Rocky Mountain Bighorn Sheep
(Ovis Canadensis Canadensis)
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Introduction
Famous for their massive horns and earth shattering head-to-head combat, the Rocky Mountain bighorn sheep (Ovis canadensis canadensis) is one of three subspecies of bighorn sheep found in North America (the other two being the desert bighorn sheep and California bighorn sheep). Rocky Mountain bighorn sheep display significant sexual dimorphism. While most people will immediately recognize males as bighorn sheep, people often mistake females, lacking the large horns, for mountain goats. The curved horns of adult males form nearly full circles at maturity and can weigh up to 14 kg. This is more than the combined weight of the rest of their skeleton and eight to twelve percent of total body mass. Meanwhile, females have small, sloped horns that are unimpressive in comparison. Sexual dimorphism is also seen in body size. Adult males weigh 80 kg on average and can be up to 140 kg, measure 1.3 to 2.0 m in length, and stand 1.0 to 1.5 m at the shoulder. Adult females average only 60 kg, measure 1.2 to 1.8 m in length, and stand 0.7 to 0.8 m at the shoulder.
In addition to their impressive horns, Rocky Mountain bighorn sheep have split, rough bottomed hooves that aid in balance and gripping which, combined with their sharp vision, enables them to inhabit rough, rocky terrain. While such an adaptation provides protection from most predators, it does not protect Rocky Mountain bighorn sheep from humans. Rocky Mountain bighorn sheep are actively hunted throughout their range. As a result, females rarely live past fifteen years of age and males (coveted as trophies for their horns) seldom live beyond age twelve. (Dewey and Ballenger, 1999) Hunters target the largest rams with the largest largest horns, both of which are highly heritable traits. A study by Coltman and his colleagues (1999) found that hunters effectively harvest the rams of highest breeding value at young ages. Many males had limited or no reproductive success. In response to this pressure, both body mass and horn size declined significantly over time and the overall vigor of the Rocky Mountain bighorn sheep may have decreased.
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