write four adaptive features of yak which have them to stay in Cold Mountain environment ?
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Answer:
Yak have many characteristics and attributes that must be regarded as adaptations to many factors: extreme cold; high altitude with low oxygen content of the air and high solar radiation; difficult, often treacherous terrain; and cyclical nutrition with short growing seasons for grazing herbage as well as a variety of herbage.
In general, temperature is the single most important factor determining the distribution, stocking density and, indirectly, the growth rate of yak. Yak survive and perform adequately if the annual mean temperature is below 5oC and the average in the hottest month is not above 13oC. They can also survive satisfactorily at ambient temperatures down to -40oC. Altitude, as such, is of lesser importance. The further north (of the equator) yak live, the lower, in general, the altitude at which they are found. Yak in North America and in animal and zoological parks in several parts of the world, may again have re-adapted, over time, to life in these, for them, non-normal situations.
Yak cope with cold by conserving heat, rather than by generating it - which would require food that may not be available. Heat conservation is effected by a compact conformation, a thick fleece of coarse outer hair and an undercoat of fine down. The proportion of down in the coat increases greatly before the onset of winter. Young calves have a fleece composed exclusively of down fibre. Normally, yak accumulate a layer of subcutaneous fat prior to winter. This also helps heat conservation and provides an energy reserve. The skin is relatively thick. It contains sweat glands, though for the most part, these are not functional. This is one reason why yak are intolerant of high ambient temperature.
Adaptation to low oxygen content of the air arises from yak having a large chest (14 - 15 pairs of thoracic ribs), large lungs and a large heart relative to their overall body size. The haemoglobin content may not be exceptionally high relative to cattle at sea level, although the content increases with altitude, but the haemoglobin of yak blood has a high affinity for oxygen. Also, anatomically, the yak is designed to be capable of breathing rapidly and take in large amounts of air.
The skin is highly pigmented and the predominant hair colour is black. Both of these attributes help to resist the effects of solar radiation. White yak exist because herdsmen in some localities prefer them.
The large rumen volume of yak relative to body size may be a useful adaptation to foraging herbage under rough grazing conditions. Yak are adapted to grazing a wide variety of plant species: grass, coarse plants and sedges and some shrubs. Yak can graze long grass using their tongues, as is common for cattle, but they can also graze very short herbage, after the manner of sheep, by using their incisor teeth and lips. When ground is covered with snow and ice, they break through the cover to the wilted grass beneath, using their hooves and heads. Yak also graze rapidly and for long hours.
To cope with precipitous terrain, yak have developed particularly suitable hooves and a temperament that is suited to potentially dangerous situations, such as marshy ground. Yak prefer to group in large herds for protection, particularly against wolves, but they are also nervous of wild animals and man and, if startled, will readily take flight.