JACKSON, Wyo. — The Tetons are home to a small, isolated herd of native bighorn sheep, a species most notable for their namesake big horns. However, upon closer inspection, these ungulates sport distinctive rectangular pupils that give them a unique advantage in the wilderness.
The National Park System (NPS) writes that prey animals evolved to have eyes on the side of their head to provide a larger field of vision for predators. According to Sarah Dewey, wildlife biologist for Grand Teton National Park (GTNP) and member of the Teton Range Bighorn Sheep Working Group, rectangular pupils are another key trait for almost all prey species, including goats, deer, horses, frogs and toads. And, of course, bighorn sheep.
“It’s truly an amazing adaptation that allows prey species to see all around them to try and avoid predators,” Dewey tells Buckrail. “The elongated, horizontal pupil increases the amount of light entering the eye on the sides and from below while decreasing the amount of light coming in from above.”
Dewey explains that increased light from below specifically enables bighorn sheep to better see the ground, critical if the animal needs to flee suddenly. Bighorn sheep pupils, like other prey species, also stay parallel to the ground regardless of if the animal is upright or if their head is down to graze. Dewey calls this “cyclovergence,” giving bighorns the ability to continuously look for predators.
Predators usually have vertical slit pupils, which Dewey says enhances depth perception to hunt food. Natural predators of bighorn sheep in the park include wolves, coyotes, eagles, foxes, bobcats and mountain lions.
Of the collared bighorn sheep that biologists are able to track, Dewey notes only one has been killed by a mountain lion. The most common bighorn sheep deaths are due to falls, avalanches and starvation.
Bighorn sheep in the Tetons are also currently facing increased risk from human development and recreation in winter territory. The Working Group has identified that this population “has permanently lost access to its traditional migration routes and low elevation winter ranges” due to human activity, necessitating new adaptations.
One way Dewey says GTNP’s bighorn sheep have adapted to human disturbances is living at higher elevations in the winter. However, Dewey emphasizes this presents bighorn sheep with a scarcity of food as a result of winter dormancy and deep snow.
“Because they must reserve their fat storage to survive the brutal winters in Wyoming, it’s vital that the bighorn sheep not be disturbed in their winter range,” Dewey says. “If the sheep are constantly on the run from disturbances, they burn life saving energy they need for their own survival and survival of their species.”
