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

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 degrees C (thermoneutral zone). We compared their food intake and reproductive output at peak lactation with animals studied previously at 21 degrees C (N=71) and 8 degrees C (N=15). Mice lactating at 30 degrees C had a significantly lower mean asymptotic food intake (12.4 g day(-1)) than those at 21 degrees C (23.5 g day(-1)) and 8 degrees C (28.6 g day(-1)). On average, mice at 30 degrees C raised significantly fewer (9.8) and smaller (6.1 g) pups than those at 21 degrees C (11.3 pups; 7.0 g per pup) and smaller pups than those at 8 degrees C (9.6 pups; 7.3 g per pup). Consequently, mean litter mass at 30 degrees C (56.0 g) was significantly lower than at 21 degrees C (77.1 g) and at 8 degrees C (68.7 g). The mean rate of litter mass increase at 30 degrees C (2.1 g day(-1)) was also lower than at 21 degrees C (3.1 g day(-1)). The reduced food intake and low reproductive output in mice lactating at 30 degrees C are consistent with the heat dissipation limit hypothesis.