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 30 degrees C (N=12), 21 degrees C (N=10; published data) and 8 degrees C (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 30 degrees C exported significantly less energy as milk (87.7 kJ day(-1)) than mice at 21 degrees C (166.7 kJ day(-1)) and 8 degrees C (288.0 kJ day(-1)). This reduction in milk energy output at 30 degrees C 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 8 degrees C, 21 degrees C and 30 degrees C, respectively) and gross energy content of milk (14.6 kJ g(-1), 13.1 kJ g(-1) and 10.5 kJ g(-1) at 8 degrees C, 21 degrees C and 30 degrees C, respectively). Milk produced at 30 degrees C contained significantly less total solids (34.4%) than milk at 21 degrees C (40.9%) and 8 degrees C (41.5%) and significantly less fat (20.0%) than milk at 21 degrees C (26.4%) and 8 degrees C (30.3%). The reduced milk energy output in mice exposed to 30 degrees C, 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.