Abstract

(1) This study was designed to understand how thermoregulatory constraints affect the timing of foraging bouts and amount of food obtained from a feeding station that was subject to an intense heat load. (2) The degu, Octodon degus, is a diurnal re-dent inhabiting hot arid environments in Chile. Two-hour trials were run in a laboratory experimental arena to determine time allocation between a food station located under a heavy radiant heat source (2500 W/m(2)) and the rest of the arena, including a cool burrow. A video camera recorded the animal's movements during the trial, and tapes were analyzed afterwards for seven degus with a heated food site and seven degus with a normal, unheated food station. (3) Degus with a food patch exposed to heat spent less time foraging and harvested less food than animals with no heat load. Over a two hour period time use of the food patch was significantly less when heated than when not heated (0.65 +/- 0.59 vs 4.90 +/- 1.73 min, respectively). Thus the degus subjected to heat above their food source minimized time exposure to a physiologically stressful condition by decreasing their visits to the heated food source, thereby reducing food (= energy) intake. (4) Oxygen consumption (VO2), evaporative water loss (EWL) and body temperature (T-b) at air temperatures (T-air) of 33-34, 36-37, and 39-40 degrees C were also determined. The metabolic response to rising T-air amounted to a doubling of VO2 and a five-fold increase of EWL at T-air 39-40 degrees C, compared with 25 degrees C; and T-b rose above 42 degrees C. (5) Using data from these metabolic measurements together with a theoretical heating model, heating curves were obtained that suggested that degus can exploit patches of food for only about 3 min at 60 degrees C and 8 min at 50 degrees C. These simulations are in agreement with our observations that degus reduced their time spent feeding when there was a thermal constraint on foraging. (C) 2000 Elsevier Science Ltd. All rights reserved.