Learning to be thirsty

Baby rats, only 5 days old and still very much reliant on their mothers for food, can be artificially dehydrated by injecting them with a saline hypertonic solution. If a source of water is placed very close to the rat's snout, it will drink. But 21-day-old rats who have just been weaned from their mothers and who readily eat and drink on their own can be injected with the same saline hypertonic and won't drink any more than non-dehydrated rats the same age. The difference is that the older rats still have to decide to drink—the water is available in their cages, but they still must actively seek it in order to consume it. It's the difference between consummatory and appetitive behavior—to have an appetite, we must seek out food and water, not just consume it when it's placed in front of us. By the age of 35 days, rats have learned to drink when dehydrated:

W.G. Hall, H. Moore Arnold, and Keven P. Myers, in a study in Psychological Science, systematically tested rats by injecting half them with the saline hypertonic and removing the water from their cages for 60 minutes. When the water was returned, the dehydrated 21-day-old rats drank at the same rate as non-dehydrated rats their same age. But dehydrated 35-day-olds quickly consumed over four percent of their body weight in water. Clearly, in the two extra weeks following weaning, somehow the 35-day-olds had learned to detect when they were dehydrated. But how? Both 35-day-olds and 21-day-olds readily consume both dry food and water without assistance. Why don't 21-day-olds know when they're thirsty?

Hall and his colleagues suspected that 21-day-olds know to drink water under normal circumstances because their mouths feel dry after eating rat chow. When they are artificially dehydrated, they have no way of knowing they're thirsty, because they've only ever had to use dry mouth as a cue to thirst. They devised a clever experiment to test this theory. They raised rats from the time of weaning to the age of 35 days on a liquid-only diet and repeated their experiment, this time comparing 35-day-old chow-reared and liquid-reared rats. Here are their results:

Now dehydrated 35-day-old rats who had been raised on a liquid-only diet didn't drink significantly more than rats who weren't dehydrated. So the dry mouth associated with eating chow must have been the cue for rats to drink. When they never had that cue, they never learned to drink when they were dehydrated.

Interestingly, if the same rats were dehydrated artificially again after this experiment, they drank as much as the chow-reared rats. It only takes one experience with dehydration for rats to figure out what to do about it.

This study offers additional support for the study we discussed earlier about how amnesia patients don't realize they are full. Clearly building up an appetite involves more than sensing how much food and water is in our bodies—memory also plays a crucial role.

W.G. Hall, H. Moore Arnold, and Kevin P. Myers (2000). The acquisition of an appetite. Psychological Science 11(2), 101-105.