The Dall Sheep (sometimes called Thinhorn Sheep)...
The Dall Sheep (originally Dall's Sheep, sometimes called Thinhorn Sheep),
Ovis dalli, is a wild sheep of the mountainous regions of northwest North
America, ranging from white to slate brown and having curved yellowish
brown horns. There are two putative subspecies: the northern Dall Sheep
proper (Ovis dalli dalli) which is almost pure white, and the more southern
Stone Sheep (also spelled Stone's Sheep) (Ovis dalli stonei), which is a
slaty brown with some white patches on the rump and inside the hind legs.
Research has shown that the use of these subspecies designations is
questionable. Complete colour integradation occurs between white and dark
morphs of the species with intermediately coloured populations, called
Fannin's Sheep, found in the Pelly Mountains and Ogilvie Mountains of
Yukon Territory. Mitochondrial DNA evidence has shown no molecular
division along current subspecies boundries, although evidence from
nuclear DNA may provide some support. Also at the species level current
taxonomy is questionable because hybrdization between Ovis dalli and Ovis
canadensis has been recorded in recent evolutionary history.
The latter half of the latin binomial dalli is derived from William
Healey Dall (1845-1927), an American naturalist. The common name Dall
Sheep or Dall's Sheep is often used to refer to the species Ovis dalli.
An alternative use of common name terminology is that Thinhorn Sheep
refers to the species Ovis dalli, while Dall's Sheep and Stone's Sheep
refer to subspecies Ovis dalli dalli and Ovis dalli stonei.
The sheep inhabit the mountain ranges of Alaska, the Yukon Territory,
the Mackenzie Mountains in the western Northwest Territories, and northern
British Columbia. Dall sheep are found in relatively dry country and try
to stay in a special combination of open alpine ridges, meadows, and steep
slopes with extremely rugged ground in the immediate vicinity, in order
to escape from predators that cannot travel quickly through such terrain.
Male Dall Sheep have thick curling horns. The females have shorter, more
slender, slightly curved horns. Males live in bands which seldom associate
with female groups except during the mating season in late November and
early December. Lambs are born in May.
During the summer when food is abundant, the sheep eat a wide variety of
plants. During the winter diet is much more limited and consists primarily
of dry, frozen grass and sedge stems available when snow is blown off,
lichen and moss. Many Dall Sheep populations visit mineral licks during
the spring and often travel many miles to eat the soil around the licks.
The primary predators of Dall Sheep are wolves, coyotes, black bears,
grizzly bears and golden eagles are predators on the young.
Dall Sheep can often be observed along the Alaska Highway at Muncho
Lake and at Sheep Mountain in Kluane National Park and Reserve, as well
as near Faro, Yukon (Fannin's Sheep).
Watchful and difficult to approach, Dall sheep challenge the hunters,
wildlife watchers, and photographers who pursue them. The sheep too
are challenged - by the harsh alpine environments of Alaska and
northwestern Canada. The animals meet this challenge because of several
unique adaptations. One place they do well is the Arctic National
Wildlife Refuge.
image of Dall sheep rams lying on mountainside - USFWSThe Refuge contains
North America's northernmost Dall sheep population. Year-round residents
of the Refuge, the sheep live mostly above timberline on ridges, dry
meadows, and steep mountain slopes. There are always rocky outcrops
and cliffs nearby. The sheep rarely venture far from this rugged terrain,
using it to escape predators, including wolves, golden eagles, bears,
and humans. Natural mountaineers, sheep negotiate this terrain with
speed and agility. They rarely fall.
Dall sheep eat grasses, sedges, broad-leaved plants, and dwarf willows.
In winter, when these foods are scarce, the sheep add lichens to their
diet. The distribution and availability of forage requires the sheep to
move seasonally between traditional summer and winter ranges. On the
Refuge, the animals supplement their diet with regular visits to mineral
licks. The sheep usually roam in small social units, either maternal
ewe, lamb and yearling groups, or groups of rams.
Sheep forage is limited by the cool temperatures and nutrient poor soils
of the northern alpine environment. Under these conditions, the sheep
mature slowly and have low reproductive rates. Females reach breeding age
at three to four years and produce only one lamb per year. Males breed
when their horns are large enough for them to establish a dominant
position in the ram hierarchy, usually at seven to nine years.
Winter weather is the main factor that affects Dall sheep numbers. In
sheep habitat, temperatures normally stay below freezing, snowfall is
light, and winds sweep many ridges and slopes, keeping snow cover light.
These conditions allow the sheep good access to winter forage. However
heavy snows, temporary thaws, and freezing rains can create a frozen
barrier preventing the sheep from digging for the plants. Conditions
like these can cause population "crashes."
Dall sheep walk a survival tightrope, although they do it rather
effectively. They have lived since the Pleistocene in places such as
the Arctic Refuge. They are one of the special wildlife assets of
this magnificent place.