Four primary components of desert bighorn sheep habitat

Obtaining resources: like all animals, appropriate food and water are essential resources for desert bighorn sheep.

1. Food
   • Bighorn sheep are primarily grazers, preferring non-woody vegetation like grasses, forbs, and sedges. However, bighorn diets vary throughout the year based on what plants are available. Bighorn graze on grasses, forbs, and sedges when available, such as during the summer, and browse on shrubs and other woody vegetation primarily during the winter. Bighorn sheep also eat cactus, such as prickly pear (Opuntia spp.). Many cacti are succulents that store water in fleshy stems, likely providing an important food-based water source for bighorn sheep and other desert wildlife.
2. Water
   • Although desert bighorn can survive for long periods of time without water (see more in the Adaptations section below), they likely always require some access to surface water within their home range. Bighorn therefore utilize areas with surface water, such as streams, springs, and desert potholes, preferentially to drier areas, especially during the hot summer months.

Evading predators: bighorn sheep are social prey animals that are hunted by a variety of predators, primarily mountain lions (Puma concolor). They have evolved strategies to reduce predation risk, such as by selecting habitats that provide refuge from predation and where predators can be more easily detected by the herd.

1. Escape terrain
   • Both desert and Rocky Mountain bighorn sheep are rarely far from habitat that provides a refuge from predation. Escape terrain is typically steep and rocky with intermixed cliffs. Bighorn are specially adapted to navigate rugged terrain, which gives them a temporary advantage over their predators who struggle to follow sheep into escape terrain. The National Wildlife Federation clearly describes their well-adapted hooves: “The outer hooves are modified toenails shaped to snag any slight protrusion, while a soft inner pad provides a grip that conforms to each variable surface”
2. Open habitat
   • Bighorn sheep primarily detect predators visually, which is easier and more effective over larger distances in habitats with open views. So, bighorn sheep tend to use "open habitats", such as rocky grasslands and shrublands, and tend to avoid "closed habitats" with more visual obstructions, such as dense forests. However, habitat selection can vary with context. For example, bighorn sheep may use more closed habitats in the summer to avoid heat, even though their risk of predation is higher. This is an example of a habitat selection tradeoff, since bighorn sheep may prioritize a "thermal refuge" (avoiding heat) at the expense of predator detection.

To summarize the foraging and predator evasion strategy of both desert and Rocky Mountain bighorn sheep:

Bighorn sheep tend to forage diurnally (during the daytime) in dispersed groups within open habitat that is close to escape terrain. This habitat selection strategy allows them to obtain food in an area where they can, as a group, effectively detect predators and quickly access escape terrain to evade predators when needed.

References: Valdez and Krausman 1999, UDWR 2012, Robinson et al. 2020.

Visualizing desert bighorn sheep habitat

Figure 5. Habitat scale 1: two mature desert bighorn rams (males) on a mesa within the Kayenta rock layer that has interspersed outcrops of Navajo sandstone and moderate density of pinyon and juniper trees, Capitol Reef National Park. Photo also illustrates a benefit of having at least one friend: one sheep forages while the other watches the photographer. Social prey animals, like sheep and deer, experience a tradeoff between foraging and being vigilant for predators. Sheep and deer often forage in dispersed groups to take advantage of "group awareness" of predators, giving each individual more time to forage than if they were alone. - Photo: J. Ceradini

Figure 6. Habitat scale 2: two desert bighorn rams (males) in the upper-center of photo. This scale shows more of the rock outcrops, open space between pinyons and junipers, and overall complicated nature of the terrain, Capitol Reef National Park - Photo: J. Ceradini

Figure 7. Habitat scale 3: no sheep in this picture, sorry. The sheep in Figures 5 and 6 were near the white circle. Broad scale habitat view showing the Kayenta pinyon-juniper mesa transitioning to large Navajo sandstone domes to the east (right), Capitol Reef National Park. This photo illustrates the broader context of the habitat in Figures 5 and 6, and also how wildlife habitat is understood at multiple spatial scales. For example, there are at least three aspects of this location that reduce predation risk: 1) at the local scale, the sheep have small rock outcrops and complicated terrain to stay hidden within, 2) at the broader scale, the Kayenta rock layer sits on top of the large Wingate sandstone cliffs (seen running north toward Thousand Lake Mountain in the background), which limit access to the Kayenta from below to only a few "breaks" in the Wingate, and 3) also at the broader scale, the rugged and steep Navajo sandstone domes serve as escape terrain for bighorn sheep, providing a partial refuge from predators - Photo: J. Ceradini

Figure 8. Broad scale view of the Fremont River/Highway 24 corridor that is dominated by the Navajo sandstone layer, Capitol Reef National Park. This area is used extensively by desert bighorn sheep because, 1) the rugged and steep Navajo sandstone provides excellent escape terrain but is still interspersed with vegetation for foraging, and 2) the Fremont River provides a year-round water source that is essential to many desert wildlife species including bighorn sheep - Photo: J. Ceradini

Figure 9. Another view of the Fremont River/Highway 24 corridor showing the juxtaposition of the Fremont River, with its flat and relatively lush riparian zone, and the rugged, dry, and sparsely vegetated Navajo sandstone above. Desert bighorn sheep frequently move between these two contrasting and essential habitat types, which also illustrates the importance of connectivity between different habitat types for wildlife - Photo: J. Ceradini

Morphological

• Desert bighorns have lighter pelage (fur) than Rocky Mountain bighorns, which reduces heat absorption since lighter colors absorb less heat than darker colors.
• Desert bighorns are smaller than Rocky Mountain bighorns, with adult rams weighing up to 35-40% less than Rocky Mountain bighorn rams. Body size represents an interesting tradeoff when it comes to heat stress. As body size increases, the surface-area-to-volume ratio of the animal decreases, which reduces the percentage of the animal's mass that is exposed to solar radiation; this means that the animal will take longer to heat up. But, this same effect means that larger animals are also slower to cool down (both of these effects are related to "thermal inertia"). So, a larger animal is slower to heat up and slower to cool down than a smaller animal. Since desert bighorns are smaller than Rocky Mountain bighorns, that suggests that desert bighorns benefit more from cooling down faster than heating up slowly, relative to their larger Rocky Mountain cousins.

Behavioral

• Adjusting the timing of activity is a common approach that animals use to reduce environmental stress. Many desert species, such as desert bighorn, reduce activity during the day when it is hot and become more active, for example to forage, at dawn and dusk, which is known as crepuscular activity. Bighorn sheep are not typically nocturnal. Additionally, in some areas relative humidity increases as temperature decreases at night, which can result in condensation on plants and increase plant water content up to 30%. Foraging in the early morning, as desert bighorn sheep frequently do, may therefore reduce heat exposure and increase water consumption due to higher plant water content.
• Habitat selection, or preferentially using certain habitats over others, is another common strategy used by desert species to reduce heat stress. Desert bighorns use habitats that are cooler than the ambient daytime temperature in the summer. Bighorn sheep may use habitats with more vegetation, such as tree cover, to avoid heat in the summer, even though their risk of predation is higher in densely vegetated areas with less visibility. This is an example of a habitat selection tradeoff, since bighorn sheep may prioritize a "thermal refuge" (avoiding heat) at the expense of predator detection. Aspect, or the compass direction something is facing, is also an important part of habitat selection in the summer. Areas with north aspects will stay cooler and more shaded throughout the day, providing another thermal refuge for animals.
• Finally, these behavioral strategies, such as timing of activity and habitat selection, can vary seasonally since environmental stressors like temperature vary throughout the year. The ideal summer habitat may not be the ideal winter habitat. The relative importance of different stressors, such as temperature and predation risk, changes throughout the year and animals adjust their behavior as conditions change.