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UPSC Prelims 1990 · Environment · Species Watch
When compared to the mammals, birds are less sensitive to temporary deprivation of water, because
- (a) The water requirement per unit weight is less in birds than in mammals.
- (b) Absorption of water from the gut is more efficient in birds than in mammals.
- (c) Urea excretion in mammals entails a greater loss of water than that in birds.
- (d) None of these
Show the answer and explanation
Answer: (c) Urea excretion in mammals entails a greater loss of water than that in birds.
Mammals excrete urea dissolved in water; birds excrete near-solid uric acid, losing much less water.
The idea
When the body breaks down protein, it is left with waste nitrogen. Animals get rid of it in three forms, and the form decides how much water is lost. Mammals make urea, which must be dissolved in water. Birds make uric acid, a white paste that needs almost none.
Fish: ammonia → needs a lot of water → Mammals: urea → needs moderate water (urine) → Birds, reptiles, insects: uric acid → almost no water
Why the others are wrong
(a) Birds actually have high metabolic rates.
Also know
The white part of a bird dropping is uric acid. This form also suits life inside an egg, where waste has to be stored without poisoning the chick.
🔒 Urea (mammals) needs water. Uric acid (birds, reptiles, insects) saves water.