Limits to sustained energy intake. VIII. Resting metabolic rate and organ morphology of laboratory mice lactating at thermoneutrality.

We have previously shown that the food intake and milk production of MF1 laboratory mice lactating at 30 degrees C, 21 degrees C and 8 degrees C increase as temperature declines. These data suggest that mice are not limited peripherally by the capacity of the mammary glands to produce milk but are limited by the capacity of the animal to dissipate body heat generated as a by-product of food processing and milk production. Here, we measure resting metabolic rate (RMR; prior to breeding and at peak lactation) and organ morphology (at peak lactation) in MF1 mice exposed to 30 degrees C (thermoneutrality) and compare these traits with the same parameters measured previously in mice at 21 degrees C and 8 degrees C. The masses of visceral organs primarily responsible for energy flux (heart, lungs, stomach, small intestine, large intestine, liver, pancreas, spleen and kidneys) increased as temperature declined. The masses of all these organs differed between mice exposed to 8 degrees C and 21 degrees C, whereas only the masses of heart, liver and kidneys differed between mice at 21 degrees C and 30 degrees C. The increases in organ masses were paralleled by increases in RMR at peak lactation above the levels measured prior to breeding, with mice at 8 degrees C and 21 degrees C having significantly higher increases in RMR than mice at 30 degrees C (29.6 kJ day(-1), 25.5 kJ day(-1) and 8.1 kJ day(-1), respectively). The observed changes in visceral organs and RMR are consistent with both the heat dissipation and peripheral limit hypotheses. However, mice exposed to 8 degrees C had substantially larger mammary glands than mice at 21 degrees C or 30 degrees C (2.450 g, 1.115 g and 0.956 g dry mass, respectively), which argues against the peripheral limitation hypothesis and is consistent with the heat dissipation limit hypothesis. In addition, cold exposure resulted in greater masses of brown adipose tissue, white adipose tissue, pelage and tail. We discuss these changes in the context of the potential thermoregulatory benefits from use of the heat generated as a by-product of milk synthesis.