白大角羊种

亚种

白大角羊其下有两个亚种：北部的Ovis dalli dalli全身呈白色；南部的Ovis dalli stonei呈浅褐色，臀部及后脚内侧有白斑。研究显示这些亚种存有疑问。在育空有一群中间形态的Ovis dalli fannini。线粒体DNA证据显示这些亚种之间在分子上没有分野，但细胞核DNA则提供些许支持亚种的说法。另外因为发现了白大角羊与大角羊的混种，白大角羊在物种的层面也有所质疑。

分布及栖息地

白大角羊分布在阿拉斯加的亚北极山脉、育空、西北地区的马更些山脉及英属哥伦比亚北部。它们栖息在相对较干旱的山地，特别喜欢留在辽阔的山脊、草坪及陡坡，因为可以躲避掠食者。

特征

雄性白大角羊有厚而弯曲的角。雌羊的较短及幼长，且轻微弯曲。雄羊联群生活，很少会与雌羊群有关联，只会在11月下旬至12月初时的交配季节才会走在一起。幼羊会于5月出生。

在食物丰富的夏天，白大角羊会吃多种的植物。到了冬天，它们会吃地衣及苔藓。在春天，很多白大角羊群落都会走到洛矶山雪羊观赏区觅食。

白大角羊的主要掠食者有狼、郊狼、美洲黑熊及灰熊。金雕会掠食幼羊。

Dall sheep or thinhorn sheep (Ovis dalli), is a species of wild sheep native to northwestern North America. O. dalli live in mountainous alpine habitats distributed across northwestern British Columbia, the Yukon, Northwest Territories and Alaska. They browse a variety of plants, such as grasses, sedges and even shrubs, such as willow, during different times of the year. They also acquire minerals to supplement their diet from mineral licks. Like other Ovis species, the rams engage in dominance contests with their horns.

Taxonomy and genetics

The specific name dalli, is derived from William Healey Dall (1845–1927), an American naturalist. The common name, Dall's sheep or Dall sheep is often used to refer to the nominate subspecies, O. d. dalli. The other subspecies, O. d. stonei, is called the Stone sheep.

Originally, the subspecies O. d. dalli and O. d. stonei were distinguished by the color of their fur. However, the pelage-based designations have been shown to be questionable. Complete colour intergradation occurs in both O. dalli sheep subspecies (i.e., Dall's and Stone's), ranging between white and dark morphs of the species. Intermediately coloured populations, called Fannin sheep, were originally (incorrectly) identified as a unique subspecies (O. d. fannini) with distributions inhabiting in the Pelly Mountains and Ogilvie Mountains of the Yukon Territory. Fannin sheep have more recently been confirmed as admixed individuals with predominantly Dall's sheep genetic origins.

Description

O. dalli stand about 3 ft (0.9 m) at the shoulder. They are off-white in color, and their coat consists of a fine wool undercoat and stiff, long, and hollow guard hairs. Their winter coats can be over 2 inches (5 cm) thick. O. dalli can live to be 12 to 16 years of age.

O. dalli are sexually dimorphic, which means rams and ewes look different. Rams are larger than ewes and typically weigh between 160 and 180 pounds (70 and 80 kg) at maturity. Ewes weigh approximately 100 to 110 pounds (45 to 50 kg) on average. During the winter, adult sheep may lose up to 16% of their body mass, and lambs and yearlings as much as 40% depending on winter weather severity. O. dalli begin growing horns at about two months old. Ewes have small, slender horns compared to the massive, curling horns of rams. Young rams resemble ewes until they are about 3 years of age. At this point, their horns begin to grow much faster and larger than ewes' horns.

Natural history

Ecology

The sheep inhabit the subarctic and arctic mountain ranges of Alaska, the Yukon Territory, the Mackenzie Mountains in the western Northwest Territories, and central and northern British Columbia. O. dalli are found in areas with a combination of dry alpine tundra, meadows, and steep or rugged ground. This combination allows for both grazing and escape from predators.

O. dalli can often be observed along the Seward Highway South of Anchorage, Alaska, within Denali National Park and Preserve (which was created in 1917 to preserve the sheep from overhunting), at Sheep Mountain in Kluane National Park and Reserve, in the Tatshenshini Park Tatshenshini-Alsek Provincial Park in northwestern British Columbia, and near Faro, Yukon.

Primary predators of this sheep are wolf packs, coyotes, black bears, and grizzly bears; golden eagles are predators of the young. O. dalli have been known to butt gray wolves off the face of cliffs.

Relationship with humans

Hunting

The Inupiat people have a long history of hunting O. dalli that dates back to at least the 16th century. Sheep are valued for their skin, which is used for warm clothing, and their meat, especially in times when caribou are not available. Historically, the sheep were hunted in summer by foot and in winter by dog sled teams. Today, the rugged terrain in which they live still requires foot travel to reach these animals. The dependence on O. dalli for meat and clothing fluctuates with caribou populations. Caribou herds declined considerably in the 1940s, and O. dalli became an important harvest species. Since the 1990s, caribou populations have been large enough to sustain people. Consequently, subsistence harvest of O. dalli is lower now than in the 1940s, but sheep continue to be an important meat source when caribou migration routes shift during the winter or between years.