies or preys. You will never see a blue rabbit or a pink panter because
this would make the animals an easy prey for (other) predators. A well-known
but special example is the chameleon, the reptile that is able to, adapt
the colour of its skin to its environment.
Birds however often have prominent feather colours: this is usually to
attract the opposite sex for mating.
Plants have evolved their flowers to make it easy for butterflies, and other pollinators to feed on their nectar. Plants attract potential pollinators in many ways, including colour, scent, reflectance, size, outline, surface texture, temperature and motion. Plants which do not depend on insect or bird pollination are unlikely to have showy or scented flowers. To attract the potential pollinator to that particular blossom, availability of nectar has to be advertised to the butterfly. This is displayed in the colour of the petals. The colour of the nectar guide changes from yellow to red when nectar is no longer in production.