DRINKING IN A PATCHY ENVIRONMENT - THE EFFECT OF THE PRICE OF WATER

Rats in a laboratory foraging paradigm searched for sequential opportu nities to drink in two water patches that differed in the bar-press pr ice of each ''sip'' (20 licks) of water within a bout of drinking (Exp eriment 1) or the price and size (10, 20, or 40 licks) of each sip (Ex periment 2). Total daily water intake was not affected by these variab les. The rats responded faster at the patch where water was more costl y. However, they accepted fewer opportunities to drink, and thus had f ewer drinking bouts, and drinking bouts were smaller at the more costl y patch than at the other patch. This resulted in the rats consuming a smaller proportion of their daily water from the more costly parch. T he size of the differences in bout frequency and size between the patc hes appears to be based on the relative cost of water at the patches. The profitability of each patch was calculated in terms of the return (in milliliters) on either effort (bar presses) or time spent there. A lthough both measures were correlated with the relative total intake, bout size, and acceptance of opportunities at each patch, the time-bas ed profitability was the better predictor of these intake measures. Th e rats did not minimize bar-press output; however, their choice betwee n the patches and their bout sizes within patches varied in a way that reduced costs compared to what would have been expended drinking rand omly. These data accord well with similar findings for choices among p atches of food, suggesting that foraging for water and food occurs on the basis of comparable benefit-cost functions: In each case, the amou nt consumed is related to the time spent consuming.