Because lactation has high energetic costs, females should vary their foraging behaviour according to reproductive status. In ungulates, however, some studies found no differences in feeding behaviour between non-reproductive (yeld) and lactating females. Despite the importance of rumination in determining digestive efficiency, no study has attempted to identify tactics involving this parameter in free-ranging ungulates. Whether or not females varied their ruminating behaviour as a function of the presence/absence of offspring was tested by observing marked bighorn ewes Ovis canadensis of known reproductive status, age, and body weight. Lactating ewes ruminated 1.21 times faster than yeld ewes and showed less inter-individual variability in rumination speed, suggesting an energetic constraint. After considering the potential physiological advantages of this behaviour, I suggest that differences in ruminating parameters may allow the synchronization of activities in groups made up of individuals with different energy requirements. Lactating females may increase rumination effort in response to increased energetic demands and risk of predation.