The interaction of hunger and thirst in the rat

Two groups of rats were given a series of trials in an enclosed runway with a food reward at the end, one group being run hungry, the other hungry plus thirsty. Then each group was split into three sub-groups: one run hungry, the second thirsty and the other hungry plus thirsty, in each case without food reward.

It was found that, whereas on the rewarded runs the extra, “irrelevant,” thirst increased running speed, on unrewarded runs it had the opposite effect and slowed up performance. Thus on unrewarded runs the two sub-groups running thirsty, and hungry plus thirsty, ran as slowly as those running hungry. Differences were found not to depend on whether the animals had been hungry or hungry plus thirsty on previous rewarded runs.

The interaction of primary needs therefore depends on the external situation. This can be accounted for in terms of the Pavlovian theories of mutual induction and conditioning, but not in terms of Hull's theory of “irrelevant drives.”