Time and energy use under thermoregulatory constraints in a diurnal rodent

(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 rodent 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 (VO 2), evaporative water loss (EWL) and body temperature ( T b) at air temperatures ( T air) of 33–34, 36–37, and 39–40°C were also determined. The metabolic response to rising T air amounted to a doubling of VO 2 and a five-fold increase of EWL at T air 39–40°C, compared with 25°C; and T b rose above 42°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°C and 8 min at 50°C. These simulations are in agreement with our observations that degus reduced their time spent feeding when there was a thermal constraint on foraging.