Bighorn sheep in winter: do rams maximize reproductive fitness by spatial and habitat segregation from ewes?

Abstract

In bighorn sheep (Ovis canadensis canadensis) the segregation of adult males from females is explained by two hypotheses: first, males exhausted by the rut must avoid females to escape culling predators; second, males ought to segregate from females in such a fashion as to minimize competition and disturbance for the mothers of their prospective offspring. Only the second hypothesis coincides closely with previously known facts about sheep. It predicts that on common winter ranges adult males ought to minimize competition for females by segregating spatially and by habitat selection. This was verified. Although both sexes may occupy a continuous range, they concentrate in different areas; rams have a greater preference for open slopes and ewes for cliffs. This segregation held true even in a winter with exceptional snow depths. Sheep occupied the wintering area in a similar manner in successive years; females were found to be more affected by snow than males. In their habitat preference, rams and ewes acted in accordance with their behavioural types, much as was found to be true for their social behaviour.