Nursing mouse mothers must be hungry all the time. Feeding multiple greedy mouths takes a lot out of a girl. So what limits the amount of energy a nursing mother can supply her litter? According to John Speakman, a mother is limited by the amount of energy she can take in, but the aspect of her physiology that imposes the limit wasn't clear. Having previously found that the gut and mammary gland did not restrict energy intake, Speakman,Elżbieta Król and Michelle Murphy decided to test whether lactating mice are constrained by the amount of waste heat generated by digestion and milk production(p. 4233). The team shaved some nursing mice to see if removing their insulation allowed them to cool off, and increase their energy intake and milk output. The team monitored the mothers' milk production and found that it was 15.2% more than unshaved mums, and the shaved mums' youngsters were 15.4% bigger too. So the amount of heat that a mammal can dispose of seems to limit the amount of energy they can take in, which could have far reaching consequences for evolution, not least in our warming climate.

Król, E., Murphy, M. and Speakman, J. R.(
). Limits to sustained energy intake. X. Effects of fur removal on reproductive performance in laboratory mice.
J. Exp. Biol.