The aerobic capacity model postulates that high basal metabolic rates (BMR) underlying endothermy evolved as a correlated response to the selection on maximal levels of oxygen consumption (VO2 max) associated with locomotor activity. The recent assimilation capacity model specifically assumes that high BMR evolved as a by-product of the selection for effective parental care, which required long-term locomotor activity fuelled by energy assimilated from food. To test both models, we compared metabolic and behavioural correlates in males of laboratory mice divergently selected on body mass-corrected BMR. VO2 max elicited by running on the treadmill did not differ between selection lines, which points to the lack of genetic correlation between BMR and VO2 max. In contrast, there was a positive, genetic correlation between spontaneous long-term locomotor activity, food intake and BMR. Our results therefore corroborate predictions of the assimilation capacity model of endothermy evolution.