A Staff Report from the Straight Dope Science Advisory Board

How do animals recognize water?

July 25, 2002

Dear Straight Dope:

Water is essential to all life as we know it, yet it is odorless, colorless and transparent. Of course, when I say "water," I mean pure H2O, which I've probably never consumed myself. This still strikes me as a paradox though. The neighbor's outdoor cat was apparently thirsty the other day and drank out of a puddle; another time, he was licking the condensation off of plants. If I were programming a cat's brain, I don't know how I would get it to recognize water when it sees it. I could see how to program it to recognize the bacteria and other life forms that were probably in the puddle, and also to recognize the peculiar bulbous forms of water droplets on leaves, but as these examples show, water can come in many forms--has evolution produced one mechanism that lets cat-sized animals zero in on water in the visual field?

Trying to guess how evolution has worked, or how another animal thinks or perceives, are both risky propositions.  But you can still make reasonable guesses if you ask the questions the right way. Ask yourself this, then: how do YOU recognize water? Most of all, how do you know when it's safe to drink?

The answer to the first question is that you assume something is or is not water based on context. To any animal in the world, the only context they know innately is the world as it would be without civilized humans in it (imagine, say, back in the time of Neanderthals). In that context, essentially the only liquid an animal could ever encounter is water, so the initial assumption any animal would make is that ALL liquids are water. How often could they be wrong?

The answer to the second question is that you get close enough to smell it. If it smells bad, you don't drink it. Evolution determines what you perceive as "bad"--water that one creature would drink might repel another, just as birds can eat habanero chilis without any trouble, but mammals cannot. That being said, you can still get deathly sick from water that smells just fine. Microbes like Giardia and E. coli don't necessarily make their presence obvious. But all that proves is that evolution never led us to develop the ability to taste Giardia. Not surprising, unless it was a matter of life and death, and for large numbers of people.

The bottom line? For simple things, nature tends to have simple solutions.

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