Thinhorn sheep (Ovis dalli)

Description

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Thinhorn sheep fact file

• Description
• Biology
• Range
• Habitat
• Status
• Threats
• Conservation
• Find out more
• Glossary
• References
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Thinhorn sheep description

The thinhorn sheep (Ovis dalli) is a medium-sized North American sheep with a stocky body and light brown, deeply ridged horns ⁽³⁾. The male and female thinhorn sheep are easily distinguished from one another. The male, known as a ram, has large, long, curly horns with sharp tips, which can grow up to a metre in length. In contrast, the female, known as a ewe, has small, slender horns, which are gently curved and grow up to around 25 centimetres ^{(2) (3) (4)}. The difference in the size of the horns accounts for a large difference in body mass between the male and female thinhorn sheep, with the male’s horns usually comprising between eight and ten percent of its total weight ⁽²⁾.

There are two recognised subspecies of thinhorn sheep, Dall’s sheep (Ovis dalli dalli) and Stone’s sheep (Ovis dalli stonei), which are differentiated by their colouration ^{(3) (5)}. Dall’s sheep has a creamy white coat and tail, whereas Stone’s sheep is black or grey with a black tail. Intermediates between subspecies are also known to occur ^{(3) (5)}. The fur of both subspecies is composed of a fine wool undercoat, covered by an overcoat of stiff, long hairs ⁽²⁾. The body of the thinhorn sheep is stocky, the legs are slender and the tail is short ⁽³⁾.

Also known as

Dall's sheep, Stone sheep, Stone's sheep, Thinhorn sheep.

Size

Total male length: 130 - 178 cm ⁽²⁾

Total female length: 132 - 162 cm ⁽²⁾

Male shoulder height: 92 - 109 cm ⁽²⁾

Male tail length: 7 - 11.5 cm ⁽²⁾

Female tail length: 7 - 9.9 cm ⁽²⁾

Male weight: 72.5 - 82.3 kg ⁽²⁾

Female weight: 46.4 - 50.4 kg ⁽²⁾

Thinhorn sheep biology

The diet of the thinhorn sheep consists mainly of grasses, shrubs and sedges ⁽²⁾, as well as lichens, willows and mosses at the beginning of autumn when other vegetation is in short supply ⁽⁵⁾. ‘Mineral licks’, rich in essential minerals like calcium, can also be important to the thinhorn sheep in certain seasons ^{(2) (5)}.

The thinhorn sheep is a gregarious species ⁽²⁾, with males forming groups of around 15 individuals, known as ‘bands’, and females gathering in larger groups which are accompanied by juveniles ⁽⁵⁾.

The mating season of the thinhorn sheep generally begins between November and December, when ‘rutting’ between males starts. This is where bands of males begin to engage in violent confrontations to compete for females, involving kicking, jumping, mounting and slamming the horns together. As an adaptation to these contests, male thinhorn sheep have air spaces in their skulls, which absorb the impact when the horns clash together. Fights are normally won by larger, older males, establishing a dominance hierarchy ⁽²⁾.

Male thinhorn sheep curl their lips to help detect the scent of any receptive females in the area. Once a female in oestrus has been detected, the male, having removed all potential mates from the area, follows and kicks the female to test her receptiveness to mating. If this is successful, mating occurs, which is followed by the male guarding the female for the next two or three days, to prevent the female from breeding with any other potential mates ⁽²⁾.

After a gestation period of around four and a half months, a single lamb is born between mid May and early June ^{(2) (5)}. The precocial lambs are able to travel with the female only 24 hours after birth, and within days the ewe and lamb rejoin the group ^{(2) (5)}. After three to five months, the lambs are fully weaned ⁽²⁾.

The male thinhorn sheep may occupy up to six different home ranges throughout the year and the female occupies up to four ⁽²⁾, although some populations are sedentary ^{(1) (2)}. As the seasons change, the thinhorn sheep move to more suitable areas. For example, the females will move to specific areas to lamb, usually in secluded, highly elevated areas ^{(2) (5)}. The ability to use upland meadows and scale near-vertical cliffs gives the thinhorn sheep an advantage when finding safe areas to give birth ⁽⁵⁾ and when escaping from predators ^{(1) (5)}.

Between April and September, the horns of the thinhorn sheep grow in length and circumference ⁽²⁾. This creates a deep ring around the horns each year, which can be used to determine the individual’s age ^{(4) (2)}. There is also a seasonal moult from March to July, with males moulting earlier than females and juveniles ⁽²⁾.

The thinhorn sheep is predated by many species, including wolves (Canis lupus), brown bears (Ursus arctos), wolverines (Gulo gulo), coyotes (Canis latrans)and lynxes (Lynx canadensis) ^{(1) (3) (2)}. Golden eagles (Aquilachrysaetos) are also known to take lambs in the first few weeks of life ⁽³⁾.

The average lifespan of the thinhorn sheep is around ten years ⁽³⁾.

Thinhorn sheep range

The thinhorn sheep ranges across northwest Canada, including British Colombia, Yukon and the Northwest Territories, as well as Alaska in the United States ^{(1) (2) (5) (6)}. Dall’s sheep is found throughout the whole of this range, whereas Stone’s sheep only occurs in northern British Colombia and southern Yukon ^{(1) (2) (5)}.

Thinhorn sheep habitat

The thinhorn sheep is found in dry, hostile and rugged areas in and around mountains ^{(1) (2)}. In these extreme alpine and Arctic regions ⁽⁶⁾, the thinhorn sheep can be found up to elevations of around 2,000 metres ⁽⁵⁾.

In spring, the thinhorn sheep occupies grassy mountainsides, moving to high alpine pastures in summer ⁽⁵⁾. In winter, this species performs altitudinal migrations, moving to areas with less snow at lower elevations ⁽⁵⁾. Areas with strong winds are preferred throughout the winter, as patches of vegetation are exposed from beneath the snow by the wind ^{(1) (2) (5)}.

Thinhorn sheep status

The thinhorn sheep is classified as Least Concern (LC) on the IUCN Red List ⁽¹⁾.

Thinhorn sheep threats

Hunting for both sport and sustenance takes small quantities of thinhorn sheep, although this is not thought to be a major threat to the species ^{(1) (3)}. High population density, disease and low-quality forage are other potential threats ⁽¹⁾. Avalanches and accidental falls have also been recorded as causes of mortality ^{(1) (2)}.

Mineral exploration, road building and other human activities within the range of the thinhorn sheep could also be potential threats ⁽²⁾. However, this species is widespread and abundant, and its population is currently believed to be stable ⁽¹⁾.

Thinhorn sheep conservation

The sport hunting of thinhorn sheep is regulated and a limited number of males over a certain age are allowed to be taken, as long as a licence is obtained and any kill is reported. Some parts of the thinhorn sheep’s range are designated as national parks, therefore banning any sport hunting and preventing industrial development within the area ⁽¹⁾.

Find out more

Find out more about the thinhorn sheep:

• BBC Wildlife Finder - Dall sheep:
  http://www.bbc.co.uk/nature/life/Dall_Sheep
• Ultimate Ungulate:
  http://www.ultimateungulate.com/

Authentication

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact: 
arkive@wildscreen.org.uk

Glossary

Gestation

The state of being pregnant; the period from conception to birth.

Home range

The area occupied by an animal during routine activities, which is not actively defended.

Lichen

A composite organism made up of a fungus in a co-operative partnership with an alga. Owing to this partnership, lichens can thrive in harsh environments such as mountaintops and polar regions. Characteristically forms a crustlike or branching growth on rocks or tree trunks.

Moult

Periodic shedding of (usually) the outermost body covering (such as feathers, fur or skin) during growth and development, or at specific times of the year.

Oestrus

The time of ovulation (release of an egg from the ovary) in female mammals, when the female becomes receptive to males. Also known as ‘heat’.

Precocial

Young that are relatively mature, mobile and independent from the moment of birth or hatching.

Subspecies

A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.

References

1. IUCN Red List (February, 2012)
   http://www.iucnredlist.org/
2. Bowyer, R.T. and Leslie, D.M. (1992) Ovis dalli. Mammalian Species, 393: 1-7. Available at:
   http://www.science.smith.edu/msi/pdf/i0076-3519-393-01-0001.pdf
3. Shackleton, D. (1999) Hoofed Mammals of British Colombia. UBC Press, Vancouver, Canada.
4. Evans, P. (2010) Yukon. Bradt Travel Guides, Connecticut, USA.
5. Hoare, B. (2009) Animal Migration: Remarkable Journeys in the Wild. Marshall Editions, London.
6. Macdonald, D.W. (2006) The Encyclopedia of Mammals. Oxford University Press, Oxford.