Birds and insects excrete uric acid rather than urea mainly because
- A. uric acid is more toxic and so must be removed faster
- B. they have no liver in which to make urea
- C. uric acid is almost insoluble and can be excreted as a paste, saving water
- D. uric acid contains no nitrogen
Explanation
Being nearly insoluble, uric acid can be voided as a semi solid with very little water, which suits animals that must conserve water and, in birds and reptiles, allows waste to be stored safely inside a shelled egg. Uric acid is in fact the least toxic of the three common nitrogenous wastes. Making it costs more energy than making urea, which is the trade off for saving water.
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