Diet

Rocky Mountain bighorn sheep are diurnal, opportunistic grazing and browsing ruminants. Their preferred food is forbs, then grasses, and finally shrubs, but bighorns have been found to consume between sixty-nine and eighty-eight plant species during the course of the year. Interestingly, Rocky Mountain bighorns are known to eat death camas (Zygadenus venenosus) and lupine (Lupinus spp.), plants poisonous to other ungulates, with no observed detriment. (Beecham et. al. 2007) In most populations, browse makes up a greater portion of the diet during the fall and winter (when graze is either dead or covered by snow) than during the spring and summer months. (Beecham et. al. 2007)

Body size sexual dimorphism leads to different foraging strategies in rams and ewes. While both sexes alternate between foraging/walking and resting, ewes spend more time foraging/walking and do so in longer intervals than do rams. Rams spend increased time laying down and ruminating, which most likely leads to more efficient and complete digestion of ingested plant matter and is beneficial for increasing body size. (Ruckstuhl 1998) Additionally, Ruckstuhl proposes that the greater time spent foraging and walking by females may be a predator avoidance strategy directed towards protecting lambs. Ruckstuhl proposes a “body-size-predation hypothesis” where males and females have different optimal foraging and movement patterns, one factor leading to segregation into separate herds.

Bighorn sheep also require additional minerals that they cannot aquire from plant matter. To solve this problem, bighorn sheep are known to use mineral/salt deposits in granite-based areas and have been seen consuming large amounts of soil. Selenium is thought to be an especially vital mineral, as selenium deficiencies have been linked to large-scale pneumonia epidemics and die-offs. (Beecham et. al. 2007)

Predators

The main predators of Rocky Mountain bighorn sheep are wolves, coyotes, and mountain lions. Additionally, golden eagles and bobcats prey on lambs. Females are at greater risk of predation than males because of smaller body size and weaponry. Bighorns attempt to mitigate predation risks through the selfish herd strategy (see Social Spacing ) and habitat selection.

Habitat

Rocky Mountain bighorn sheep are found in mountainous habitats from Alberta, Canada to northern New Mexico. Their habitat is characterized by alpine meadows, grassy mountain slopes, and foothill country near rugged cliffs and rocky outcroppings. (ZooMontana 2007) Bighorn sheep prefer open habitats that facilitate earlier predator detection and provide ample graze and browse. Rocky Mountain bighorn sheep are found on slopes of thirty-six to eighty percent and avoid slopes less than twenty percent. (Beecham et. al. 2007) The reason for such slope selection is most likely predator avoidance, as bighorn sheep have a significant advantage over their predators on steep, rugged terrain. Escape terrain (rocky cliffs, canyons, etc) are important features in any bighorn habitat. Males are found to venture further from escape terrain than females, most likely to pursue optimal foraging ground to support their greater body mass and because they are at less risk of predation and have no young to protect. (Beecham et. al. 2007)