The title of this very short post is widely known in our trade as “Orgel’s Second Rule” after evolutionist Leslie Orgel. Of course the Rule doesn’t mean that natural selection is conscious or has a pre-planned goal or outcome: simply that sometimes natural selection can achieve a result so wild and unexpected that it looks as if there was a clever mind behind it. (I say this so that the ID types won’t attribute to me a “mind” behind evolution.)
I lectured about this during this trip when explaining how, in Antarctic “icefish”, the gene making the enzyme trypsinogen—produced by the pancreas to help digest food in the intestine—became duplicated over evolutionary time, and thereafter one duplicate became rearranged by natural selection so it produced a bizarre glycoprotein having a small amino-acid sequence repeated many times over. That sequence allows the new protein (the old one’s still there) to glom onto the small particles that would enable ice to form and grow in the body. It is an anti-freeze protein that, produced in large quantities, reduces the freezing point of the fish’s blood below the -1.9° C of Antarctic waters. Therefore the fish’s cells don’t freeze and it can survive in water that would kill most other fish.
We know the evolutionary source of this antifreeze protein because it bears, at the beginning and end, the “start” and “stop” bits of the DNA that makes trypsinogen. Those “vestigial sequences” are evidence of evolution—of the ancestry of the antifreeze protein.
But I’ve digressed. I wanted to talk about something I just realized. I took my daily half-hour constitutional on the top deck (daily when the weather’s good, that is), and came back thirsty. It was gloriously warm and sunny outside, and I grabbed my aluminum water bottle from the cabin fridge. (We all get these bottles to save plastic, as Hurtigruten is green.)
I don’t like “hydrating”, but this time I needed to. And this time, because I had a thought, I timed how long it took me to take the several deep swallows needed to slake my thirst. It was about five seconds.
Now over those five seconds, my body didn’t have time to absorb and use the water that it needed. The thirst, of course, is a signal that your body needs water. But what struck me is this: I drank sufficient water for my body’s needs before those needs had even begun to be satisfied!
In other words, not only is thirst a way of telling you need water (and hunger for food and so on), but the slaking of thirst is a way of telling you when you’ve had enough water. It’s as if by simply ingesting a medicine you need, you’re cured before the medicine even does its thing. Or it’s as if you have an infection, and the infection starts to go away five seconds after you swallow your antibiotic. Or it’s as if your headache went away the minute you swallowed your aspirin.
Of course, we didn’t evolve to take antibiotics, but we did evolve to drink water when needed. And natural selection has been clever enough to find a way to tell us that we’ve drunk enough before that water has entered our system.
Presumably the “that’s enough” reflex evolved because you don’t want to drink too much water. You could get bloated, or perhaps a predator is lurking nearby to snatch you as you guzzle from the water hole. I don’t know how it happened, but it did happen.
Maybe this seems tedious and obvious to you, but it still amazes me. What mechanism operates in your throat and stomach to let you know that you can stop drinking?
UPDATE: Well of course biologists have thought about this problem before; I certainly didn’t think I was the first. And, sure enough, in the first comment below Cyrus Martin, senior editor of Current Biology, steered me to a paper in his journal that dealth with the issue, “Thirst,” by David Leib et al.
Here’s the most relevant part, but the article has lots of information about the generators of thirst and the physiological distress it signals:
Drinking quenches thirst in anticipation of water absorption
There is a delay of tens of minutes between the ingestion of water and its full absorption into the bloodstream. However, drinking can quench thirst within seconds, long before the ingested water has had time to alter the blood volume or osmolality. Thus, thirst is not quenched by the reverse of the process that generates it; instead, the brain terminates thirst by using sensory cues from the oropharynx to track ongoing water consumption and then estimate how this water intake will alter fluid balance in the future, after the water has been absorbed. These anticipatory signals are transmitted from the oral cavity to the SFO [JAC: the “subfornical organ” i the brain that detects changes in blood osmolarity and partly generates the “thirst signal”] where they inhibit the same thirst neurons that respond to change in the blood volume and osmolality. This circuit organization allows SFO thirst neurons to make a comparison between the physiological need for water, which they measure directly by monitoring the blood, and the amount of water that has recently been consumed, which they measure by tracking oropharyngeal signals of fluid intake. SFO thirst neurons compare these two values to decide when drinking should be terminated. It is likely that a similar integration occurs within other structures of the lamina terminalis that control drinking behavior and hormone release.
That IS clever, isn’t it? But wait—there’s more:
The specific oropharyngeal mechanisms that are used to track water consumption are not well defined. One signal appears to be temperature, because cold liquids inhibit SFO thirst neurons more efficiently than warm liquids, and oral cooling alone can reduce both thirst and the activity of these SFO cells. One explanation for this temperature-dependence is that water ingestion tends to cool the oropharynx, and as a result animals may learn to associate changes in oral temperature with the post-ingestive effects of water consumption. In addition to temperature, other somatosensory signals that report on the sensation of water in the oral cavity are likely to be important. There is also evidence that signals from further down the gastrointestinal tract, such as stretch receptors and osmosensors in the stomach, may play a role in thirst satiation. In none of these cases, however, is the identity of the relevant sensory neurons and the neural pathway by which they transmit information to the lamina terminalis clear.
I was just wondering last night why cold water is so much more desirable when you’re thirsty, and why people don’t just guzzle lukewarm water when they’re thirsty. This temperature-dependence, as it says above, may be a phenomenon involved with learning the relationship between temperature of water and how well your body is satisfied with the water. But, as usual, we don’t know the full answer.
I was just wondering last night why cold water is so much more desirable when you’re thirsty, and why people don’t just guzzle lukewarm water. This temperature-dependence, as it says above, may be a phenomenon involved with learning the relationship between temperature of water and how well your body is satisfied with the water. But, as usual, we don’t know the full answer.















































































