Sunday, June 30, 2002		Nature

Rabbit is in the habit of fleeing into burrow
Nutan Shukla

CAMOUFLAGED animals resemble living or dead vegetation, soil, rocks and, in few instances, the dropping or faeces of other animals. The reason is their colours and patterns match the background well either by blending or by resemblance to a specific structure, such as a leaf.

Most camouflaged animals are active at night and remain motionless during daytime: indeed keeping still is probably as important in escaping detection as the camouflage itself. Rabbits run away the moment danger threatens, scampering as fast as they can into their nearby burrows, but hares, out in the middle of the field, do not. Instead, they freeze, relying on their ability to blend in with the grass. The young of many other herbivores do the same. They only flee at the last moment, when they are certain they have been spotted.

Camouflage is a very common means of defence, the simplest being countershading. Pelagic fish tend to be dark above the pale below. When they are seen from below their white underside matches the light from the surface, and if viewed from above the grey back blends in with the darkness of the deep. Grey-and-black stripes, like those on the mackerel, mimic the patterns made by scattered light as it passes through disturbed water.

In high altitudes or latitudes, animals adopt white fur or feathers in snow and mottled brown patterns when the snow has melted. Arctic animals, such as hares and ptarmigan, a stocky bird, change their coats with the season. In the spring when the snow cover is patchy, the pattern is matched by the fur colour on their backs.

The willow grouse goes one better: it not only takes on the colours of its background but, in the face of danger, shuts down its entire body. Heartbeat is reduced to a mere tick-over and breathing rate drops dramatically. In the resting state, the willow grouse’s heart pumps away at a comfortable 150 beats per minute, but if an Arctic fox approaches, the motionless bird reduces it to 20 beats per minute. Breathing rate drops by 70 per cent. This serves to reduce all signs of life to a minimum and suppresses the accidental release of any scent that might alert the predator. Dogs trained to seek out birds for research purposes have been seen to walk straight over a sitting bird. If, however, the predator discovers the grouse, a remarkable thing happens. In just one second, the heart rate suddenly accelerates from 20 beats to 600 and the bird explodes into the air.

No matter how well an animal is camouflaged, there is still the problem of concealing its shape. Animals are three-dimensional and since light normally comes from above the lower part would appear darker, making the animal conspicuous.

Camouflaged animals are generally palatable and although often common, they are easily overlooked by the human observer and presumably by their predators. Predators find camouflaged prey by constantly searching and repeatedly capturing those individuals which, for some reason, do not match their background as well as other individuals: they are quick to detect movements, and learned by experience. Familiarity with the local environment is useful to the predator in finding camouflaged prey.

There are a few species of camouflaged plants too that blend with dead vegetation and by doing so are overlooked by grazing and browsing animals that locate suitable food-plants by sight. Virtually all camouflaged plants are succulents or parasitic mistletoes growing in arid regions where browsing mammals occur. It is possible that there are many examples than at present known: the difficulty is in finding the plants in barren, arid surroundings and in appreciating the subtleties of their colouration and growth form.

Camouflage is used not only for the purpose of hiding from predators but there are predators that use this strategy to aid them in stalking the prey. In many predators (for example, mantids), the camouflage probably plays a dual role in that it both deceives their predators and conceals them from their prey.