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First published online May 21, 2007
Journal of Experimental Biology 210, 2013-2024 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.001974
Behavioural and physiological responses to increased foraging effort in male mice
1 University of Groningen, Department of Behavioural Biology, Kerklaan 30,
9751 NN Haren, The Netherlands
2 University of California, Riverside, Department of Biology, Riverside, CA
92521, USA
3 University of Groningen, Centre for Isotope Research, Nijenborgh 4, 9747
AG Groningen, The Netherlands
* Author for correspondence (e-mail: l.m.vaanholt{at}rug.nl)
Accepted 13 March 2007
Free-living animals must forage for food and hence may face energetic constraints imposed by their natural environmental conditions (e.g. ambient temperature, food availability). Simulating the variation in such constraints, we have experimentally manipulated the rate of work (wheel running) mice must do to obtain their food, and studied the ensuing behavioural and physiological responses. This was done with a line of mice selectively bred for high spontaneous wheel running and a randomly bred control line that vary in the amount of baseline wheel-running activity. We first determined the maximum workload for each individual. The maximum workload animals could engage in was around 23 km d–1 in both control and activity-selected mice, and was not associated with baseline wheel-running activity. We then kept mice at 90% of their individual maximum and measured several physiological and behavioural traits. At this high workload, mice increased wheel-running activity from an average of 10 to 20 km d–1, and decreased food intake and body mass by approximately 20%. Mass-specific resting metabolic rate strongly decreased from 1.43 to 0.98 kJ g–1 d–1, whereas daily energy expenditure slightly increased from 2.09 to 2.25 kJ g–1 d–1. Costs of running decreased from 2.3 to 1.6 kJ km–1 between baseline and workload conditions. At high workloads, animals were in a negative energy balance, resulting in a sharp reduction in fat mass as well as a slight decrease in dry lean mass. In addition, corticosterone levels increased, and body temperature was extremely low in some animals at high workloads. When challenged to work for food, mice thus show significant physiological and behavioural adjustments.
Key words: mouse, daily energy expenditure, doubly labeled water technique, energy balance, resting metabolic rate
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