Monday, May 19, 2014

Ewe Gotta' Want It

Bighorn sheep ewes butting heads (c) John Ashley
Bighorn sheep ewes butting heads in spring

Last week I spent a little time watching a small band of eight female bighorn sheep, ewes, feeding placidly in a level meadow. The first few snippets of green, spring grass lured the sheep down from the steep, rocky talus slopes where they spend most of their time. They seemed to have no personal space issues or food guarding behaviors that I could detect. Old and young moved along at random intervals, at times feeding within a few inches of each other. All seemed rather peaceful and placid, at least for the first 20 minutes.

Out of nowhere, two ewes suddenly turned and bashed their heads together three times in quick succession. This behavior caught me off guard - I didn't know the females would, at times, act the way that the petulant big-boy rams are famous for. In just a couple of seconds they shattered my peaceful but mistaken impression. I needed to read up on this.

Bighorn sheep live in separate groups for most of the year, rams in one herd, adult females and young in a separate herd. Young males live with the females until they're three years old, when they become dominant to the adult females and move over to the big-boy herd.

Bighorn rams (c) John Ashley
Subordinate ram (left) halfheartedly "pushes" an older, dominate ram    
We've all seen how the dominant rams rear up and bash their horns together, repeatedly, in the weeks leading up to the rut. According to Dr. Foresman, in "Mammals of Montana," the rams' social hierarchy is linear and closely tied to their age, only to be reinforced by battle each fall. The strongest and least-exhausted ram "tends" the female herd until individual ewes come into heat, when he'll breed with them. Challengers will try to distract the dominant ram just long enough to sneak in a breed with a female. Younger, subordinate rams use a third strategy. During the breeding season, they'll try to separate a female and keep her away from the herd long enough to breed when she comes into estruses.

But what about bighorn dominance and hierarchy within the female herd?

Unlike rams, ewes have a non-linear social hierarchy. Female status is only loosely related to age, while dominance and rank among females is achieved primarily through physical competition. Apparently, female status is open to challenge during any time of the year. The ewe head-butting I witnesses was not the half-hearted, out-of-season, practice head-butting that the younger males tend to try year-round in their all-male herd.

These ewes were serious. Female bighorns have to fight their way to the top. Real life is seldom as peaceful and easy as I keep trying to make it out to be.