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Taking in water by mouth to quench thirst. Many aquatic animals take water in through the mouth, but this may play no role in homeostasis. True drinking applies to terrestrial animals that take in water to maintain their internal fluid balance. The water that is continuously lost through thermoregulation and excretion must be replaced. Many species are able to conserve water by reducing the rate of loss when they are dehydrated, but this does not make up for water already lost. Some desert-living species, such as the budgerigar (Melopsittacus undulatus), kangaroo rat (Dipodomys sp.), and gerbil (Gerbillus sp.), can live entirely on dry food without drinking water. They live on seeds that contain about 10% water.

Although most animals drink in response to various forms of dehydration, this is not the only, or even the most usual, everyday stimulus for drinking. Many animals drink with their meals, feeding behaviour being a direct stimulus for drinking. In this way, animals drink in anticipation of the possible dehydrating effects of food intake. Animals may also drink in direct response to temperature changes. The processes of thermoregulation often involve water loss, and some animals drink in anticipation of this loss. Many animals also drink out of habit, at a particular time of day, or in a particular familiar place.

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