The stocky brown body, large white rump patch and small black tail, are diagnostic features of Bighorn Sheep. The characteristic massive, brown ridged horns of adult males are tightly spiralled and especially large at the base, while those of females are much smaller, narrower, and more upright, curving only slightly up and back from the head. Although female Bighorn Sheep are sometimes confused with Mountain Goats, the two species can be readily distinguished by body coloration and horn features. Female Bighorn Sheep have a brown upper body, and a large white rump patch with a short black tail, in contrast to the all-white Mountain Goat. Female Bighorn Sheep horns are dull brownish with rounded, blunt tips, quite different from the sharp-tipped, shiny black horns of Mountain Goats.
The species most closely resembling the Bighorn Sheep is the Thinhorn Sheep. Dall’s Sheep is easily separated by its all-white body, and the body of Stone’s Sheep is black or grey rather than brown like the Bighorn Sheep. Stone’s Sheep males also have a lighter coloured neck that contrasts with the rest of the body, whereas all Bighorn Sheep, even those with dark brown coats, have the same colour neck as the body.
The skull of a mature Bighorn Sheep can only be confused with that of Thinhorn Sheep males, and it is distinguishable by the horn cores. Those of Bighorn Sheep males taper less and end in relatively blunt, jagged tips compared to the narrower, pointed tips of the Thinhorn’s horn cores. If horn sheaths are present on the skull, examine the keel (outer edge) of the horn: in Thinhorn Sheep it is more prominent and the general cross-sectional shape is more clearly triangular than in Bighorn Sheep. The horn sheaths of Bighorn Sheep also tend to be pale dull brown rather than amber as in Thinhorn Sheep.
Female Bighorn Sheep skulls are not easily separated from those of female Thinhorn Sheep, but can be readily separated from the similar-sized skulls of Mountain Goat. Horn cores of female Bighorn Sheep are oval in cross-section, being longer in the anterior-dorsal axis, rather than round as in the Mountain Goat, and sheep cores have blunt not pointed tips. The female Bighorn Sheep skull is also wider across the orbits giving the skull a somewhat triangular shape in dorsal view, compared to the narrower profile of the Mountain Goat skull.
Tracks of Bighorn Sheep are the same size but not as pointed as those of Mule Deer and White-tailed Deer, and the separate hoof prints are not as narrow as those of the Mountain Goat. Separating the faecal pellets of these four species can be difficult.
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The birth period is in early spring, and coincides with new vegetation growth and warmer temperatures. After a gestation period of around 175 days, most young are born between May and June. In some California Bighorn Sheep populations in the southern Okanagan and Ashnola, young are born as early as April. Females almost always produce only one young each year, but twins have been verified in some B.C. California Bighorn Sheep populations (e.g., the Vaseux herd). Newborns weigh from 2.8 to 5.5 kg. When about to give birth, a female usually leaves her group and moves to steep, rugged areas where her newborn will be relatively safe from most predators. Once the young is able to run fast, the pair return to the group, usually after four to seven days. Pregnancy rates in healthy B.C. populations are high, up to 90 per cent of adult females. Young of the year are followers, but by mid summer, one or two adult females may be seen grazing near several young of the year, suggesting that they may be baby-sitting. But this may simply be because older young are more independent, and instead of tagging along near their mothers, they spend more time playing and interacting with each other: butting heads, chasing, mounting and even playing King of the Castle on boulders.
Bighorn Sheep reach puberty as early as 18 months of age, but most females mate for the first time when 2.5 years old, and males much later, sometimes not until 9 or 10 years old. Young sheep do most of their growing between April and mid-October, capitalizing on the new vegetation growth. Horn growth also takes place during this period. Females grow little after 3 to 4 years of age, while males continue to grow until at least 6 years or older. Some biologists have suggested that Rocky Mountain Bighorn Sheep are heavier and larger than the California subspecies, but available data shows no consistent differences between them. Each winter, most individuals lose body mass, sometimes as much as 20 per cent, or even more under harsh conditions. For adult males, their mating activities can add to the over-winter losses in weight and body condition.
| Primarily a grazing species, the Bighorn Sheep is opportunistic, adapting its diet to the local and seasonal changes in available plants. Besides grasses, forbs and sedges, it will also consume browse such as willows and Douglas Maple. Most browse is consumed in spring when the buds and young leaves are most nutritious. California Bighorn Sheep may browse more often throughout the year than the Rocky Mountain subspecies, eating shrubs such as Saskatoon, Antelope-bush and Mock Orange in the southern Okanagan. Bighorn Sheep seem able to survive for long periods without free-standing water if necessary, meeting much of their water requirements from succulent vegetation in summer and from snow in winter. |
The age of a Bighorn Sheep can be fairly accurately determined by counting the distinct rings or annuli on the horns. The first horn ring can be difficult to recognize because it is often indistinct, and for females this method works only up to about 5 or 6 years of age. In old males, allowance must be made for where the terminal part of the horn is lost by brooming, which occurs during horn clashes between males when the tips split and eventually break off. With a little practice, a male’s age can be estimated fairly accurately in the field with the aid of binoculars or a spotting scope. Tooth succession and cementum annuli counts are other methods for telling a sheep’s age. The average longevity of adults varies between populations. The oldest recorded ages for individuals in the wild are from three marked Rocky Mountain Bighorn Sheep – a male and female from Banff National Park, Alberta, and a female from Stoddart Creek, B.C. All three were 20 years old, and the Banff female was accompanied by a young of the year. But, on average, female Bighorn Sheep live for 10 to 15 years and males for somewhat fewer years.
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Mortality is usually high during the first year of life due to disease, inclement weather, poor nutrition, poor mothering, human disturbance or predation. The impact of these factors depends on the date of birth, range condition, population density and suitability of security cover. Coyotes are one of the most important predators of Bighorn Sheep, preying especially on young of the year. Mortality of first-year young can reach 80 per cent in areas where security cover is limited. Eagles also prey on young of the year, and Wolves, Cougars and bears are known to take sheep of any age. In the Junction California Bighorn Sheep herd, local Cougars specialized on adult males after the rut, and together with Coyotes, caused a high level of mortality.
Disease has infrequent but often devastating effects on Bighorn Sheep populations in British Columbia, and over 50 per cent of the population can be lost in one year. Less dramatic losses are more often seen where young of the year or other age classes may be selectively affected. Large-scale die-offs of Bighorn Sheep have been reported since the 1800s and seem to occur about every 20 years in the East Kootenay region and in some other herds in British Columbia. The cause of these die-offs is still not completely understood. They are thought to originate from an interaction of population and environmental stress factors along with infectious micro-organisms (such as bacteria and viruses) and lungworm, and result in acute or chronic pneumonia. Mortality of Bighorn Sheep has often been associated with the presence of Domestic Sheep that can carry several infectious organisms capable of causing mortality in wild sheep. Die-off investigations report a range of infectious agents, including several respiratory viruses and a number of bacteria of which some species of Pasteurella are the most significant.
A combination of many population and environmental factors seem to play a role in a herd’s overall vulnerability to die-offs. These include contact with Domestic Sheep and Goats, trace mineral deficiencies, diet changes, poor nutrition, high population densities, inclement weather and other stress-producing events. There has been much research devoted to the Bighorn Sheep pneumonia syndrome, and while it has resulted in various management recommendations – such as proactive attempts to treat for lungworm, medication for bacterial pneumonia and improving habitat quality – so far, treatments and management to prevent die-offs have not been completely effective.
In 1987, an introduced herd of Rocky Mountain Bighorn Sheep near Chase suffered a die-off, dropping from about fifty individuals to fewer than five. Agriculture Canada diagnosed this as an outbreak of haemorrhagic disease caused by Epizootic Haemorrhagic Disease virus (EHD), which is similar to Blue Tongue virus and causes identical pathological lesions. This herd has a history of occasional less severe pneumonia-related declines.
All Bighorn Sheep populations examined in B.C. have been infected with one or two types of protostrongylid lungworm. Protostrongylus stilesi and P. rushi are uniquely adapted to Bighorn Sheep. Adult P. stilesi live and lay eggs in lung tissue, and when present in large numbers can cause severe damage that often leads to secondary bacterial infections. Protostrongylus rushi is found in the air passages and appears to be a lesser problem. Both parasites use terrestrial snails as intermediate hosts, but P. stilesi is also transmitted directly to the young by the mother before birth, so that infections may occur as early as 6 weeks of age. While lungworm in wild sheep may contribute to outbreaks of respiratory disease, they are believed to be more often responsible for reducing respiratory efficiency. This results in mortality of young animals, and heavily infected adults are less capable of extreme effort (e.g., running to escape from predators).
Another disease of Bighorn Sheep is contagious ecthyma or soremouth, an acute to chronic eruptive dermatitis seen on the skin of the lips, nostrils, eyelids, teats or coronary bands of the hooves. It is caused by a parapox virus and is most severe in young animals less than 6 months old, but occasionally infects older animals. Contagious ecthyma is transmitted by direct contact, or by contaminated feed and salt sources. It is not fatal, but can reduce feeding ability and lead to debilitating secondary bacterial infections. Following an infection, most individuals are immune for life.
Bighorn Sheep occasionally show tooth problems, most often in the lower jaw. Bacteria normally found in the mouth cause a bony growth or deformity of the jaw called actinomycosis, often referred to as lumpy jaw. Infection is believed to begin when the gum is damaged, such as when a hard, dry grass stalk or other coarse material damages the mucosa around the base of the tooth. The infection can attack the ligament around the tooth root and eventually cause the tooth to fall out. When this happens, teeth in the opposing row can become distorted if they are still erupting. Chronic jaw infections and tooth losses can reduce the animal’s ability to eat because of malocclusions, and so result in poor body condition and thus hasten death. Some Bighorn Sheep may be missing one or more of the first premolars, usually in the lower jaw, but this seems to be a genetically determined condition rather than the result of actinomycosis.
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Like most ungulates living in open habitats, Rocky Mountain and California Bighorn Sheep spend their lives in groups, with adult males and females living separately for most of the year. Group sizes vary from 2 to more than 40 sheep; the largest group reported in British Columbia was of 110 California Bighorn Sheep. But most females and young live in groups of around 25 members, and males generally live in smaller groups. The two sexes join together mainly in the rut to form mixed groups. Many populations use geographically separate areas at different seasons of the year – these are their seasonal ranges – often returning to the same locations each year. But a few populations, such as the California Bighorn Sheep population at Riske Creek, may stay on the same general range year round.
Adult male Bighorn Sheep use their massive curled horns to fight each other. Not only do they bash heads, but they rear up and run towards each other on their hind legs, hurling themselves at their opponents at the last minute. This creates a combined clash force in the order of 900 kg. This tremendous impact is absorbed not only by the mass of their large horns, but an adaptation of the skull: enlarged sinuses create a cavity within the skull above the brain, a condition called pneumation. The resulting double bone plates and the flexible bone connections between them are believed to cushion some of the clash force.
Besides the spectacular horn clashes, Bighorn males use various displays to intimidate each other. They may threat jump – rear up on their hind legs as if to clash – or make a horn threat, usually following a clash – holding the head high and staring at each other. In extreme cases, a dominance fight can last several hours. Once a male has established himself as dominant, he treats the subordinate male like a female, using the same behaviour patterns he does when courting, even including mounting. During the height of the rutting season, prolonged dominance fights are rare, because males have little time to waste in disputes over social status. Instead, they are intent on courting as many females as possible.
The mating season usually begins in early November and lasts until mid or late December; in some Okanagan herds, it can begin in late October. Shortly before the rut, males may gather together, probably to establish dominance relationships. Most of the spectacular fights between male Bighorn Sheep are noted for take place in this pre-rut period. The onset of the rut is signalled by males moving from female to female in a low stretch. They sniff each female’s ano-genital region and she usually responds by squatting and urinating. The male will sniff the urine before raising his head to perform a lip-curl, testing her oestrous condition. Once he finds a female in oestrus, a dominant male stays with her, defending her against all other males, while also courting and copulating with her.
Courtship consists of a series of patterns that gradually decrease the distance between the male and female, while increasing the degree of physical contact between them, until the female accepts copulation. A typical sequence of male courtship patterns is: nosing the female’s flanks and rump, often while twisting his head and flicking his tongue in and out and making a soft ululating growl; gently kicking or stroking her rump with his foreleg; placing his chin on her rump and pushing his chest against her rear; and finally, rising up on his hind legs in a copulation attempt.
Females show little if any courtship towards males. Often when several males are in a group, they will surround the courting pair, and sometimes, chases will ensue. Such chases also happen early in the rut when younger over-eager males try to copulate with females before they come into oestrus.
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