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Dall
Sheep Life Cycle
Like
all species, Dall sheep need water, food, shelter, and space to
survive. These four needs define the species’ habitat. Disease,
habitat destruction, predators, weather, food availability, pollution,
and human interference can all be limiting factors that affect
population size and health. In this unit, students will learn how
habitat affects sheep behavior over the course of a year.
No
single habitat provides everything Dall sheep need to survive throughout
the year, so sheep move between habitats as the seasons change.
Moving with the seasons is one type of migration.
In spring, after spending the winter at lower elevations, Dall sheep
migrate to open slopes at higher elevations as the snow melts. They
can often be seen around mineral licks. Scientists believe that
the mineral licks help re-supply the sheep with micronutrients
that were lost during the winter. Pregnant ewes leave the small
groups, or bands, to find isolated and protected spots for
giving birth. The birthing sites are often high, rocky outcroppings
or caves. Lambs are born in May or June just as the new plants and
grasses emerge. Ewes and lambs form an immediate bond, recognizing
each other by both smell and sound. The ewe and lamb will stay by
themselves for a few days before joining small nursery bands
consisting of ewes, lambs, and yearlings, last year’s lambs.
These bands stay together for protection against predators.
 During
the summer, Dall sheep are still in separated into nursery bands
and ram bands. They spend their time grazing on the alpine ridges
to build up fat stores that help them through the winter. Alpine
ridges with access to steep slopes and rocky outcroppings provide
safety from predators, as well as refuge from storms and wind. While
in the nursery band, they move with the young rams and ewes to various
summer ranges. By the time they are weaned in autumn, they will
weight 10 times as much as they did at birth.
The
summer is also a time for the lambs to learn from the ewes the necessary
behaviors for survival. The lambs quickly learn to eat grass and
escape danger. Common lamb predators are golden eagles, bears, and
wolves. The first year of a sheep’s life is perilous and many will
not survive.
In autumn, as snow begins to fall at the higher elevations, the
small bands of ewes and rams are forced down to lower subalpine
habitats. As they migrate to these lower, more accessible areas,
they come together in larger groups for the rut, or mating season.
The ram with the largest horns tops the dominance scale and treats
all other sheep, regardless of sex or age, as subordinates. During
the rut, older rams will challenge the dominant ram for mating rights
by clashing horns. Fighting can last several hours and ends when
one ram is either exhausted, hurt, or behaves in a subordinate manner.
The winning ram will breed with the ewes in the group.
 With
winter, Dall sheep continue their migration to the lower elevations,
seeking areas with the least amount of snow covering the plants
they forage. Rams that are over three years old move away from the
nursery bands and spend the winter in small groups, referred to
as bachelor or ram groups. Winter is a difficult time
for all the sheep. Dominant rams that were active during the rut
may now be undernourished or injured and may die. Some lambs might
still be too small to survive the winter. Winter diets are limited
to dry, frozen grasses and plant stems. Snow depth and air temperature
are critical in sheep survival. Sheep can dig through snow, up to
12 inches deep, but prefer areas that the wind has swept clear.
Winter can also have an effect on the unborn lambs. The gestation
period (how long they are pregnant) for the ewes averages 180
days (6 months). Ewes need both quality food for the developing
fetus and good weather in the spring in order to have a successful
pregnancy.
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