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First published online October 27, 2003

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The Journal of Experimental Biology 206, 4255-4266 (2003)
doi: 10.1242/jeb.00674

Limits to sustained energy intake VI. Energetics of lactation in laboratory mice at thermoneutrality

E. Król^1,* and J. R. Speakman^1,2

¹ Aberdeen Centre for Energy Regulation and Obesity (ACERO), School of Biological Sciences, University of Aberdeen, Aberdeen AB24 2TZ, UK
² ACERO, Division of Appetite and Energy Balance, Rowett Research Institute, Bucksburn, Aberdeen AB21 9SB, UK

^* Author for correspondence (e-mail: e.krol{at}abdn.ac.uk)

Accepted 14 August 2003

The limits to sustained energy intake are important because of their implications for reproductive output, foraging behaviour and thermoregulatory capabilities. Recent attempts to elucidate the nature of the limits to sustained energy intake have focused on peak lactation, which is the most energetically demanding period for female mammals. The hypothesis that performance of lactating animals is limited peripherally by the capacity of mammary glands to produce milk has received the most attention. However, some empirical data cannot be explained by the peripheral limitation hypothesis. Here, we present a novel hypothesis that the limits to sustained energy intake at peak lactation are imposed by the capacity of the animal to dissipate body heat generated as a by-product of processing food and producing milk. To test the heat dissipation limit hypothesis we challenged reproducing MF1 laboratory mice (N=67) with a reduced potential heat flow between the animal and the environment by exposing them to 30°C (thermoneutral zone). We compared their food intake and reproductive output at peak lactation with animals studied previously at 21°C (N=71) and 8°C (N=15). Mice lactating at 30°C had a significantly lower mean asymptotic food intake (12.4 g day^-1) than those at 21°C (23.5 g day^-1) and 8°C (28.6 g day^-1). On average, mice at 30°C raised significantly fewer (9.8) and smaller (6.1 g) pups than those at 21°C (11.3 pups; 7.0 g per pup) and smaller pups than those at 8°C (9.6 pups; 7.3 g per pup). Consequently, mean litter mass at 30°C (56.0 g) was significantly lower than at 21°C (77.1 g) and at 8°C (68.7 g). The mean rate of litter mass increase at 30°C (2.1 g day^-1) was also lower than at 21°C (3.1 g day^-1). The reduced food intake and low reproductive output in mice lactating at 30°C are consistent with the heat dissipation limit hypothesis.

Key words: asymptotic food intake, digestibility, reproductive output, peripheral limit, heat dissipation limit, laboratory mouse, Mus musculus

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