Sunday, March 28, 2004

Masters of a waterless existence
Nutan Shukla

SOME small mammals are found in places where no drinking water is available. They meet their need for water from the moisture present in their food. Their physiology and behaviour ensure that their need for water as well as its loss from their body are minimal. The best known example is the kangaroo rat, which lives in the driest of deserts and often spends a lifetime without drinking water. Yet, it leads a far more active life than many animals.

The giant kangaroo rat of America collects seeds and buries them almost an inch deep in the soil over a large area. They remain there until they dry up, after which they are gathered and transferred to larders in the animalís burrows. They are then stored in cool subterranean tunnels. These air-dried seeds act as edible sponges by attracting and absorbing moisture from the earth and the atmosphere inside the burrow. Also, the bulk of the seed consists of carbohydrates. During the metabolic process, they release energy, carbon dioxide and water. This way, these small rodents obtain precious water from their food itself.

Now the question is: when they do not drink water how do they prevent loss of body fluid? Careful observation of these rodents, both in the wild and captivity, has shown that they adapt to a waterless life in several ways. First, they avoid moving out during daytime, when the outside heat demands the use of body water for temperature regulation. When the air temperature rises over 50`B0 C and the surface sand temperature touches 90`B0 C, these small mammals lie in their deep burrows where moisture level is high and the temperature oscillates between 25`B0 to 35`B0C. They only venture out during the cool desert night. This eliminates the need to sweat. Thus, these rats do not have sweat glands at all and lose virtually no water through evaporation from the skin.

Their other water conservation strategies include reduction of fluid loss from the lungs to a minimum during normal breathing. They achieve this by keeping the temperature of exhaled air very low. The exhaled air is passed over a cool nasal passage whereby the condensed moisture is absorbed and dry air is thrown out. The kidneys of kangaroo rats also conserve water by producing highly concentrated urine in which the salt content is twice as much as in seawater. They also pass extremely dry faeces due to which the rate of water loss is only one-fifth of that from the faeces of ordinary rats.

Deserts provide very few hiding places for animals. Therefore, an early warning system is essential for them to avoid being eaten or killed. Kangaroo rats have very fine hearing power. This is due to a big middle ear cavity, which enables the eardrum to vibrate more freely, magnify sounds by 100 times and also detect low-pitched sounds. This quality makes kangaroo rats an unusual breed among small rodents, who normally communicate through high-pitched sounds. The kangaroo ratís sensitive ears are able to catch the quietest movements of a predator, even the almost silent flapping of an owlís wings.

Different species of this rodent communicate differently. For instance, if a banner-tailed kangaroo rat, found in the southwestern deserts of the USA, wants to send information or warn other members of the group about impending danger, it starts drumming noisily with its feet on top of the sandy mound that contains its burrow. Each signal is a rapid tattoo, repeated so as to produce a recognisable sequence.

These small nocturnal rodents, which have many enemies and form the food of numerous desert predators, have thus mastered the art of surviving in a hostile, waterless environment.