Limits to sustained energy intake VII. Milk energy output in laboratory mice at thermoneutrality

The limits to sustained energy intake at peak lactation could be imposed peripherally, by the capacity of the mammary glands, or centrally, by the capacity of the animal to dissipate body heat generated as a by-product of processing food and producing milk. To distinguish between the two hypotheses, we examined milk energy output at peak lactation in MF1 laboratory mice exposed to 30degreesC (N=12), 21degreesC (N=10; published data) and 8degreesC (N=10; published data). The peripheral limitation hypothesis predicts that milk energy output will remain constant at different temperatures, while the heat dissipation limit hypothesis predicts a decline in milk energy output as temperature increases. Since estimates of milk energy output in small mammals can vary depending on the calculation method used, we evaluated the milk energy output of mice (N=24) using four different methods: (1) as the difference between metabolizable energy intake and daily energy expenditure of the female, (2) from female water turnover, (3) from pup water turnover and (4) from the energy budget of the litter. We assessed these four methods by comparing their accuracy, precision and sensitivity to changes in parameters involved in the calculations. Methods 1, 3 and 4 produced similar estimates of milk energy output, while those derived from female water turnover were significantly lower and more variable. On average, mice at 30degreesC exported significantly less energy as milk (87.7 kJ day(-1)) than mice at 21degreesC (166.7 kJ day(-1)) and 8degreesC (288.0 kJ day(-1)). This reduction in milk energy output at 30degreesC was caused by a significant decline in both milk flow (20.0 g day(-1), 12.9 g day(-1) and 8.5 g day(-1) at 8degreesC, 21degreesC and 30degreesC, respectively) and gross energy content of milk (14.6 kJ g(-1), 13.1 kJ g(-1) and 10.5 kJ g(-1) at 8degreesC, 21degreesC and 30degreesC, respectively). Milk produced at 30'C contained significantly less total solids (34.4%) than milk at 21degreesC (40.9%) and 8degreesC (41.5%) and significantly less fat (20.0%) than milk at 21degreesC (26.4%) and 8degreesC (30.3%). The reduced milk energy output in mice exposed to 30degreesC, paralleled by their reduced food intake and low reproductive output, argues against the peripheral limitation hypothesis and provides strong support for the heat dissipation limit hypothesis.