Drinking

• 1. Introduction

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  Drinking is the act of ingesting water or other liquids into the body, and in humans it happens through the mouth. The swallowing that begins it is finished by peristalsis, the muscular squeeze that pushes liquid down the esophagus to the stomach. Gravity does much of the rest of the work. But this familiar act is only one version of a process that looks radically different across the animal world. A cat and a giraffe both need water, yet almost nothing about how they get it is the same. Some creatures never drink at all. What follows asks how different bodies solve the same problem, why thirst exists in the first place, and what happens when a person drinks far too little or far too much.

• 2. How Mammals Drink

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  Cats, canines, and ruminants all lower the neck and lap water with their powerful tongues, yet the mechanics diverge sharply. A canine scoops water by shaping its tongue into a ladle and pulling the liquid into its mouth. A cat is stranger still. Only the smooth tip of its tongue touches the surface, then the tongue snaps back so fast that a column of liquid rises with it, secured when the mouth closes. Ruminants and most other herbivores take a different route entirely. They partly submerge the tip of the mouth and draw water in with a plunging action, the tongue held straight. Pace separates these animals too. Cats drink markedly slower than ruminants, animals that face greater natural predation hazards and cannot linger at the water's edge. The elephant abandons the tongue altogether. It draws water up into its trunk and squirts the load into its mouth, a method no other animal shares.

• 3. Living Without A Drink

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  Many desert animals never drink even when water becomes available, relying instead on the moisture inside succulent plants. Most free-roaming animals in general stay hydrated through the fluids in fresh food, and they learn to actively seek out foods with high fluid content. Captivity changes this. Terrestrial animals held by humans grow accustomed to drinking water by necessity rather than instinct. Cold and frozen places force their own solutions. Hares, tree squirrels, and bighorn sheep turn to snow and icicles when liquid water is locked away. The giraffe's method on the savanna has puzzled observers for its apparent defiance of gravity. The most recent theory suggests the animal's long neck works like a plunger pump. Even insects bend the rules. Onymacris unguicularis, a desert beetle, has evolved to drink substantially from nighttime fog, harvesting water out of the air itself.

• 4. Bills, Cuticles, And Gills

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  Most birds scoop or draw water into the buccal areas of their bills, then raise and tilt their heads back to let it run down. The common pigeon breaks this pattern. It can suck water in directly by inhalation, with no head-tilt required. Insects mostly skip drinking too, pulling enough water from their food. When dehydrated by a lack of moist food, many species will turn to standing water. All terrestrial insects also absorb a steady amount of the air's humidity straight through their cuticles. Water dwellers split along the line between fresh and salt. Amphibians and freshwater animals do not need to drink, taking water in continuously through the skin by osmosis. Saltwater fish face the opposite pressure. They drink through the mouth as they swim and purge the excess salt through their gills, taking in plenty of water and excreting only a small volume of concentrated urine.

• 5. Thirst And Its Failures

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  Thirst is the body's alarm, a desire to drink regulated by the hypothalamus in response to subtle shifts in electrolyte levels and blood volume. A falling level of total body water is called dehydration, and left unchecked it leads to death by hypernatremia. Care for it ranges from assisted drinking to oral rehydration therapy. The opposite danger is real too. Overconsumption of water causes water intoxication, dangerously diluting the body's salts. Overhydration turns up among athletes and outdoor laborers, but it can also signal disease or damage to the hypothalamus itself. A persistent craving for inordinate amounts of water is a psychological condition called polydipsia. It often comes with polyuria, and it may be a symptom of diabetes mellitus or diabetes insipidus.

• 6. How Much Water

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  The USDA frames the daily requirement as total water, counting moisture from beverages and foods rather than drinking alone. Its recommendation is 3.7 liters, roughly a gallon, for an adult male, and 2.7 liters, about three-quarters of a gallon, for an adult female. A competing figure circulates widely. Many nutritionists advise eight servings of eight fluid ounces of fresh drinking water per day, about 1.8 liters, though no scientific evidence supports that rule. Evidence-based hydration experts reject fixed numbers. They say the amount depends on ambient temperature, activity level, body size, and sweat rate. Research shows that simply drinking when thirsty keeps a person within about 2 percent of the needed level. Drinking beyond thirst may still help certain people, including those facing tasks of intense concentration, those with kidney disease, kidney stones, or urinary tract infections, and older people with a weakened sense of thirst.

• 7. The Other Meaning

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  The word drinking often stands in for something narrower, the consumption of alcoholic beverages. Most cultures throughout history have folded strong drink into their meals, celebrations, ceremonies, and toasts. The roots run deep. Evidence of fermented drinks reaches back to the Neolithic Period, and the first pictorial evidence appears in Egypt around 4,000 BC. From those beginnings grew the many drinking cultures established around the world today. The cost has grown alongside them. Alcohol consumption poses significant health risks, and alcohol abuse together with the addiction of alcoholism are common maladies in developed countries. Heavy consumption can lead to cirrhosis, gastritis, gout, pancreatitis, hypertension, and various forms of cancer, a list that places this everyday word at the center of serious illness.