| Yak | |
|---|---|
| |
| A yak in the Nepalese Himalayas. | |
Domesticated
| |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Family: | Bovidae |
| Subfamily: | Bovinae |
| Genus: | Bos |
| Species: |
B. grunniens
|
| Binomial name | |
| Bos grunniens
Linnaeus, 1766
| |
The domestic yak (Bos grunniens) is a long-haired domesticated bovid found throughout the Himalayan region of the Indian subcontinent, the Tibetan Plateau, Northern Myanmar, Yunnan, Sichuan and as far north as Mongolia and Siberia. It is descended from the wild yak (Bos mutus).
Etymology
The English word "yak" is a loan originating from Tibetan: གཡག་, Wylie: g.yag. In Tibetan and Balti it refers only to the male of the species, the female being called Tibetan: འབྲི་, Wylie: 'bri or Tibetan: གནག, Wylie: g.nag in Tibetan and Tibetan: ཧཡག་མོ་, Wylie: hYag-mo
in Balti. In English, as in most other languages that have borrowed the
word, "yak" is usually used for both sexes, with "bull" or "cow"
referring to each sex separately.
Taxonomy
Yaks belong to the genus and are therefore related to cattle (Bos primigenius species). Mitochondrial DNA analyses to determine the evolutionary history of yaks have been inconclusive.
The yak may have diverged from cattle at any point between one
and five million years ago, and there is some suggestion that it may be
more closely related to bison than to the other members of its designated genus. Apparent close fossil relatives of the yak, such as Bos baikalensis, have been found in eastern Russia, suggesting a possible route by which yak-like ancestors of the modern American bison could have entered the Americas.
The species was originally designated as Bos grunniens ("grunting ox") by Linnaeus in 1766, but this name is now generally considered to refer only to the domesticated form of the animal, with Bos mutus ("mute ox") being the preferred name for the wild species. Although some authors still consider the wild yak to be a subspecies, Bos grunniens mutus, the ICZN made an official ruling in 2003 permitting the use of the name Bos mutus for wild yaks, and this is now the more common usage.
Except where the wild yak is considered as a subspecies of Bos grunniens, there are no recognised subspecies of yak.
Physical characteristics
A domestic yak at Yamdrok Lake.
Yaks are heavily built animals with a bulky frame, sturdy legs,
rounded cloven hooves, and extremely dense, long fur that hangs down
lower than the belly. While wild yaks are generally dark, blackish to
brown in colouration, domestic yaks can be quite variable in colour,
often having patches of rusty brown and cream. They have small ears and a
wide forehead, with smooth horns that are generally dark in colour. In
males (bulls), the horns sweep out from the sides of the head, and then
curve forward. They typically range from 48 to 99 cm (19 to 39 in) in
length. The horns of females (cows) are smaller, only 27 to 64 cm (11 to
25 in) in length, and have a more upright shape. Both sexes have a
short neck with a pronounced hump over the shoulders, although this is
larger and more visible in males.
Males weigh 350 to 585 kg (772 to 1,290 lb), females weigh 225 to
255 kg (496 to 562 lb). Wild yaks can be substantially heavier, bulls
reaching weights of up to 1,000 kilograms (2,200 lb).
Depending on the breed, domestic yak males are 111–138 centimetres
(44–54 in) high at the withers, while females are 105–117 centimetres
(41–46 in) high at the withers.
Both sexes have long shaggy hair with a dense woolly undercoat
over the chest, flanks, and thighs to insulate them from the cold.
Especially in bulls, this may form a long "skirt" that can reach the
ground. The tail is long and horselike rather than tufted like the tails
of cattle or bison. Domesticated yaks have a wide range of coat
colours, with some individuals being white, grey, brown, roan or piebald. The udder in females and the scrotum in males are small and hairy, as protection against the cold. Females have four teats.
Yaks grunt and, unlike cattle, are not known to produce the
characteristic bovine lowing (mooing) sound, which inspired the
scientific name of the domestic yak variant, Bos grunniens (grunting bull). Nikolay Przhevalsky named the wild variant Bos mutus (silent bull), believing that it did not make a sound at all.
Physiology
Yak rider near Tsomgo Lake, Sikkim (3700 m)
Yak physiology is well adapted to high altitudes,
having larger lungs and heart than cattle found at lower altitudes, as
well as greater capacity for transporting oxygen through their blood due to the persistence of foetal haemoglobin throughout life. Conversely, yaks have trouble thriving at lower altitudes,
and are prone to suffering from heat exhaustion above about 15 °C
(59 °F). Further adaptations to the cold include a thick layer of subcutaneous fat, and an almost complete lack of functional sweat glands.
Compared with domestic cattle, the rumen of yaks is unusually large, relative to the omasum.
This likely allows them to consume greater quantities of low-quality
food at a time, and to ferment it longer so as to extract more
nutrients. Yak consume the equivalent of 1% of their body weight daily while cattle require 3% to maintain condition.
Odour
Contrary to popular belief, yak and their manure have little to no detectable odour
when maintained appropriately in pastures or paddocks with adequate
access to forage and water. Yak's wool is naturally odour resistant.
Reproduction and life history
Ten-day-old yak.
Yaks mate in the summer, typically between July and September,
depending on the local environment. For the remainder of the year, many
bulls wander in small bachelor groups away from the large herds, but, as
the rut
approaches, they become aggressive and regularly fight among each other
to establish dominance. In addition to non-violent threat displays,
bellowing, and scraping the ground with their horns, bull yaks also
compete more directly, repeatedly charging at each other with heads
lowered or sparring with their horns. Like bison, but unlike cattle, males wallow in dry soil during the rut, often while scent-marking with urine or dung. Females enter oestrus up to four times a year, and females are receptive only for a few hours in each cycle.
Gestation lasts between 257 and 270 days,
so that the young are born between May and June, and results in the
birth of a single calf. The cow finds a secluded spot to give birth, but
the calf is able to walk within about ten minutes of birth, and the
pair soon rejoin the herd. Females of both the wild and domestic forms typically give birth only once every other year, although more frequent births are possible if the food supply is good.
Calves are weaned
at one year and become independent shortly thereafter. Wild calves are
initially brown in color, and only later develop the darker adult hair.
Females generally give birth for the first time at three or four years
of age,
and reach their peak reproductive fitness at around six years. Yaks may
live for more than twenty years in domestication or captivity, although it is likely that this may be somewhat shorter in the wild.
Hybrid yak
In Nepal, Tibet and Mongolia, domestic cattle are crossbred with yaks. This gives rise to the infertile male dzo མཛོ། as well as fertile females known as མཛོ་མོ། dzomo or zhom, which may be crossed again with cattle. The "Dwarf Lulu" breed, "the only Bos primigenius taurus type of cattle in Nepal" has been tested for DNA markers and found to be a mixture of both taurine and zebu types of cattle (B. p. taurus and B. p. indicus) with yak. According to the International Veterinary Information Service, the low productivity of second generation cattle-yak crosses makes them suitable only as meat animals.
Crosses between yaks and domestic cattle (Bos primigenius taurus) have been recorded in Chinese literature for at least 2,000 years. Successful crosses have also been recorded between yak and American bison, gaur, and banteng, generally with similar results to those produced with domestic cattle.
Relationship with humans
Domesticated yaks have been kept for thousands of years, primarily for their milk, fibre and meat, and as beasts of burden.
Their dried droppings are an important fuel, used all over Tibet, and
are often the only fuel available on the high treeless Tibetan Plateau.
Yaks transport goods across mountain passes for local farmers and
traders as well as for climbing and trekking expeditions. "Only one
thing makes it hard to use yaks for long journeys in barren regions.
They will not eat grain, which could be carried on the journey. They
will starve unless they can be brought to a place where there is grass." They also are used to draw ploughs. Yak's milk is often processed to a cheese called chhurpi in Tibetan and Nepali languages, and byaslag in Mongolia. Butter made from yaks' milk is an ingredient of the butter tea that Tibetans consume in large quantities, and is also used in lamps and made into butter sculptures used in religious festivities.
Yak racing
Husbandry research
The
Indian government established a dedicated centre for research into yak
husbandry, the ICAR-National Research Centre on Yak, in 1989. It is
located at Dirang, Arunachal Pradesh, and maintains a yak farm in the Nyukmadung area at an altitude of 2750 m above MSL.
Yak sports
In parts of Tibet and Karakorum, yak racing
is a form of entertainment at traditional festivals and is considered
an important part of their culture. More recently, sports involving
domesticated yaks, such as yak skiing or yak polo, are being marketed as tourist attractions in Central Asian countries, including in Gilgit-Baltistan, Pakistan.
