Sunday, August 11, 2002		Nature

Scent of an animal
Nutan Shukla

WE think of the nose primarily as an organ through which we breathe, but the origin of the nose and nostrils was undoubtedly for smell. If we look at fish, particularly sharks, who have a very good sense of smell, we see that the nostrils lead into a small blind-end chamber — all the breathing in fishes is done through the mouth and the gill slits. Inside the chambers, the ‘surface epithelium’, which is the skin layer that has the olfactory cells, may be in a series of folds. By this means the number of sensory cells that can be contained is increased enormously. The cells are also very well supplied with nerves from the olfactory lobes in the brain. These lobes lie at the front of the brain and they are relatively large in animals with a good sense of smell, like the man-eating shark, the blacktip, where they are clearly the largest portion of the brain.

In the amphibians, the nostrils connect with the roof of the mouth. As the animal breathes, it is at the same time, taking in, a sample of air to be tested for any interesting smells. Frogs, and most reptiles, are, however, lacking a ‘true palate’ and the air and food intermingle in the mouth. The true palate, where the air is almost entirely shut off from the mouth and filters through a network of fine porous bones, is only found in the mammals. However, amphibians and many reptiles have developed a small pouch called ‘Jacobson’s organ’ leading off from the main respiratory passages. This small organ contains many olfactory cells.

The tongues of some snakes lie on pads in the mouth. When testing their environment, these snakes flick their tongues out of their mouths and pick up traces of scent. When they retract their tongues, the scent is brushed off on the pads, from where it passes into ‘Jacob’s organ’.

The sensation of taste in many vertebrates can be confused with that of olfaction. The catfish has a number of antennae-like barbels projecting in all directions from its head. These barbels contain assemblages of taste and touch sensors, and using them the catfish can follow a food source upstream. However, in most vertebrates the sense of taste is confined to areas within the mouth.

In contrast to catfish, an adult man has 100,000 taste buds, some 9,000 taste receptors, placed mainly on the peripheral parts of the top of the tongue. In children, the taste buds maybe much more numerous and distributed widely over the tongue and even on the cheeks.

How we taste is not really known, although it has been suggested that a substance which has a characteristic taste somehow depresses or excites the activities of enzymes — biological catalysts — which are known to be secreted by the cells surrounding the taste buds.

There are some strange anomalies with taste. Although all normal people can taste the four basic flavours, some people are unable to taste substances which to others have a distinct flavour; this is governed by inherited characteristics. Substances like saccharin, which we think of as sweet, are very bitter to the cat, which can not taste sweet substances. Animals are often sensitive to smells of their own species; the dog, like the tom cat that goes round its neighbourhood spraying musk, leaves its ‘visiting card’ to warn other dogs in the area of its presence. In one sense, the dog is staking out his territory. Many animals use musks or scents to indicate their presence.

Scent is also very important to colonies of animals and the animals itself. For instance, rats, too, live in colonies, and each member of the colony emits a distinct smell of the colony by which it is recognised by his fellow rats. Should he be an intruder or have his scent masked in some way, he is likely to be driven away or even killed.

Some social insects, like the ants, when they have found a source of food, leave a pheromone trail as their abdomen brushes the ground on the return journey to the nest. This trail attracts other ants of the nest to the food source; they themselves leave trails, and by this method maximum ant-power can be brought to play in the search for food.

Bees also taste the food brought back by foraging bees, and this stimulates them to join in an all-out effort to bring back food for the hive from the source. Bee communities depend on substances being passed all the time between one bee and another. The smell and taste of these substances determine the social behaviour of the hive. Indeed, the queen bee retains her status by secreting a special substance which is tasted by all the bees of the hive.

Going by the above-mentioned facts, the close relationship between smell and taste means that it is often more accurate to speak of a single’s sense of flavour’.

They are part of the same phenomenon, and man depends very much on both operating at the same time for a sense of flavour. Certainly, people who lack a sense of smell (anosmics) seem to have a different sense of taste from other people.

This feature was published on August 4, 2002