Drinking

Most animals drink water to maintain bodily hydration, although many can survive on the water gained from their food. Water is required for many physiological processes. Both inadequate and (less commonly) excessive water intake are associated with health problems.

Methods of drinking

In humans

When a liquid enters a human mouth, the swallowing process is completed by peristalsis which delivers the liquid through the esophagus to the stomach; much of the activity is assisted by gravity. The liquid may be poured from the hands or drinkware may be used as vessels. Drinking can also be by sipping or sucking, typically when imbibing hot liquids or drinking from a spoon. Infants employ a method of suction wherein the lips are pressed tight around a source, as in breastfeeding: a combination of breath and tongue movement creates a vacuum which draws in liquid.

In other land mammals

By necessity, terrestrial animals in captivity become accustomed to drinking water, but most free-roaming animals stay hydrated through the fluids and moisture in fresh food, and learn to actively seek foods with high fluid content. When conditions impel them to drink from bodies of water, the methods and motions differ greatly among species.

Cats, canines, and ruminants all lower the neck and lap in water with their powerful tongues. Cats and canines lap up water with the tongue in a spoon-like shape. Canines lap water by scooping it into their mouth with a tongue which has taken the shape of a ladle. However, with cats, only the tip of their tongue (which is smooth) touches the water, and then the cat quickly pulls its tongue back into its mouth which soon closes; this results in a column of liquid being pulled into the cat's mouth, which is then secured by its mouth closing. Ruminants and most other herbivores partially submerge the tip of the mouth in order to draw in water by means of a plunging action with the tongue held straight. Cats drink at a significantly slower pace than ruminants, who face greater natural predation hazards.

Many desert animals do not drink even if water becomes available, but rely on eating succulent plants. In cold and frozen environments, some animals like hares, tree squirrels, and bighorn sheep resort to consuming snow and icicles. In savannas, the drinking method of giraffes has been a source of speculation for its apparent defiance of gravity; the most recent theory contemplates the animal's long neck functions like a plunger pump. Uniquely, elephants draw water into their trunks and squirt it into their mouths.

In birds

There are three general ways in which birds drink: using gravity itself, sucking, and by using the tongue. Fluid is also obtained from food.

Most birds are unable to swallow by the "sucking" or "pumping" action of peristalsis in their esophagus (as humans do), and drink by repeatedly raising their heads after filling their mouths to allow the liquid to flow by gravity, a method usually described as "sipping" or "tipping up". The notable exception is the family of pigeons and doves, the Columbidae; in fact, according to Konrad Lorenz in 1939:

one recognizes the order by the single behavioral characteristic, namely that in drinking the water is pumped up by peristalsis of the esophagus which occurs without exception within the order. The only other group, however, which shows the same behavior, the Pteroclidae, is placed near the doves just by this doubtlessly very old characteristic.

Although this general rule still stands, since that time, observations have been made of a few exceptions in both directions.

In addition, specialized nectar feeders like sunbirds (Nectariniidae) and hummingbirds (Trochilidae) drink by using protrusible grooved or trough-like tongues, and parrots (Psittacidae) lap up water.

Many seabirds have glands near the eyes that allow them to drink seawater. Excess salt is eliminated from the nostrils. Many desert birds get the water that they need entirely from their food. The elimination of nitrogenous wastes as uric acid reduces the physiological demand for water, as uric acid is not very toxic and thus does not need to be diluted in as much water.

Most birds scoop or draw water into the buccal areas of their bills, raising and tilting their heads back to drink. An exception is the common pigeon, which can suck in water directly by inhalation.

In insects

Most insects obtain adequate water from their food: When dehydrated from a lack of moist food, however, many species will drink from standing water. Additionally, all terrestrial insects constantly absorb a certain amount of the air's humidity through their cuticles. Some desert insects, such as Onymacris unguicularis, have evolved to drink substantially from nighttime fog.

In marine life

Amphibians and aquatic animals which live in freshwater do not need to drink: they absorb water steadily through the skin by osmosis. Saltwater fish, however, drink through the mouth as they swim, and purge the excess salt through the gills. Saltwater fishes do drink plenty of water and excrete a small volume of concentrated urine.