SUMMARYThe capacity of animals to dissipate heat may constrain sustained energy intake during lactation. We examined these constraints at peak lactation in MF1 mice that had ad libitum access to food, or that had to run a pre-set target on running wheels to obtain ad libitum access to food. The voluntary distance run decreased sharply during pregnancy and peak lactation. When lactating females were provided with 80% of their estimated food requirements, and had to run pre-set distances of 2, 4 or 6km before given access to additional ad libitum food, most of them did not complete the running target during late lactation and the mice with the highest targets failed to reach their targets earlier in lactation. There were consequently significant group differences in asymptotic food intake (2km, 16.97±0.40gday ) and weaned litter masses (2km, 71.11±2.39g; 4km, 54.63±4.28g and 6km, 47.18±2.46g). When the females did run sufficiently to gain ad libitum food access, their intake did not differ between the different distance groups or from controls that were not required to run. Thus, despite being physically capable of running the distances, mice could not exercise sufficiently in lactation to gain regular ad libitum access to food, probably because of the risks of hyperthermia when combining heat production from exercise with thermogenesis from lactation. Supplementary material available online at

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