James Mitchell Jr., age 14, of Gary, Indiana, for his question:

How does the kangaroo rat live without water?

The kangaroo rat is a rodent of the pocket mouse family. His pair of pockets are fur lined cheek pouches, used to tote food to his private storage bins. He survives on the arid prairie and thrives on a diet of the driest seeds. This is remarkable, because he drinks no water, perhaps throughout his entire life.

All living cells need constant streams of water to carry on their biochemical activities, and most animals need at least one good drink a day, usually after the main meal. But not the kangaroo rat. He can survive indefinitely without a sip. His home on the open prairie is a maze of sandy burrows among the tangled roots below a grassy tuft. Obviously he is specially adapted to survive without drinking, and his remarkable secrets have been investigated from all angles.

Actually he depends on a system of several adaptations. He performs certain moisture conservation chores for himself. But the major miracle depends on physical adaptations of his breathing, digestion and metabolism.

The kangaroo rat comes forth to forage at night, when the air is cooler and more humid. He packs his pockets with dry seeds and returns home to dine underground. Before the day warms up, he stuffs sand into his burrow exits, sealing in supplies of cool, humid air. These chores conserve the scanty moisture available on the arid prairie. But they do not provide his daily drinking water.

This part of the program depends on his special physical adaptations. Other animals lose waste body water through the kidneys and by breathing out warm moisture from the lungs. The kangaroo rat has superior kidneys that concentrate waste chemicals and recycle surplus water. His nose and lungs manage to cool the air and hence reduce loss of moisture. But even these features do not provide that needed drinking water.

Water is the main ingredient in most foods, and all animals extract moisture from their meat and vegetables. But apparently the kangaroo rat can do what they cannot. Biological evidence suggests that his remarkable systems of digestion and metabolism can create molecules of water from oxygen and certain foods. These biochemical miracles are highly complex and not fully understood.

The oxygen used to build water molecules seems to be taken from the air he breathes; the hydrogen comes from certain foods. The metabolism of protein foods takes a lot of energy, and the kangaroo rat selects a minimum of these diet items. He prefers starchy carbohydrates, and many of them have molecules that can be broken apart and used with oxygen to create molecules of water. So the kangaroo rat does not need to drink because he manufactures his own water.