Plasticity in the rumination behaviour of bighorn sheep: contrasting strategies between the sexes?

Although activity budgets have been studied extensively in ruminants, information on the plasticity of rumination among individuals is rare. Generally, time spent bedded is considered a proxy for time spent ruminating. Researchers rarely include measures of interindividual differences in the intensity and time spent ruminating. We explored individual variation in rumination behaviour in Rocky mountain bighorn sheep, Ovis canadensis. We expected rumination to be a plastic behaviour affected by various intrinsic and extrinsic factors such as body mass, age, sex, reproductive status and environmental conditions. We investigated rumination behaviour based on (1) mean number of chews per bolus of food (chews/bolus), (2) mean number of chews per second per bolus (chewing rate) and (3) mean time spent chewing per bolus (bolus processing time) for sheep of known age, sex, body mass and reproductive status. Our results show that both intrinsic and extrinsic factors affected rumination behaviour. Females tended to spend less time processing each bolus than males of similar body mass, resulting in a significantly higher chewing rate for females. There was no difference in chews/bolus between sexes. Environmental factors and group size also affected rumination behaviour: cold temperatures were associated with fewer chews/bolus; wind was associated with more chews/bolus. Sheep in larger groups had a slower rate of rumination than those in smaller groups. We conclude that rumination behaviour is very plastic and, therefore, that assumptions of uniform rumination rates across individuals may be misleading and may miss an important piece of ruminants' foraging ecology.