A Little Natural History: Bighorn SheepJanuary 13, 2020
By Herm Hoops
A Little Natural History: Bighorn Sheep. If you go down the Green River through Dinosaur National Monument and Desolation and Gray Canyon you are likely to see bighorn sheep (Ovis canadensis). They get their name from the large, curved horns on the males. They are legendary for their ability to climb high, steep, rocky cliffs.
When you pass by bighorn close to the river, or approach them it is important that you remain quiet and move slowly. They may appear docile and content, but the presence of human intrusions causes increased vigilance and their heart rate increases as they become increasingly aroused. Studies show that an approach 500 feet by humans cause ewes heart rate to increase from a resting rate of 55 to 135 beats per minute (b.p.m.), it continued to climb for 60 minutes following the initial contact and reached peaks of over 200 b.p.m. Heart rate remained elevated for 2 hours afterward. Studies also show that crossing a highway creates high levels of stress in the animals, which can reduce their resistance to disease thereby increasing sheep mortality. Because bighorn sheep are sensitive to human disturbance, please help in protecting the sheep by viewing them quietly from a distance.(8)
Wild sheep crossed the Bering Land Bridge from Siberia into Alaska during the Pleistocene(11) (about 750,000 years ago) and spread through western North America as far south as Baja California and Mexico. Divergence from their closest Asian ancestor (snow sheep) occurred about 600,000 years ago. In North America, wild sheep developed into two extant species – Dall sheep in Alaska and northwestern Canada, and bighorn sheep.(1)
The population in North America peaked in the millions, and the bighorn sheep entered into the mythology of Native Americans.(2) By around 1900, hunting, competition from ranching, and diseases had decreased the population to several thousand. A program of reintroduction, National Parks and Wildlife Refuges, and reduced hunting, together with a decrease in domesticated sheep near the end of World War II, allowed the bighorn sheep to make a comeback.(3)
The bighorn’s compact body is muscular, with brown hair trimmed in white on the muzzle, rump, and belly. Most weigh 160 to 250 pounds, but males may weigh more than 350 pounds and stand around 40 inches at the shoulder. Ewes (females) also have horns, but they are shorter with less curvature.
Because they are a prey animal, sheep require excellent senses to enhance their chances of survival. Their keen eyesight, hearing, and sense of smell help them detect and avoid predators. They prefer wide open areas like fire burns, sparsely vegetated cliffs and places they can use their specialized senses to their advantage from cougars and other predators. 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. Because of their amazing balance, bighorn sheep can stand on ledges that are only 2 inches wide, they can jump 20 feet and race up a cliff at a brisk 15 mph.
Sheep depend heavily on their vision, their wide-set eyes are situated well forward on the head, providing excellent peripheral vision and they can see behind themselves without turning their heads. However, they have poor depth perception, they cannot see well immediately in front of their noses. Contrary to previous thought, sheep and other livestock perceive colors, though their color vision is not as well-developed as it is in humans. Sheep will react with fear to new colors.
They have excellent hearing, and can direct their ears in the direction of a sound. Sound arrives at each ear at slightly different times, with a small difference in amplitude. They are frightened by high-pitched and loud noises, such as barking dogs or people yelling.
They have an excellent sense of smell and are very sensitive to what different predators smell like. Smell helps rams locate ewes in heat and ewes locate their lambs. Sheep also use their sense of smell to locate water and determine subtle or major differences between feeds and pasture.
The sense of taste in sheep is probably not as important as the other senses. However, they have the ability to differentiate between feedstuffs. Bighorn’s are herbivores, and primarily graze on grasses, sedges and forbs(4) and they seek minerals at natural salt licks. Their digestive system acts as a survival mechanism. They eat large amounts of vegetation quickly. Females tend to forage and walk, possibly to avoid predators and protect lambs, while males tend to eat and then rest and ruminate, which lends to more effective digestion and greater increase in body size.(5)
Unlike ungulates (cows, deer and elk), bighorn sheep rams start growing their horns at birth and continue to grow them throughout their lifespan. Although the male horns are “hollow” they can weigh up to thirty pounds. The horns are made of keratin, which is the same material that fingernails and hooves are made of, and is a fast-growing substance.
Bighorn’s are gregarious and live in social groups, but rams and ewes usually only meet to mate. Rams live in bachelor groups and ewes live in herds with younger lambs. Young females generally remain in their mother’s group (led by an older ewe) for life. Males depart their mother’s group around two to four years of age and join a group of rams. This is sometimes a tough time of wandering until the young rams find a male group, and they will sometimes take up with other species out of loneliness.
Prior to the mating season or “rut”, the rams attempt to establish a dominance hierarchy to determine access to ewes for mating by exhibiting agonistic behavior: two competitors walk away from each other and then turn to face each other and charge at speeds around 40 miles before jumping and lunging into headbutts clashing their curled horns, which produces a sound that can be heard a long way off. Rams’ horns frequently exhibit damage from repeated clashes.(6)
Females exhibit a stable hierarchy that correlates with age. Bighorn ewes have a six-month gestation. Lambs born earlier in the season are more likely to survive than lambs born later because lambs born late may not have access to sufficient milk.(7) Newborn lambs weigh from 8 to 10 pounds and can walk within hours. The lambs are then weaned when they reach four to six months old.
The lifespan of rams is typically 9–12 years, and 10–14 years for ewes. They are highly susceptible to diseases carried by domestic sheep, such as scabies and pneumonia. Additional mortality occurs as a result of accidents like falling off cliffs.(9) Predation primarily occurs with lambs, which are hunted by coyotes, bobcats, lynxes, and golden eagles. Bighorns of all ages are preyed on by cougars, which are perhaps best equipped with the agility to prey on them in uneven, rocky habitats. They are considered good indicators of land health because the species is sensitive to many human-induced environmental problems.
National Wildlife refuges, National Parks and Monuments, Public Lands (BLM) and State Wildlife Departments manage and fund bighorn populations from hunter fees. To learn more about bighorn sheep read Wild Sheep Country very readable and enjoyable book by Valerius Geist.
(1) Recent genetic testing indicates three distinct subspecies of Ovis canadensis in the U.S. one of which is endangered: “meta-population” within the California Desert. Unfortunately some subspecies, such as Ovis canadensis auduboni of the Black Hills, were driven into extinction. Historic 1930s campaigns to save the desert bighorn sheep have resulted in the establishment of two bighorn game ranges in Arizona: Kofa National Wildlife Refuge and Cabeza Prieta National Wildlife Refuge. Ovis canadensis is one of three species of mountain sheep in North America and Siberia; the other two species being O. dalli, which includes Dall sheep and Stone’s sheep, and the Siberian snow sheep, O. nivicola.
(2) Peoples of First Nations – Many historic cultures, including the Fremont and Anasazi People hunted bighorn sheep. They also left petroglyphs (painted) and pictographs (pecked) on the rocks. While this art may look like a bighorn sheep or hunting them, we do not know what they really represent. Just imagine someone discovering the Statue of Liberty in a thousand years with no Rosetta Stone to explain what it meant to us. Might they not think we worshiped a large woman who read to us by torch light. Appreciate the petroglyphs for what they are, a master control of the medium and environment to last so long, and a symbol that The People had enough free time to create.
(3) Recruitment & Stress – Forage/water competition with livestock, potential for disease transmission from livestock, collision mortality on highways, herbicides/fatal plants on golf courses, climate change increasing temperatures, fractured habitat associated with oil and gas extraction, housing and recreational vehicle use in increasingly steep terrain (including lambing habitat) are continuing to devastate bighorn sheep populations. Recruitment, especially of rams can cover great distances. In 1979 a collared ram traversed almost 120 miles from the Rocky Boy Reservation to the Charles M. Russell National Wildlife Refuge in Montana across open range.
(4) Forbs – Forbs are herbaceous (not woody), broadleaf plants that are not grass, sedge, or rush-like. Bighorn sheep eat different foods depending on the season, forbs are especially important for lambs during spring and summer. During the summer, Bighorn subsist on grasses or sedges. During the winter they eat more woody plants, such as willow, sage and rabbit brush.
(5) Rumen – Bighorn sheep have a complex four-part stomach that enables them to eat large portions rapidly before retreating to cliffs or ledges where they can thoroughly rechew and digest their food. They regurgitate and grind the plants to make more surface area on the plants and reswallow the cud where bacteria take over, breaking down plant fibers for digestion. The bacteria are then passed to the reticulum (also known as tripe) that has a honeycomb-like structure. The large surface area of the reticulum absorbs electrolytes, volatile fatty acids, minerals, and the fermented food. The sheep also absorb moisture during this digestive process, enabling them to go for long periods without water. Absorption of volatile fatty acids continues in the omasum, which has contractions that squeeze fluid out of the food before allowing it to continue into the abomasum. The abomasum is the fourth chamber in the ruminant. It functions similarly to the carnivore stomach as it is glandular and digests food chemically, rather than mechanically or by fermentation like the other three chambers of the ruminant stomach.
(6) Horns and Dangers to Humans – The sound of two rams butting heads can be heard a long distance. Think of a drum full of water and another that is empty, beating on both drums, the sound of the empty drum is louder and travels further. Male bighorn sheep have large horn cores, enlarged sinuses, and internal bony septa. These adaptations serve to protect the brain by absorbing the impact of clashes. They have preorbital glands on the anterior corner of each eye, inguinal glands in the groin, and pedal glands on each foot. Secretions from these glands may support dominance behaviors. Within each horn is a living core that provides a continuous flow of blood beneath the hard sheath. Rams’ horns typically grow tremendously until the ram gets old and fully mature when the horn growth slows down. Yet, as the ram becomes fully mature, the horns grow more in mass, becoming thicker.
As the bighorn rams age a growth ring is created. For each year that passes, a ring is reflected in a ram’s horn and is often referred to as “growth rings” or “annuli rings.” The ring is created when the animal is under stress, which is usually caused by rutting or mating rituals in which they are not thinking about feeding and maintaining their nutrition. Rocky Mountain bighorn rings are created in the winter. Counting rings is the best way to age a bighorn ram. The most important thing to look for when aging a bighorn ram is the four-year ring. the most predominant dark and discolored ring. Four years old is when bighorn rams have matured enough to start mating. This creates the first major stress on their body, which then creates a deeply dark and discolored ring. To age a ram look for the four-year ring and then count the rings toward the skull.
If you encounter a bighorn ram while hiking, especially during the rut, do not make eye contact, turn slightly sideways and walk away down slope. If you face the ram or are on higher ground the ram may see you as a challenger and charge you.
(7) Milk – The composition of bighorn ewe milk varies with the stage of lactation. Initially, it contains Colostrum, a milky fluid that comes from mammals the first few days after giving birth, before true milk appears. Colostrum contains proteins, carbohydrates, fats, vitamins, minerals, and antibodies that fight disease-causing agents such as bacteria and viruses. Bighorn ewe milk ranges around: fat 8%; lactose 2%; protein 13%. Domestic sheep milk contained considerably lower fat and protein than bighorn sheep milk. The butterfat is considerably higher than cow’s milk, which averages around 3%.
(8) Heart Rate – The heart rate recorded from animals moving at night or through timber by day are higher than during daytime movement across open slopes. The appearance of free-ranging canids evoked maximal increases in HR in all ewes. Vehicular traffic and aircraft elicited high heart rate responses. The appearance and continued presence (1–10 minutes) of a human within 200 feet resulted in a 20% rise in mean heart rate. Harlow et al. (1987) reported that if a bighorn experienced a combination of medium or heavy stressors in a single day, the animal would be assessed as being in a state of chronically elevated blood cortisol. It has been suggested that stress-related release of cortisol may inhibit reproductive mechanisms, depress immunosuppression, and account for widely observed epidemics of pneumonia.
(9) Falls – Not all Bighorn die from predation or hunting. In 1988 on an early March river trip through Split Mountain in Dinosaur National Monument I observed a ram on the ice just below Inglesby Rapid. The ram had obviously fallen and had broken its neck. It appeared that the ledge the ram had jumped to had broken off as there were signs of rockfall on the snow and rock debris on the ice.
(10 Disease – Many bighorn sheep populations in the United States experience regular outbreaks of infectious pneumonia, which result from the introduction of bacterial pathogens (in particular, Mycoplasma ovipneumoniae, and some strains of Mannheimia haemolytica) carried in domestic sheep. Once introduced, pathogens can transmit rapidly through a bighorn population, resulting in all-age die-offs that sometimes kill up to 90% of the population. In the years following pathogen introduction, bighorn populations frequently experience multiple years of lamb pneumonia outbreaks.
(11) The Pleistocene is the geological epoch which lasted from about 2,588,000 to 11,700 years ago, spanning the world’s most recent period of repeated glaciations. The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Paleolithic age used in archaeology.
– Geist, Valerius, (1993), Wild Sheep Country.
– Geist, Valerius, (1971). Mountain sheep: a study in behavior and evolution. Chicago, IL: Univ.
– Grubb, P. (2005). Wilson, D.E.; Reeder, D.M. (eds.). Mammal Species of the World: A Taxonomic
and Geographic Reference (3rd ed.). Johns Hopkins University Press.
– Yoshida, Kate (2014). “A Symbol of the Range Returns Home”. New York Times.
– Chung-Hsu-Chen, E., (1965) Milk Composition of Rocky Mountain bighorn Sheep, Department of
Agricultural Chemistry McGUl University, Montreal.
– Valdez, R.; Krausman, P.R. (1999). Mountain Sheep of North America. The University of Arizona
– Hass, C. C. (1991). “Social status in female bighorn sheep (Ovis canadensis): expression, development, and
reproductive correlates”. Journal of Zoology. London.
– Dassanayake, R.P.; et al. (2010). “Mycoplasma ovipneumoniae can predispose bighorn sheep to
fatal Mannheimia haemolytical pneumonia”. Veterinary Microbiology.
– Coates and Schemnitz: (1989), Cardiac Telemetry, Natality, and Food Habits of Bighorn Sheep, Published by
Wyoming Scholars Repository.
– Follman, E. H., A. E. Manning, and J. L. Stuart. (1982). A long range implantable heart rate
transmitter for free ranging animals. Biotelemetry and Patient Monitoring.
Herm Hoops’ life has always been associated with water: from bucolic farm ponds and awe-inspiring rivers to the endless ocean, and he’s always had an interest in history.
Herm, the son of farmers, grew up on a large dairy cattle and Morgan horse farm. After attending the University of Vermont he taught Vocational Agriculture and Forestry in the Northeast Kingdom of Vermont.
He began running Western rivers in 1966. In 1972 Herm left Vermont and headed West for the better part of a year to run any rivers he ran across. In 1975 he began a career with the National Park Service and U.S. Fish and Wildlife Service. Herm retired from the Dinosaur National Monument in 1996, but he has continued following his love of rivers as a guide, naturalist, historian, and he is proactive in protecting the river canyons of the Colorado Plateau. Over the years he has been acknowledged for his contributions to a large number of river guides and books. Herm has written articles for historical journals and magazines. He is a lifetime member of the Colorado Plateau River Guides, the Grand Canyon River Guides and recently was made an honorary life member of the Utah Guides and Outfitters Association. Herm has served on the boards of several organizations, including Plateau Restoration and Conservation Adventures (Moab) and Colorado Plateau River Guides.
The rivers have been good to him, and perhaps, he has been good to them.
Herm and his wife Valerie live in Jensen, Utah.