Science Update: Habitat Preferences of Desert Bighorn Sheep

I have no real personal connection with bighorn sheep (Ovis canadensis). I’ve never seen one, eaten one, and know relatively little about them. Perhaps because of this lack of opportunity to interact with them on some personal level, I’m somewhat fascinated by them. At least a part of this fascination has to do with some pretty remarkable life history, physical characteristics, and habits of the species. I’ve also been reading some pieces by Canadian biologist Valerius Geist in the last little while. Geist spent a great deal of time studying bighorn sheep and I recently bought his book Mountain Sheep and Main in the Northern Wilds, so maybe this post is just the result of the convergence of a few individual interests and information trails. In any case, I came across a recent study on the habitat preferences of female desert bighorn sheep and found that it offered an interesting glimpse into the lives of these species.

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Desert bighorn sheep. Source: U.S. National Park Service

Wild sheep arrived in North America sometime around 750,000 years ago, during the Pleistocene (the ice age period that preceded our current epoch, the Holocene). There are currently two species of wild sheep in North America, Dall sheep (Ovis dalli) and bighorn sheep, with the latter also comprised of a number of subspecies. Historically, the range of bighorn sheep covered much of the western portion of North America from Canada to Mexico. As with many other large mammal species on this continent, wild sheep population abundance and range have fluctuated throughout their history, and much of this has to do with the availability of suitable habitat. Though wild sheep are listed as “least concern” by the International Union for Conservation of Nature and remain unlisted in both Canada and the United States (two subspecies resident to California are listed as “endangered” under the U.S. EPA), the Wild Sheep Foundation continues to work on a variety of initiatives and programs to enhance sheep habitat and distribution throughout wild sheep range. Currently, bighorn sheep still exist across their historic range, but their numbers and the extent of continuous populations has been fairly dramatically reduced.

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Historic range of bighorn sheep from 1850-2012. Source: Wild Sheep Foundation

It’s perhaps not difficult to see why someone would be intrigued by this animal. I’ve never been a fan of the “man vs. wild” or “conquering nature” discourses, but there is something primally attractive about the prospect of being able to navigate and survive in the kind of perilous places wild sheep live that makes hunting them somewhat irresistible to me. Wild sheep live in some of the most precipitous habitat on this continent, generally avoiding predators by spending their time in terrain so steep, rocky, dangerous, and difficult to navigate that it is virtually inaccessible to other species, including many of their predators. As with many other ungulates, male sheep (rams) use their thick, curled, sometimes 30-pound horns to fight one another. I’ve heard that sheep can deliver blows with their horns with a force 40 times what it would take to fracture a human skull.

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Rams fighting. Source: Pinterest

It’s long been understood that the habitat selection preferences of wild sheep is an important factor in their ability to avoid predation, including protecting their young (lambs) from these risks. Predation is the leading cause of mortality in neonate (newborn) ungulates, so appropriate habitat selection by pregnant females is critical to neonate survival. For instance, the amount of visibility within a habitat is an important factor in a female’s ability to avoid predation, particularly during times when young are less mobile and therefore are highly susceptible to predation. Areas of low visibility (e.g. high shrub cover) reduce a predator’s ability to see young, whereas areas of high visibility, while allowing increased visibility for predators, also allow females to detect predators. A new paper published in The Journal of Wildlife Management, Desert bighorn sheep lambing habitat: Parturition, nursery, and predation sites, conducted by researchers in New Mexico has provided some new insight into the parturition and nursery habitats used by female sheep (ewes).

Existing knowledge of desert bighorn sheep parturition habitat had been largely based on observation of the presence of lambs – the belief was that the locations in which lambs were observed was likely reflective of the kind of habitat ewes use for lambing. However, through the use of radiocollars and implants that detect when ewes gave birth, the researchers able to identify more precisely the differences in parturition and nursery habitats used by ewes. It was previously thought that both parturition and nursery habitats were in areas of steep, rugged terrain, high elevation, and high visibility. In contrast, the current study found that parturition sites were more likely to occur in habitats at intermediate slopes and intermediate elevations, whereas nursery sites were more likely to be located in areas with steeper slopes. In terms of visibility, parturition sites were more likely to be located in areas of low visibility and nursery sites were associated with habitats at both ends of the visibility spectrum (but not intermediate visibility).

Bighorn lambs are only immobile for 2-3 days after birth, but are classified as a follower species. On a hider-follower classification, a “hider” species leave their young to hide from predators, whereas “follower” species are mobile earlier after birth and are able to escape from predation. Therefore, this study indicates that ewes and their lambs move from parturition sites to nursery sites at higher elevations and steeper slopes shortly after lambs are born to occupy habitat less accessible to predators. It’s pretty amazing that only days after birth, here is a species that is capable of moving deeper into habitat that predators such as mountain lions find difficult to navigate.

Again, while I don’t have any personal experience with bighorn sheep at this point in my life, I find them to be a fascinating and remarkable species and one I hope to connect with more directly at some point. As someone who is also interested in new knowledge on wildlife, I found this paper particularly interesting as it is the first study to definitively describe desert bighorn sheep parturition habitat characteristics. It’s sometimes easy to think that the answers to all of our questions are simply one Google search away, and it’s exciting to realize that just as we are still discovering entirely new species, we are also uncovering details of species that we have interacted with for thousands of years.

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