Multidimensional aspects of drinking in the rat.

In 60-min test sessions, thirsty rats licked an empty metal spout for 0.2-milliliter shots of water delivered audibly to a neighboring spout that remained accessible after the first delivery. As the instrumental empty-lick requirement increased they drank less water, in accordance with the demand law, but made the same number of licks at the water delivery spout. In terms of water licks/milliliter they, therefore, licked less efficiently at higher behavioral prices, opposite to the relation obtained when rats respond for licks at a spout full of water. Given a continuous supply of free water they licked more efficiently, and licks varied in strict proportion to the amount drunk. Yoked controls with no instrumental requirement drank the same amount and made the same number of water licks as their price-paying experimental partners. Thus, intermittent delivery of a small amount of water induced rats to lick the delivery spout at a relatively high average rate that was unaffected by the behavioral price of the delivery or the interdelivery interval. Opposite efficiency effects suggest that external constraint on any dimension of drinking will affect that dimension more than any other left unconstrained.