Lesson Plan

Bighorn Sheep

Desert bighorn ram
Desert bighorn rams live on a knife edge of survival.

NPS photo by Neal Herbert

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Grade Level:
Sixth Grade
Subject:
Biology: Animals, Ecology
Duration:
30 mins
Setting:
outdoors
National/State Standards:
Utah State Science Corps Curriculum Topic, Standard Five: Students will understand that microorganisms range from simple to complex, are found almost everywhere, and are both helpful and harmful.
Keywords:
virus, bacteria, Desert Bighorn Sheep, habitat, parasites

Overview

Students first play a board game describing the habits and hardships of desert bighorn sheep. Outside, students explore the tracks and track patterns of animals that live in bighorn sheep habitat, learn about plants bighorns eat, play a bighorn trivia game, and learn to identify the birds that share bighorn habitat. Back in the classroom, students put clues together to solve the mystery of how microorganisms could wipe out a herd of sheep.

Objective(s)

PRE-TRIP ACTIVITY
A Bighorn Adventure


Students will be able to:
a. Describe the habitat of desert bighorn sheep.
b. Name some adaptations that help desert bighorn to survive.


FIELD TRIP ACTIVITIES
Hike the Habitat: Bighorn Sheep, Plants, Tracks and Birds


Students will be able to:
a. Describe the habitat of desert bighorn sheep.
b. Name two plants that desert bighorns eat.
c. Name two other creatures that share the habitat of desert bighorn sheep.


POST-TRIP ACTIVITY
Murder Ewe Wrote


Students will:
a. Analyze information about a complicated wildlife population event, and apply the analysis to answering a number of questions.
b. Name three factors that can lead to a crash in a bighorn sheep population.
 

Background

(from Hauke, 1998; U.S. Forest Service, Rocky Mountain Region 1995)

Bighorn sheep are the most highly evolved and most widely distributed of all bovines. They are thought to have evolved in North Africa and migrated to the Americas during the Pleistocene. Desert bighorn sheep live in the Canyonlands of Utah and the deserts of Arizona New Mexico, and some parts of California and northern Mexico. Desert bighorns are very sensitive to changes in the environment; therefore, they are often referred to as an “indicator species.” A healthy, thriving herd of bighorns is indicative of a healthy, thriving ecosystem. An unhealthy herd of bighorns may indicate that the ecosystem is overused. Recent on-going studies by biologists indicate that for a herd of bighorns to survive long-term, there must be a minimum of 100 animals.

Bighorns are generally a medium gray-brown with white on the rump, backs of legs, and muzzle. This coloring allows them to blend in with the rocky landscape that surrounds their habitat. Desert bighorn depend primarily on their sense of sight to detect danger. They also have good hearing. Their sense of smell is used to distinguish between foods, detect enemies, and identify their young.

Group at Mesa Arch Adult desert bighorns are 30-39 inches tall atthe shoulders. Males are normally larger than females. An adult male, called a ram, averages 160 to 200 pounds or more early in the summer. Rams have a thick, blocky appearance. They have thick necks and large curled horns that measure up to 30 to 40 inches along the outside of the curl. An adult female bighorn, called an ewe, averages 105 pounds. Ewes are more slender than males; they have especially slender necks. Ewes have small horns measuring 10 to 13 inches long. Wildlife managers categorize bighorns into four size classes, using the curl of the horn to determine the class. Rams recognize their elders by the size of their horns; dominant sheep having the biggest horns. Average life expectancy is 10-12 years.

Desert bighorns need food, water, escape terrain, and space. Their first food preference is Indian ricegrass. They also browse brush and plants with woody stems, especially blackbrush, and sometimes ephedra. They also feed on a few forbs (green leafy plants other than grasses). Water is the primary limiting factor for desert bighorn. Rams can go a long time without drinking; getting all the water they need from their food. But ewes, especially during lactation, require a regular source of water. Sources of water include streams, potholes, dew, springs, and water found in food. Desert bighorn prefer open space around their drinking holes so they can see approaching threats. They won’t venture more than a few hundred meters from their escape terrain to get water.

Bighorns use escape terrain to get away from predators, since few animals are able to move as quickly as bighorns through such rugged terrain. Bighorns do not run long distances, but escape their enemies by climbing and hiding in this rugged terrain. Caves and the shelter of trees are used during poor weather and to escape aircraft and eagles. Predators of bighorns are coyotes (most common), eagles (feed on lambs), gray foxes, bobcats, and mountain lions. Predation is not a primary cause of death for bighorns, due to their escape terrain and the variety of wildlife that these predators prey upon.

The area across highway 191 from the Atlas Tailings pile has all the components of good desert bighorn sheep habitat. It has an open space for the animals to graze and spot predators, and it is close to steep rocky escape terrain. Unfortunately, cars hit approximately one sheep a year, a danger the sheep are not genetically adapted to avoid.

Ewes have their first lambs at about age three,and produce one, rarely two, lambs each year. Lambs are born in the spring and gain weight quickly. By two to three months of age they have sleek, well-proportioned bodies. At six months of age they are weaned from their mothers. Lambs are usually born in rough terrain with caves or overhanging rocks for protection from predators and weather. Nighttime bedding grounds are often near the top of ridges or long spurs, from which ewes can see much territory. Locations like this allow for a quick escape over the ridge or down the mountain.

Desert bighorns have a “nursery system.” Two ewes remain with all of the lambs on the edge
of escape terrain. The other ewes in the band move into open areas to feed on succulent spring plants. Lambs are obedient to the ewes in charge. The feeding ewes return to the lambs on occasion to nurse and exchange places with the nursery ewes. As lambs become older and begin to eat solid foods, they begin to travel with their mothers.

Cause of death can be hard to determine in bighorns. Causes include diseases caused by bacteria and viruses, parasites, tumors, and mineral and dietary deficiencies, as well as accidents, poisonous plants, and extreme climate conditions. Often several factors interact to cause death. Domestic sheep are the biggest threat to bighorn. They eat similar foods and carry parasites and diseases detrimental to bighorn. Bighorns are known for precipitous herd die-offs. Investigations have attributed the causes to parasites and diseases. Recent management efforts to separate domestic sheep from bighorn range have resulted in decreased die-off. Ear mites cause the most common disease among the bighorn of southeastern Utah. These mites spread down from the head, cause skin problems, and weaken the sheep, making them more likely to succumb to other stresses.

When settlers arrived in the western U.S., there were probably 1.5 to 2 million desert bighorns. In 1975, there were only about 1000 in the state of Utah. The drop can probably be attributed to diseases brought in by European domestic sheep. In the 1970s, grazing leases
within Canyonlands and Arches national parks expired. Around this time, the BLM also ended sheep grazing leases on BLM lands near the bighorn range in the parks. Consequently, the populations of bighorn sheep in this area have vastly increased. When Canyonlands was established in 1964, there were 80 bighorn livingin the park. Most bighorn were in the Island
in the Sky District; a few were in the Needles District. Now, Island in the Sky is home to approximately 350 bighorn. Some Island in the Sky sheep were transplanted to produce the Arches herd, now 125 head, and the Maze herd, now 50-100 head. The population of the Needles herd dropped from 125 to 15 in the mid 1980s, after domestic sheep were grazed just outside the park. The domestic sheep were removed and the bighorn population has since rebounded to 50 head.

Individual bighorns can become physically rundown or nervous if harassed by other bighorns,
large animals, or people, which can contribute to improper diet. Bighorns are sensitive to people on foot, especially in areas where people seldom travel. Bighorns seem less bothered by people on bikes or in cars, even when vehicles stop for a look.

Bighorns commonly use pathways dictated by the topography of the land. The Colorado and Green rivers provide natural barriers that divide herds, and prevent the spread of disease. Unfortunately, roadways, fences, and canals built by people tend to cross bighorn travel routes. This can limit movement from feeding grounds to water, causing herds to become isolated. Inbreeding can occur in smaller herds, which weakens the immunity and health of the herds and creates a serious concern for the Bighorn sheep at Arches National Park herd’s longevity. Fortunately, much of the land occupied by bighorn is under the protection of government agencies. These agencies are attempting to implement new regulations on bighorn range that will benefit these animals.

Procedure

Additional Resources

Elmore, F. (1976). Shrubs and trees of the southwest uplands. Globe, AZ: Southwest Parks and Monuments.

Fagan, D. (1998). Canyon country wildflowers: A field guide to common wildflowers, shrubs, and trees. Helena and Billings, MT: Falcon Publishing.

Hauke, Craig. (1998). Bighorn Sheep: Monitoring and management of desert bighorn in the national parks of southeast Utah. Lecture, Dan O’Laurie Canyon Country Museum and the Moab Information Center Lecture Series. (Video)

Stall, C. (1990). Animal tracks of the southwest. Seattle, WA: The Mountaineers. U.S. Forest Service, Rocky Mountain Region. (1995). Ecosystem matters: Activity and resource guide for environmental educators. U.S. Government Printing Office: 1995-577-064.

Williams, D. (2000). A naturalist’s guide to canyon country. Illus. by G. Brown. Helena, MT: Falcon Publishing.