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Bos grunniensyak

By Matthew Oliphant

Geographic Range

While their domesticated counterparts can be found in a much more varied area in the oriental region, the main geographic range of wild B. grunniens is limited to the Tibetan Plateau, which includes "...the western edge of Gansu Province, Qinghai Province, the southern rim of the Xinjiang Autonomous Region, and the Tibet Autonomous Region." (Schaller & Wulin, 1995).

Habitat

The habitat of B. grunniens can vary, but mainly consists of three areas with different vegetation: Alpine meadow, alpine steppe, and desert steppe (Schaller & Wulin, 1996). Each habitat features large areas of grassland, but differ in the type of grasses/small shrubs, amount of vegetation, average temperature, and precipitation.

The habitat for B. grunniens can also be dependent on the season. Some herds will migrate large distances seasonally to feed on grass, moss, and lichens. However, B. grunniens by most accounts does not care for warm weather and, preferring the colder temperatures of the plateaus, will return when seasonal temperatures start to warm (Buchholtz, 1990).

In more recent years, B. grunniens has been increasingly confined to the desert steppe. This is in part due to the fact that farmers moving into the region don't find the land and minimal precipitation (50-100 mm/yr) desirable. Yaks are thus not disturbed by human activities in this region (Schaller & Wulin, 1996).

  • Range elevation
    3200 to 5400 m
    10498.69 to 17716.54 ft
  • Average elevation
    4500 m
    14763.78 ft

Physical Description

Wild yak males can be up to 3,250 mm in length and more than 2,000 mm in height. Average weight for male B. grunniens is 1,000 kg with the female being around 300 kg (Nowak, 1999). Domestic yaks can be considerably smaller in weight, with males ranging from 350 to 580 kg and females between 225 to 255 kg (Buchholtz, 1990).

Other physical features of B. grunniens include: a blackish brown pelage, large black upward curving horns, and long hair covering the body including the tail (Nowak, 1999). In contrast, domesticated populations have shorter legs, broader hooves, more varied pelage coloration, and weaker horns which sometimes can be absent altogether (Buchholtz, 1990).

Both wild and domesticated yaks possess large lungs, a high red blood cell count, and higher concentration of hemoglobin than most other bovids. All of these factors allow the yak to live and thrive in higher elevations that would give other, non-acclimated animals elevation sickness (Summers, 1997).

  • Range mass
    300 to 1000 kg
    660.79 to 2202.64 lb
  • Range length
    3250 (high) mm
    127.95 (high) in

Reproduction

For most of the year, male and female wild yaks spend their time in separate herds. While females and young (occasionally including some young males) usually stay in large herds, males tend to either spend their time alone or in small groups. During the mating season, males leave their groups and join with the female herds. Males compete for access to receptive females, often violently. (Nowak, 1999; Buchholtz, 1990)

The mating season for B. grunniens starts in September, with births usually occuring in June (Buchholtz, 1990).

In the wild, the female B. grunniens have one calf every other year. Gestation is about 9 months, weaning occurs at one year, and full size is obtained in 6-8 years. At this age sexual maturity is also attained. For the domesticated yak the reproductive cycle is more varied, with the cow sometimes giving birth to more than one calf per year (Nowak, 1999).

  • Breeding season
    In wild yak the mating season begins in September, with births occurring in June (Nowak, 1999).
  • Range number of offspring
    1 (low)
  • Average number of offspring
    1
  • Average number of offspring
    1
    AnAge
  • Average gestation period
    9.33 months
  • Average gestation period
    274 days
    AnAge
  • Range weaning age
    5 to 9 months
  • Range age at sexual or reproductive maturity (female)
    6 to 8 years
  • Range age at sexual or reproductive maturity (male)
    6 to 8 years

Most of the parental care of young is done by the female due to the nature of wild female and male B. grunniens to spend the majority of the year in separate groups (Buchholtz, 1990). Young are born able to stand and walk within several hours after birth.

Lifespan/Longevity

In the wild, the maximum lifespan of B. grunniens is limited to about 25 years.

Behavior

Wild yak spend most of their time grazing, sometimes moving to and from various areas depending on the season. They are mainly active during the day. They live in herds which can vary in size from 20 to 200 individuals (Buchholtz, 1990). However, a more recent survey of the Chang Tang Reserve by Schaller & Wulin (1995) found smaller herds; Usually 2 to 5 individuals for solitary male herds, and 6 to 20 individuals in mainly female herds. Females and males live separately for most of the year. Male bulls usually band together in groups of 10-12, while females and young can comprise a much larger group (from as little as 10-12 to perhaps 200 individuals).

Another interesting behavior of wild B. grunniens is that, for unknown reasons, they will sometimes attack an kill their domesticated counterparts (Buchholtz, 1990).

Communication and Perception

Food Habits

Bos grunniens is a grazer, with a diet composed mainly of various low-lying grasses and grass-like plants, including shrubs, forbs, cushion plants, etc., found on the Tibetan plateau, though they will also consume lichen, mosses, and forbs (Schaller & Wulin, 1995).

  • Plant Foods
  • leaves
  • bryophytes
  • lichens

Predation

At signs of danger, wild herds of B. grunniens will run from the threat. Other actions that they will take include loud snorting and charging at the perceived threat (Buchholtz, 1990).

Ecosystem Roles

Through their grazing activities, yaks play an important role in nutrient recycling and in generating intermediate levels of disturbance in their ecosystems. They are also an important prey species for wolves.

Economic Importance for Humans: Positive

Domestic:

With wide hooves and the ability to carry large weights at high elevations, domesticated yak serve as beasts of burden for many inhabitants of the Tibetan plateau. The finer fur of the young is used for clothing, while the longer fur of the adult is used in making blankets, tents, etc. Also, in some areas where firewood is particularly sparse, the dung is used as fuel. In some areas, milk from the cow is used to produce large amounts of butter and cheese for export (Buchholtz, 1990).

Wild:

The wild counterpart of B. grunniens serves many of the same economic functions, although to a lesser degree due to their rather sparse availability and obvious non-domestication. While penalties have been set by China, hunting of the wild yak still takes place, especially in the winter where some local farmers find them the only source of meat during harsh winter months (Schaller & Wulin, 1996).

  • Positive Impacts
  • food
  • body parts are source of valuable material
  • produces fertilizer

Economic Importance for Humans: Negative

There are several negative economic impacts on humans. Where wild and domesticated B. grunniens live in close proximity to one another, wild yak have been known to break down fences. In some extreme circumstances, wild yaks have been known to kill domesticated yak (Buchholtz, 1990).

The possibility of the transmission of disease between domestic and wild B. grunniens is also a concern in areas where the two live fairly close to one another, and perhaps have limited contact (Schaller & Wulin, 1996).

Conservation Status

There are many factors that are currently leading to a decline in the number of wild B. grunniens, which is currently estimated at around 15,000. Perhaps one of the largest has been hunting by humans. According to Schaller and Wulin (1995), while the Tibet Forest Bureau is making substantial efforts to protect yak (including fines of up to $600), "Hunting is difficult to suppress without a mobile patrol force, as the recent decimation of wildlife in the Arjin Shan Reserve has shown."

Also, as pastoralists are starting to change their habits from a nomadic to sedentary lifestyle their habit is beginning to become fenced off. The introduction of domesticated yaks (via the pastoralists) also presents problems in regards to the transmission of disease (e.g. brucellosis), and possible increased competition for the same grazing land (Schaller & Wulin, 1995).

Domesticated yak are not listed as endangered due to their large numbers.

Contributors

Matthew Oliphant (author), University of Northern Iowa, Jim Demastes (editor), University of Northern Iowa.

Glossary

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

chemical

uses smells or other chemicals to communicate

dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

food

A substance that provides both nutrients and energy to a living thing.

herbivore

An animal that eats mainly plants or parts of plants.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nomadic

generally wanders from place to place, usually within a well-defined range.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynous

having more than one female as a mate at one time

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tundra

A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Buchholtz, C. 1990. True Cattle (Genus *Bos*). Pp. 386-397 in S Parker, ed. Grzimek's Encyclopedia of Mammals, Volume 5. New York: McGraw-Hill Publishing Company.

Nowak, R. 1999. Walker's Mammals of the World, 6th Edition, Volume II. Baltimore: Johns Hopkins University Press.

Schaller, G., L. Wulin. 1995. Distrobution, Status, and Conservation of Wild Yak (*Bos grunniens*). Biological Conservation, 76: 1-8.

Summers, D. March-April 1997. Mountain Machine. International Wildlife, 27 no. 2: 36-42.

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