BAT­TLE OF THE BIGHORNS

Male BIGHORN SHEEP are built to fight, ex­plains BBC zool­o­gist Ben Gar­rod.

BBC Wildlife Magazine - - In Focus -

Some an­i­mals lead a per­ilous life. Though bighorn sheep are the largest wild sheep in North Amer­ica, size alone is no guar­an­tee of pro­tec­tion against at­tack by moun­tain lions, wolves and coy­otes. So they have made their home in some of the steep­est, rock­i­est, most in­hos­pitable places imag­in­able, from Mex­i­can deserts to the Rocky Moun­tains.

One might as­sume that her­bi­vores that spend so much time on sheer rock faces would be timid crea­tures pre­oc­cu­pied with keep­ing their foot­ing. Yet cliff-liv­ing bighorn sheep in­dulge in some of the most vi­o­lent, at­ten­tion-grab­bing dis­plays in the an­i­mal king­dom. Un­like the an­ces­tor of do­mes­tic sheep, the Euro­pean mou­flon, bighorn sheep do not have a clear hi­er­ar­chy in their groups. In­stead, males bat­tle for dom­i­nance at the start of each breed­ing sea­son.

Two males will ap­proach, then walk away from one an­other. They turn, rear up on their hind legs and rush for­wards, crash­ing horns to­gether at an es­ti­mated 20–40kph. And they re­peat this head-bang­ing – some­times for hours – un­til one com­bat­ant sub­mits. Signs of dam­age can of­ten be seen on males’ horns, but they rarely suf­fer se­ri­ous in­juries, even though sci­en­tists have es­ti­mated that the force gen­er­ated in th­ese fights might be up to 60 times greater than that needed to frac­ture our skulls.

The bighorn sheep skull has un­der­gone sev­eral adap­ta­tions to sur­vive such bru­tal clashes. The bone is ex­tremely thick, so harder to dam­age. To help cush­ion mighty blows, the si­nuses at the front of the skull are en­larged, and there are ex­tra lit­tle sec­tions of scaf­fold­ing-like bone called septa. Ev­ery mam­malian skull is made from many smaller bones joined to­gether, but the skull of the bighorn sheep does not fuse fully; in­stead th­ese lines of fu­sion, or su­tures, re­main partly open and act as nat­u­ral shock ab­sorbers. Force from the im­pact is dis­trib­uted along th­ese un­fused su­tures, mak­ing the en­tire skull slightly flex­i­ble yet in­cred­i­bly strong.

Ben Gar­rod is an evo­lu­tion­ary bi­ol­o­gist and BBC science pre­sen­ter.

Ri­val ma­ture male bighorn sheep face off dur­ing the breed­ing sea­son. The species’ bru­tal bat­tles re­quire a suite of cra­nial adap­ta­tions.

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