The terrestrial habitat lacks both water and salts in the surrounding medium (air), therefore, the terrestrial animals often face the problem of both water and salt losses.
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Water loss may be due to evaporation of water from the general body surface or through sweat or through urine.
Salt loss may be due to sweat and urine. This problem can only be solved by remaining in water and salt balance that is by maintaining a balance between water and salt loss and water and salt gain.
Therefore, the terrestrial animals usually drink large amount of water and develop various devices for the conservation of tissue water and feed on such food materials which are rich in salts so that the deficiency of salts may be compensated.
In terrestrial animals such as reptiles, birds, mammals and some crustaceans and annelids, the physiological adaptations to meet the osmotic problem are the following:
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(i) For water conservation the body is covered by means of a water proof covering which prevents the evaporation of water from the body.
The water proof body covering may be of horny scales (as in certain reptiles), feathers (as in birds) and hair or fur (as in mammals).
(ii) The loss of water through urine and faeces is checked by various mechanisms. In reptiles and birds the water from the faeces is absorbed by rectum and cloaca.
Further, in birds and mammal the water from the urine is absorbed by Henle’s loop in the uriniferous tubules of the kidneys.
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The terrestrials animals such as birds, snakes and lizards excrete a semisolid urine containing uric acid crystals, thus minimizing water loss.
Uric acid is quite insoluble in water, and it can be excreted without, the use of much water. The amount needed is the necessary to flush the uric acid into cloaca. Within the cloaca most of the water is reabsorbed and the waste is given off in paste form.
(iii) The animals of extreme terrestrial habitat, the dry hot desert, have developed special means of water conservation.
They have adapted to a lack of drinking water as they obtain water entirely from the metabolic reactions of which the water is by product, and the water contained in foods. For example, the kangaroo rat is found in some of the hottest and driest regions of the south western United States.
It eats only dry seeds and never drinks water. It has no sweat glands, so loses no water in sweat, but it must remain in cool burrows during the day to minimize the loss of moisture through the lungs.
It produces very dry faeces and excretes a very concentrated urine. With such adaptation, oxidative metabolism supplies all the water needed. Similarly camels can go for long periods without water in hot, dry deserts.
Their primary adaptation is their ability to withstand extreme body dehydration. A camel can lose as much as 40 per cent of the water in its body fluids and still survive, yet a man will die if he loses as much as 20 per cent.
A dehydrated camel appears very thin as if from starvation, but when he drinks, the body fluids are restored to their normal volume and his body size will return to normal in a short time.
(iv) In other animals the osmoregulation is performed by sweat glands, mouth, tongue, lungs and kidneys.