Natural selection is nature's mechanism for selecting characteristics that best adapt the animal to its environment. Mammals have evolved a sophisticated mechanism for eliminating nitrogenous wastes that involves

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Mammals have evolved the mechanism of forming urea in the liver and storing it in a concentrated solution in the urinary bladder before excreting it through the urethra. This mechanism is considered sophisticated because it allows mammals to excrete nitrogenous wastes efficiently without losing excessive amounts of water. Option A is incorrect because although some waste products can be secreted through cell membranes, this mechanism is not specific to mammals, nor is it relevant to the elimination of nitrogenous wastes. Option C is incorrect because producing and excreting uric acid is a mechanism commonly used by birds and reptiles, but not mammals. Option D is incorrect because while urea can indeed be recycled, it is not used as an energy source by mammals.

Natural Selection Is Nature's Mechanism for Selecting Characteristics That Best