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Bighorn Sheep - Ovis canadensis

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Inna Slobodyan

on 22 April 2014

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Transcript of Bighorn Sheep - Ovis canadensis

Bighorn Sheep
Ovis canadensis

Diet
Behavior
Live in herds or bands of about 5 to 15 ewes, lambs, yearlings, and 2 year olds
Groups of males are smaller usually consisting of 2 to 5 males
In the fall the males compete for ewes by having a butting contest
They charge at each other at a speed of more than 20 mph and crash directly into the horns of the competitor
Last as long as 24 hours
The Way out
Background
One of two species of mountain sheep in North America
Get their name from the large horns on the males
Horn length is more than 30 inches and weigh 30 lbs
Males live 9-12 years while females live 10-14 years
Beginning of the 19th century there were between 1.5 million to 2 million bighorn sheep in North America
Today, there are less than 70,000
Range & Population
Eat different foods depending on the season
Summer diet
Grasses or sedge
Winter diet
Woody plants such as willow, sage, and rabbit brush
*Desert bighorn sheep will eat brushy plants such as desert holly and desert cactus
Male (Rams)
Female (Ewes (Pronounced /Yous/))
Dall sheep (Ovis dalli)
Stone sheep (Ovis dalli stonei)
Hey there
Reproduction

Mating Season: November and December
Gestation: 5-6 months
Offspring: 1 lamb
Lambs are born with soft woolly light colored coats and small hornbuds
Within a day, a lamb can walk and climb as well as its mother.
A lamb will stay with its mother for the first year of its life.
Threats
Predators
Mountain lions, coyotes, wolves and bears will all kill mountain sheep if given the opportunity
Disease
Disease usually takes a far higher toll among bighorns than predators
Accidents
Bighorn sheep sometimes slip and fall from the cliffs or are struck by falling rocks
Poaching of large, trophy males.
Study 1: Mating in Bighorn Sheep: Frequent Male Reproduction via a High-Risk "Unconventional" Tactic
Rocky Mountain bighorn rams use three distinct tactics in competition for mates.
Tending, blocking, and coursing
Coursing is a rare example of an unconventional alternative mating tactic that is high-gain and high-risk
Injury from falls and horn blows during coursing may cause death, handicap future mating competition or increase risk of predation
Study 1: Methods
Study was done in the National Bison Rage (NBR) bighorn population (Montana) during 1979-1993 and in the River Sanctuary (SR) herd (Alberta) in 1989

Used eartags or phenotypes to recognize and analyze data on bighorn sheep

Tissue for genetic analysis was collected by a biopsy dart or after chemical immobilization
Uh Oh
Study 1: Results
Male mating tactics in bighorn are not restricted to discrete genetic, developmental or social classes
Coursing combat is made less risky by the evolution of concussive rather than piercing weaponry
Injuries from falls or horn blows were common and the damage compromised future ability to course and increased vulnerability to predation
Study 2: Age-Sex Differences in the Diets of Wintering Rocky Mountain Bighorn Sheep
Rocky mountain sheep in Banff National Park, Canada segregate sexually onto separate portions of their winter range & occupy cliff and meadow terrains in different portions
This separation suggests ecological separation for purpose of competition reduction
Study 2: Methods
All observations were made on the Palliser Range in the southeast corner of Banff National Park during the winters of 1975-1976 and 1978
The range was divided into 12 spatial units
Sheep were censused from one to four times weekly
Total of 75 censuses were completed yielding 2153 sheep sightings
Used fecal samples to determine diet in different ranges

Study 2: Results
Fig 1 shows that rams were more present in the northern range and non-rams exhibited an inverse relationship

Table 3 also showcases the similarity comparing the mean diets of non-rams and rams based on their range
Darwin stated that competition is most intense between individuals that are ecologically similar
Thus, intraspecific competition is likely to be the most severe

The first study focused on competition between male bullhorn sheep (coursing) and their competition for dominance and ewes

The second study focused on intraspecific competition between non-rams and rams
Resources
Buechner, H. (1960). The Bighorn Sheep in the United States, Its Past, Present, and Future. Wildlife Monographs, (4), 3–174.

Hogg, J., & Forbes, S. (1997). Mating in Bighorn Sheep: Frequent Male Reproduction via a High-Risk “Unconventional” Tactic. Behavioral Ecology and Sociobiology, 41(1), 33–48.

Shank, C. (1982). Age-Sex Differences in the Diets of Wintering Rocky Mountain Bighorn Sheep. Ecology, 63(3), 627–633.


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