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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.


Features common to the environment in which yak live are extreme cold, mountainous terrain, high altitudes with reduced oxygen in the air, high solar radiation and short growing seasons for herbage and a variable assortment of herbage, sparse in some areas. Plant growing seasons vary from 120 to 180 days, but the periods of relatively vigorous plant growth are even shorter than that. Wilted herbage provides some sustenance for the yak at other times of year, but not in sufficient quantity for their requirements. There is, of course, some variation in these features. Some "compensatory" factors have also to be taken into account. For example, the more northerly the latitude at which the yak are found, and hence, in general, the colder the climate, the lower the altitudes at which the yak will live. These points will be discussed in more detail later in this chapter. Many of the characteristics of the yak can be regarded as adaptations to these conditions, in which cattle of other species have difficulty in surviving.

Distribution in relation to environmental factors

Several studies in China have analysed the distribution of the yak using multiple regression approaches with the stocking density of the yak, in selected areas, as the dependant variable and various factors of the environment as the independent variables. The factors most commonly included in these studies (for example, Wen Zhenzhong and Xie Zhonglun, 1985) are: altitude above sea level, yearly average air temperature, annual precipitation, average relative humidity, average annual sunshine and, in some studies, the type of plant cover.

These factors are not independent of each other. For example, altitude and air temperature are related, as are annual precipitation and annual sunshine, and all these factors impinge on the length of the growing season for plants and the type of plant cover likely to be found. With these limitations in mind, authors of various studies concluded that air temperature was of major importance for the distribution and body size of the yak and more important than altitude (Cai Li, 1992; Chen Zhihua, 2000). In analyses where the type of plant cover was also included as a variable (Huang Wenxiu and Wang Sufang, 1980; Dou Yaozong, et al. 1985) its importance ranked alongside temperature.

However, the quantity of available herbage itself must be strongly influenced by the climate of the area. Annual precipitation was generally of less importance to the distribution of the yak, and altitude, as such, of lesser importance still.

Some environmental factors in relation to areas of yak distribution are shown in Table 4.1. It can be seen that yak are found generally above 3 000 m above sea level (a.s.l.), with cold and semi-arid to semi-humid climate (yearly average temperature ranging from -5°C to 5.5°C, and average relative humidity from 50 percent to 65 percent). The main natural feeds are grass, sedges and forbs.

The effect of air temperature

In the native regions of yak in present times, the stocking density of yak declines as average annual temperature increases. The greatest concentrations of yak are found at average annual air temperatures between -3oC and +3oC. In Qinghai province, the yak are concentrated in areas with annual mean temperatures between -3oC and -4oC; those in Tibet are densest at the range of -3oC to -5oC; and in Sichuan province between -1.6oC and +3.4oC (Table 4.1).

Table 4.1 Environmental factors in yak distribution areas


Altitude (m a.s.l.)

Yearly average temp (°C)

Annual precipitation (mm)

Type of plant cover



4 500 - 5 500

-3 - -5

600 - 700

Grass, sedges and forbs

Lu Zhonglin, 1999


4 000 - 4 500

-3 - -4

500 - 600

Grass and sedges

Cai Li, 1994

Southern Gansu

3 000 - 4 500

1.4 - 3.2

300 - 860

Grass and sedges

Lu Zhonglin, 1999

Western Sichuan

3 000 - 5 000

-1 - 5

600 - 800

Grass, sedges and forbs

Cai Li, 1989

Zhong Dian, Yunnan

3 000 - 4 000

5.2 - 5.5

500 - 700

Grass and sedges

He Shaoyu et al., 1997

Bazhou, Xinjiang

2 400 - 4 000


Around 284.6

Grass and shrubs

Lu Zhonglin, 1999


3 600 - 4 600


Below 650

Grass and sedges

Tshering et al., 1996


4 000 - 5 000

Grass and forbs

Pal et al., 1996


3 000 - 5 000

350 - 500

Grass and shrubs

Davaa, 1996


3 000 - 5 000

Grass and sedges

Sherchand et al., 1996

Li Shihong et al. (1981) reported that above an ambient temperature of 13oC the respiration rate of the yak starts to rise, and at 16oC the heart rate and body temperature start to rise. When environmental temperatures reach 20oC, yak will stand near water or in shade, if available, without moving, grazing, drinking or ruminating. At the other extreme, yak can feed and move normally on grasslands with air temperatures ranging as low as -30oC to -40oC, or even lower as in the Tibetan Naqu area where a minimum air temperature in the cold season was recorded as -42oC. The lowest temperature that yak can tolerate has not been recorded.

It appears from these studies that air temperature is the single most important environmental factor influencing the distribution of the yak. Yak survive and perform adequately provided the annual mean temperature is below 5oC and the average in the hottest month does not exceed 13oC, though daily maximum temperatures can rise in the summer to much higher levels before falling again at night. (Nonetheless, yak kept commercially in North America and those in zoological and wild animal parks in many countries appear to have adapted to different sets of conditions - see Chapter 11, part 3). It is a matter of observation that the farther north of the equator that yak live, the lower is, on average, the altitude of the terrain on which they are found because average temperature declines with increasing latitude (cf. Table 4.2).

Air temperature has also been reported as the most important environmental factor influencing the growth and body size of yak (Cai Li, 1992; Yao Yubi and Li Yuqing, 1995; Chen Zhihua et al., 2000). Yao Yubi and Li Yuqing, (1995) derived an equation of meat production from its relation with air temperature by using multiply regression as follows: ? = 2.452 + 7.166T7 + 1.726 R2 (r=0.907, P<0.01). Where ? is average carcass weight of yak, T is the average temperature of the mid-ten days in July and R is the precipitation in February.

The effect of altitude

Subject to the availability of adequate grazing, the distribution and stocking density of yak increases with altitude, but as already noted, this is also dependent on latitude. Thus, at the more southerly latitudes as, for example, Qinghai province, yak seek out higher altitudes than in more northerly areas, such as Mongolia. The few yak introduced to Canada and Alaska in far northern latitudes existed at relatively low altitudes (see Chapter 11, part 3). It is likely that, as previously suggested, the relationship between altitude and latitude is mediated through air temperature.

The highest altitude where yak live normally is at 5 500 m in the Tibetan Rongbusi region in the lower ranges of the Himalayas. Yak steers used as pack animals are quite capable of traversing terrain at 7 200 m. Low oxygen content of air and high solar radiation are not therefore barriers to the yak's survival.

The effect of precipitation and relative humidity

Yak live in two distinct zones. One is semi-arid with an annual precipitation of 500 - 600 mm and a relative humidity of 50 - 60 percent. In these areas the potential for evaporation tends to exceed the level of precipitation (e.g. Qinghai and much of Tibet). The other zone has an average annual rainfall of 600 - 700 mm and a relative humidity of 60 - 65 percent and is described as semi-humid (e.g. eastern Tibet and southwest Sichuan). The two zones differ in the predominant types of vegetation found and, as noted in Chapter 2, the two zones are associated with different types of yak.

The effect of sunshine

In general, yak live in areas with more than 2 000 hours of annual sunshine. In Qinghai province, the highest densities of yak are in areas with between 2 500 and 2 700 hours sunshine (Wen Zhengzhong and Xie Zhonglun, 1985), while in Sichuan province the greatest density of yak is in the districts with between 2 000 and 2 200 hours of sunshine per year.

Adaptive characteristics

The ability of the yak to live in conditions in which other bovines will not survive, or, at least, not thrive, suggests that the yak has developed specially adapted characteristics. These are described in the following sections.

Resistance to cold


The yak's body is compact with short neck, short limbs, no dewlap, small ears and a short tail. (The limb length index, defined as {[height at withers-chest depth]/height at withers} x 100, is small at 40 - 42 percent, relative to other cattle in the region.) The scrotum of the male is small, compact and hairy, and the udder of the female small and also hairy. The skin has few wrinkles, and the surface area of the yak is relatively small per unit of bodyweight (0.016 sq m per kg [Li Shihong et al. 1984]). The yak has only a few functional sweat glands (see section, Skin thickness and sweat glands). All these factors result in a minimized dissipation of body heat.

In addition in terms of appearance though not relevant to heat dissipation, yak can be either horned or polled. The distribution of these two forms appears to depend on the regional preferences of the herdsmen. Thus, yak in Mongolia, for example, are predominantly polled (or dehorned) while those of Tibet or Nepal are nearly all horned. The distribution of the horn types is shown in Table 4.2.

Coat of the yak

Heat conservation is enhanced by a thick fleece on the whole body, composed of an outer coat of long hair and an undercoat of a dense layer of fine down fibres that appear in the colder season. As already noted, hair colours vary from black and brown through variegation to white. Since white absorbs less heat, the existence of this colour must be attributed to herders' preference rather than to adaptive usefulness. It is significant, however, that white yak are more prevalent at more northerly latitudes. In such areas, solar radiation is not as intense as in the more southerly latitudes, or at the higher elevations where black is the predominant colour (see Table 4.2). Hair production of white yak is higher than that of the black and brown ones (Wang Yuchang, 1995). The thick and dense coat of white yak may partly compensate for less heat absorption of the white colour in winter.

Details of fleece production and physical and structural properties of the fibres are given in Chapter 6. The purpose here is to discuss the coat as a feature of the adaptation of the yak to its environment.

In general, the coat of the yak seems well suited to insulating the animal from cold, protecting it from heat and repelling moisture. All these factors are important to survival in the prevailing climate. As Yousef (1985) noted, a thick winter coat is a general adaptation of animals living in extreme cold, e.g. arctic mammals. Thus, conservation of heat takes precedence over extra generation of heat. To generate extra heat would ultimately require additional feed, which is in short supply throughout the winter. It is of interest in the present context to observe that one of the most successful of all the yak breeds, the Jiulong yak of Sichuan province, has a fibre strain that produces, on average, between three and five times as much fleece as other types of yak. This strain also inhabits one of the coldest, dampest and most fog-bound areas of all yak territory. It is probable that it is the dense, heavy coat that has helped the Jiulong yak to survive in these conditions.

Insulation from the fleece

The coat consists of three types of fibre: coarse, long fibres with a diameter in excess of 52 m, down fibre with a diameter below 25 m and mid-type hairs with diameters between these two values. The down fibre is a particular attribute of the winter coat of the yak to provide the additional insulation then required. Coats with a mixture of fibre types have been shown to maintain a stable air temperature within the coat. Ouyang Xi and Wang Qianfei. (1984) measured temperatures at the skin surface and at the middle and top of the staples of various parts of the yak body, namely the ear, forehead, sides of neck, shoulder, rump, the back, belly, tail and legs. Measurements were made on ten animals on three successive days in the cold season in February (mean ambient temperature -18oC) and on nine animals on three days in the warm season at the end of May (mean daytime ambient temperature 22oC - though somewhat windier at that time than in February). Figures 4.1 and 4.2 show the measurements.

The results (Figures 4.1 and 4.2) show that the gradient in temperature between the skin surface and the top of the staple is far greater in winter than in summer for parts of the body trunk, such as the shoulder, rump and belly.

Figure 4.1 Temperature oC in summer (mean daytime ambient temperature 22oC) at the skin surface (blue), mid-staple (purple), and top staple (yellow) of various body parts: 1 back, 2 ear, 3 neck, 4 forehead, 5 tail, 6 shoulder, 7 rump, 8 belly [Source: Ouyang Xi et al., 1985]

Figure 4.2 Temperature oC in winter (mean ambient temperature -18oC) at skin surface (blue), mid-staple (purple), and top staple (yellow) of various body parts: 1 leg, 2 back, 3 ear, 4 neck, 5 forehead, 6 tail, 7 shoulder, 8 rump, 9 belly [Source: Ouyang Xi et al., 1985]

However, the seasonal difference in the temperature gradient from skin surface to top of staple is much less at extremities of the body, such as the ears, where vasoconstriction occurs during cold. For this reason, temperature varies at the skin surface, during the cold season, between the different parts of the body, as shown in Figure 4.1. The results also show clearly the insulation from cold provided by the fleece (though these features are not unique to the yak; the effect of fleece on heat regulation, and consequently on the energy metabolism of the animal, have been demonstrated, e.g. in relation to sheep by Blaxter, et al. 1959). No case of frostbite has been recorded in the yak, even at the extremities of its body.

The function of the coat in helping yak to survive in very cold and wet conditions is enhanced by the low water absorption of the coat (Xue Jiying and Yu Zhengfeng, 1981).

The arectores pilorum muscles are highly developed in the dermis of the yak (Ouyang Xi and Wang Qianfei, 1984). Their contraction makes fibres stand up and effectively increase the depth of the coat and reduce heat loss under stress from cold.

Seasonal changes

Hair growth and the composition of the coat change with season. As air temperature falls with the approach of winter, down fibres grow densely among the coarser hairs, especially on the shoulder, back and rump. Ouyang Xi et al. (1983) found in a herd studied both in summer and in winter that the proportion of down fibre increased by between 17.5 percent and 30 percent in winter, through the activation of down follicles that had lain dormant. The proportion of coarse hairs correspondingly decreased. As air temperature rises with the onset of the warm season, down fibres begin to be shed from the fleece (see Chapter 6).

As a consequence of the abundant grazing in summer and early autumn, yak are normally able to develop a layer of subcutaneous fat that also provides them with insulation from cold as well as an energy reserve during the period of nutritional deprivation over winter and spring.

Breed and location differences

The amount of down fibre on the yak's back may vary with breed. From different studies it appears that down fibre is more than twice as dense in Tianzhu White yak in Gansu province as in Luqu yak of southern Gansu, with the Maiwa yak of Sichuan province somewhat intermediate (Zhang Rongchang, 1977; Lu Zhongling et al., 1982; Wang Jie et al., 1984). However, here again, breed type is confounded with the area in which the different breeds are kept. To establish that it is breed and not location that is responsible for the differences would require a comparison of the different breeds at the same location.

Studies also indicate that fibre density declines when yak are moved to areas with warmer summers and longer frost-free periods. Thus, yak introduced in the 1970s to the Jingbei Plateau of northern Weichang county of Hebei province had an average density of fibres of 3 167 per sq cm at that time. The density subsequently declined to 1 870 fibres per sq cm. The changes occurred through a decrease in the density of down fibre in particular, but the coarse hairs decreased in length. These changes allowed better heat dissipation at the warmer times of the year and can be assumed to have occurred in response to the environmental changes directly affecting the animal rather than due to selection among the yak.

Age effects

In calves younger than six months, the coat consists almost entirely of down fibre. Thereafter, the proportion (by weight) declines to 62 percent at one year old and 52 percent by year two, 44 percent at year three and 43 percent at four and five years old (Zhang Yingsong et al., 1982). There is a corresponding increase in the deposition of subcutaneous fat as the animals grew older.

Skin thickness and sweat glands

Xiao Wangji et al. (1997) found Zhongdian yak produced more hide and possessed a higher proportion of skin to body weight (P<0.01), when compared with Zhongdian cattle and yak-cattle hybrids. Skin thickness is greater on the back of the yak than on other parts of the body. This is associated with the fact that the back is the part of the animal most exposed to wind, rain and snow. Li Shihong et al. (1984) measured skin thickness on the shoulder blade, the back and the knee of 70 live female yak and found the average thickness at the three positions to be 5.6 ± 0.36, 7.5 ± 0.83 and 5.6 ± 0.40 mm, respectively. Ouyang Xi et al. (1984), using histological sections, measured thickness of epidermis and dermis combined. Again, the back had the thickest skin (average 5.13 mm) and the densely haired parts had a thickness of as little as 2.36 mm. Averaged over the different parts of the body, the skin thickness was 3.37 ± 1.38 mm, but the epidermis itself was very thin at 0.044 ± 0.019 mm.

Sweat glands are distributed in the skin over the whole body and are of the apocrine type. Density per sq cm was found to be greatest on the forehead (891 sq cm) and least on the rump (138 sq cm), with an overall average of 399 ± 251 sq cm (Li Shihong et al., 1984; Ouyang Xi and Wang Qianfei, 1984). However, the function of the sweat glands is poorly developed. Tests made by these authors, using different methods, agree in detecting sweat secretions only on the muzzle and not on other parts of the body. The absence of sweating in the yak assists cold tolerance but helps make the yak intolerant to heat.

Energy metabolism

Hu Linghao (1994) studied the energy metabolism of growing yak at three different ages (one, two and three years old) and compared it with that of yellow cattle, both kept at three different altitudes (2 261 m, 3 250 m and 4 271 m). He reported that the fasting heat production of the yak remained fairly constant irrespective of altitude, whereas that of the yellow cattle rose markedly. This could well point to an adaptive response of the yak to life at high altitude and to the nutritional deprivation that yak experience in winter and early spring.

At the lowest of the three elevations in the trials conducted by Hu Linghao, the absolute fasting heat production of the yak was higher than that of the yellow cattle, but not so at the higher altitudes. In another experiment by Hu Linghao, the yak generated a little more heat in the course of walking than did the somewhat larger yellow cattle. The author attributes the difference in heat production to the difference in body size, as smaller animals are expected to generate more heat. Clearly, these and similar experiments are important for understanding the factors that lead to adaptation and may, in due course, provide a means for devising improved grazing and management strategies.

Adaptation to low atmospheric oxygen and high solar radiation

At elevations of 3 500 m above sea level, where most yak live, the oxygen content of the air is some 35 percent lower than at sea level. On even higher grazing pastures, at an altitude of 5 000 m, the oxygen content is halved. Also, in most of the areas there is more than 2 000 hours of sunshine and levels of solar radiation are between 130 and 165 kcal per sq cm (540 - 690 kJ per sq cm) annually, depending on elevation.

The yak has adapted to these conditions: It is considered to take in larger volumes of air than most other cattle, to retain a higher proportion of the oxygen breathed in and to be protected against harmful effects of solar radiation by the colour of its coat and skin.

Vertebrae, thorax, heart and lungs

Vertebrae. Yak have 14 thoracic vertebrae and 14 pairs of ribs - one more than in other cattle - although several authors report 15 ribs, two more than in other cattle. This gives the yak a larger chest capacity. There are five lumbar vertebrae, one less than in other cattle. The number of coccygeal vertebrae is variable in the range from 12 - 16 (other cattle have 16). The yak has five sacral vertebrae and seven cervical ones, the same as for other cattle. The total number of vertebrae are thus fewer than for other species of cattle.

Thorax and organs.Yak ribs are narrow and long with a relatively large distance between them. There is also a good development of muscle between them. Relative to local cattle, the yak has a large thorax (heart girth index = 150 [heart girth x 100/height at withers]), allowing the development of large lungs and a large heart. For example, according to Xiao Wangji et al. (1997), the lung of Zhongdian yak (n=12) weighs 3.7 ± 0.54 kg, more (P<0.05 than that of Zhongdian cattle 1.8 ± 0.57 kg (n=5) and yak-cattle hybrids 2.5 ± 0.39 kg (n=10). Similarly, these authors recorded the heart of Zhongdian yak, weighing on average 1.3 ± 0.29 kg, heavier (P<0.01) than the cattle heart (0.87 ± 0.24 kg) or the yak-cattle hybrids heart (0.84 ± 0.11 kg). The larger sizes of these organs help the yak to achieve adequate intake and circulation of oxygen in conditions where the supply is low. Lung weights of the Chinese yak vary among the different breeds, from 1.1 percent to 1.5 percent of body weight, and heart weights vary between 0.5 percent and 0.78 percent of body weight (Li Shihong et al., 1984; Zhang Rongchang, 1985).

Denisov (1958) found that the alveolar area occupied 59 percent of the cross-sectional area of the yak lungs, compared with 40 percent for Jargas cattle located nearby. This suggests that the yak lung also has a relatively large surface area from which to absorb air in order to compensate for the lower oxygen content of the air.

The trachea of the yak is also particularly large to allow a high rate of intake of air. Zhang Rongchang et al. (1994) stated that the trachea length in Tianzhu White yak is shorter than in other cattle but that the diameter is appreciably greater. Wang Yuchang (1995) confirmed that finding and also referred to large nostrils in the yak that further assist air intake. Li Shihong et al. (1984) measured five females and reported 43 cm for the length of the trachea and 5.5 cm for the diameter. Apart from variation in the trachea dimensions among individual animals, the size of the trachea varies with the general body size of the yak, as affected by breed and location. The annular cartilage of the trachea was found to be narrow and adjacent cartilages of the trachea to be about 4 cm apart. This allows the yak to breathe rapidly and to quickly increase air intake into the lungs when conditions demand it.

Circulation and oxygen intake and absorption

Heart and pulmonary pressure. A study of five yak at high altitude (Ladakh, India, 4 500 m) and six yak at low altitude (Whipsnade Park, England, 150 m) by Anand et al. (1986) found that the pulmonary arterial pressure was not significantly different in the two groups. But the pulmonary arterial resistance was slightly higher in the yak at high altitude than in those at virtually sea level (0.58 vs. 0.34 mm Hg l-1 min). A higher resistance would be expected if vasoconstriction has occurred in the pulmonary arterial system.

Vasoconstriction commonly occurs in order to reduce blood supply to under-ventilated areas of the lung and maintain homeostasis in other respects (Anand et al. 1986).

For example, in an animal with a pneumonic lung, the vasoconstriction reflex will shut off oxygen to the damaged area even at low altitude, making more oxygen available to the functional areas of the lung. At high altitude, as Anand et al. (1986) argued, such vasoconstriction would not be a good long-term response to permanently hypoxic conditions. Because the whole lung becomes a low-oxygenated area at high altitudes, the vasoconstriction reflex would be very damaging, as it would then affect the whole lung. As shown by these authors from comparisons of yak with cattle, it seems the yak has adapted to prevent this vasoconstriction happening to all but a very small extent.

Thus, when comparing yak with Himalayan (hill) cattle and hybrids of these with yak, all at the high altitude, Anand et al. (1986) found that while arterial pulmonary pressure in the cattle was somewhat higher than in the yak, the pulmonary arterial resistance was more than three-fold greater. In these respects, the first-generation hybrids of the yak with these cattle were intermediate in their pulmonary haemodynamics, but considerably closer to the yak. Backcrosses to cattle (three quarter cattle, one quarter yak), however, had a bi-modal distribution - with some animals closer to cattle and others closer to yak, especially in respect to the resistance trait. Anand et al. (1986) concluded from this study that there was an inherited basis to pulmonary arterial resistance and that the yak had gone a long way towards eliminating the vasoconstrictor response to high-altitude-low-oxygen living.

Anand et al. (1986) provided a cautionary comment to their conclusions by saying that they cannot be certain to what extent the differences in resistance between the hill cattle and the yak are an expression of the differences in the size of the animals (the cattle were much smaller). If the results are confirmed, a genetically attenuated vasoconstrictor response to low-oxygen conditions is clearly an adaptation of importance.

In an effort to explore the subject further, Anand et al. (1988) did another study in which sheep and goats were also included. Their later results supported the earlier thesis, in relation to the reduced vasoconstriction response in the yak, and also added data (albeit from only one yak), suggesting that the yak has a relatively larger right ventricle of the heart than what is found in hybrids of yak with cattle. Moreover, the yak, unlike the cattle, had a smaller medial thickness of the small pulmonary arteries (further suggesting a reduced capacity for vasoconstriction).

Belkin et al. (1985) reported, on the basis of a study of 40 hearts from mature yak, that there was a higher degree of capillarization of the right ventricle of the heart compared with the left. This suggests a further adaptive response of the yak to high altitude conditions that require the right ventricle to cope with increased loading.

Respiration. Cai Li et al. (1975) observed 48 adult female yak at pasture at an altitude of 3 450 m in July and August. Respiration rate was between 20 and 30 per minute when the air temperature was below 13oC, but above that temperature the respiration rate rose rapidly. Respiration rate was significantly higher in the evening than in the morning, but was not significantly correlated with humidity, wind speed or the prevailing weather. Zhang Rongchang (1989) reported a respiration rate in adult yak of 80 per minute at 28°C, 49 per minute at 10°C and 25 per minute at 5°C. For one-year-old yak, the respiration rate at the high temperature was as high as 130 per minute but declined to 7 - 15 breaths per minute at -6°C. A study by Wang Yuchang (1995) on Tianzhu White yak found both respiration rates and pulse rates to be higher in the females than in males.

As expected, respiration rate was also found to be higher during periods of activity than during inactivity.

Zhao Bingyao (1982) examined seasonal differences in the respiration rate in five adult female yak at an altitude of 3 400 m on cold grassland. Over a period of a year, the animals were observed each day between 0600 hrs and 0800 hrs and again between 1800 hrs and 2000 hrs. The respiration rate was found to be highest in August and pulse rate highest in June. Both rates declined gradually after the warm season ended and were at their lowest in March. Body temperature was virtually unaffected by season and averaged 37.6°C in the morning and 38.5°C in the evening. All of this suggests that yak alter their respiration rate not only in response to a changing need for oxygen, but also to regulate body temperature. The yak, with its thick skin, absence of sweating and a heavy coat, has few means at its disposal for heat dissipation, other than respiration rate. The lowest pulse rate in March corresponds to the time of year when yak are in their poorest condition and often at a point of exhaustion. At this time, they have a low metabolic rate due to the prolonged period of a shortage of feed over winter that leads to near starvation.

Blood cells and haemoglobin. The capacity to take in sufficient air by virtue of anatomical features, respiration rate and physiological response is clearly an important aspect of yak adaptation to life at high altitudes. It is also important that absorption and retention of oxygen from the air should be adequate for the need. This, too, may be specially adapted in the yak. In this regard, the evidence from red blood cells and haemoglobin (Hb) content is not totally conclusive. Data from 21 different sources are presented in Table 4.3. These results suggest that, relative to adult cattle (Bos taurus) at or around sea level, the yak in these various studies do not have exceptionally high numbers of erythrocytes per unit volume of blood. The values range from 5.2 to 10.3, with an average of 6.9 (1012 per l) for the 16 mean values shown. This compares with a mean of 7.0, and a range from 5.0 to 9.0, given as normal values for other cattle in a review article by Doxey (1977).

The overall average of the 21 Hb values (g per dl) in Table 4.3 is 11.8 (range of averages from 8.3 to 18.4). These values are only slightly higher than the overall average of 11.0 (8.0 - 14.0) given as the normal values by Doxey - and the mean values of only two of the groups of yak fall outside the range for the cattle examined. The data in Table 4.3 indicate that the haemoglobin concentration in blood increases, in general, with increasing altitude, particularly if only the data from yak at the several highest altitudes are considered. Interestingly, the values for yak at Whipsnade Park, a little above sea level, were similar to the values for yak from China, Bhutan and India. Taking account of the altitude effect, it seems that yak are not exceptional relative to cattle. (There is no particular explanation for the fact that two of the values quoted by Zhang Rongchang et al., 1994, for yak in Tibet at altitudes of 4 366 m and 4 500 m, are markedly higher than the other values from that area. Sampling errors cannot be ruled out because the number of animals involved is not given). Only a few authors provide data on packed cell volume (PCV). These are, on average, higher than the normal mean given in the article by Doxey, previously mentioned. None are outside the range he quotes. A useful parameter, which can be derived from a combination of PCV and red-cell count, is the mean corpuscular volume (MCV), which provides an indication of red-cell size. The average of the seven estimates available is 59.8 and puts this at the top of the range quoted by Doxey for cattle. This, then, may indicate that yak have larger red cells with a greater surface area and a higher capacity for the retention of oxygen. (Larrick and Burck, 1986, in a general article on yak in Tibet, give a contradictory view by suggesting that yak have very small red cells in relation to sea-level bovines but have vastly more cells per unit volume of blood; unfortunately no actual data or references are provided to verify this claim.)

An intriguing paper by Lalthantluanga, et al. (1985) showed that two types of a and two types of b chains are found in the yak haemoglobin, and that there has been a substitution of valine at position 135 of the bII-chain, in place of the more usual alanine. This was considered by the authors to be the reason for the intrinsically higher oxygen affinity of yak haemoglobin, compared to that of lowland cattle, which is quoted by them and other authors as an established finding in the yak.

It seems, therefore, that factors concerned with air intake, combined with a high oxygen affinity of yak haemoglobin, provide the basis for the yak's adaptation to life at high altitudes.

One final note regarding Table 4.3: Attention is drawn to the difference in blood values at Whipsnade Park between yak manually restrained and those sedated with xylazine. The act of struggling by the animals in the course of manual restraint was shown by Hawkey et al. (1983) to release reserves of red cells from the spleen and hence raise the values of several of the blood parameters above those of sedated animals. This point was noted by Winter et al. (1989) who also sedated their animals. It has to be assumed, in the absence of information to the contrary, that all the other estimates presented in Table 4.3 are based on manually restrained animals. Therefore, the values from the majority of the sources are likely to be higher than they would have been from sedated animals. It seems possible that the degree of struggling by the animal in the course of restraint may also affect the results, though there are no data presented on that point.

Seasonal variation in Hb content. There is some seasonal variation in the Hb content of yak blood. It is relatively low in May (10.5 g per dl) and higher in October, after the end of the summer grazing season (14.6 g per dl, based on some observations of female yak in Menyuan county of Qinghai province (Research Co-operative Group, 1980 - 1987). Similar observations were made on yak in parts of Siberia by Belyyar (1980), who recorded an Hb content of 10.2 g per dl in the spring and 12.8 g per dl in the autumn of the same year. He also noted that the diameter of the erythrocytes in these yak was 4.83 m, which was larger than for contemporary Yakut cattle (4.38 m) in the same area.

Cai Li et al. (1975) also provided some evidence on age differences and the difference between lactating and dry adult females. Age effects were not significant on the numbers involved (groups of 17 to 58 for female yak). However, lactating yak had lower red cell counts than dry cows (as shown in Table 4.3 for data from Ruoergai).

Table 4.2 External characteristics of yak at different locations varying in altitude



No. observed

Polled (%)

Black/ brown (%)

Black with white patches (%)

Variegated (%)

White (%)

Latitude (N)

Longitude (E)

Altitude (m)

Tibet, Pali



4 300





Yunnan, Zhongdian



3 300





Sichuan, Muli



3 500





Sichuan, Jiulong



3 800





Sichuan, Liuba



3 800






Tibet, Pengbuo



4 000






Tibet, Dangxiong



4 400





Tibet, Jiali



4 500






Sichuan, Shachong



3 200





Sichuan, Ganzi



3 800






Tibet, Naqu



4 570







Sichuan, Seda



3 893







Sichuan, Hongyuan



3 500







Gansu, Gannan



3 400







Qinghai, Tongde



3 300





Qinghai, Haiyan



3 500







Qinghai, Menyuan



3 300







Qinghai, Gongda



3 500







Gansu, Shandan



3 000

463** 109






Xinjiang, Bazhou



2 500







* Median latitude and longitude for the area in question
** 463 observed for horned/polled, 109 for hair colour

Table 4.3 Red cell counts, haemoglobin (Hb) concentration, packed cell volume (PCV), and estimated mean corpuscular volume (MCV) in adult yak from various sources


Altitude (m)


Red cell count [1012/l]

Hb count [g/dl]

PCV [l/l]

MCV [fl]









UK, Whipsnade Park











Hawkey et al., 1983

UK, Whipsnade Park











Hawkey et al., 1983

USSR, Yakutia










Zhang Rongchang et al., 1989 [quote]


1 500



Katzina, 1997 [quote]

Xinjiang, Bazhou

2 500




CCOYSR, 1982

India, Dirang

2 750









Mondal et al., 1997

Gansu, Tianzhu

3 000









Zhang Dasou et al., 1985

Tibet, Linzhi

3 000



Zhang Rongchang, 1994 [quote]

Yunnan, Zhongdian

3 300




CCOYSR, 1982)

India, Sikkim

3 300









Sahu et al., 1981

Qinghai, Gonda

3 400




Li Jinxuan, 1984

Sichuan, Ruoergai

3 450







Cai Li et al., 1975

Sichuan, Ruoergai

3 450







Cai Li et al., 1975

Sichuan, Hongyuan

3 500




Liu Qibui, 1983

Bhutan, (east)

4 000







Winter et al., 1989

Qinghai, Huzhu

2700 - 4100




10.3 (40)*


0.32 (70)**


Ma Sen, 1997

Qinghai, Darri





Xu Rongchan & Wu Zhiqiang, 1984

Tibet, Naqu




Zhang Rongchang et al., 1994 [quote]

Tibet, Dangxiong





Tang Zenyu et al. 1982

Tibet, Longzhi




Zhang Rongchang et al., 1994 [quote]

Tibet, Yagao





Huang Wenxiu & Wang Sufang, 1980

* Number of yak is 40.
** Number of yak is 70. (S) = Sedated with xylazine; (m) = manually restrained; others: form of restraint not specified, manual restraint assumed; dry = dry adult females; lact = lactating adult females.

Other blood cells and constituents

Most of the papers quoted in Table 4.3 also provide values for white-cell content of the blood. They average 10.2 (106 per litre) (8.6 - 12.5), suggesting that the animals were in normal health at the time of bleeding. White cell content of the blood of Tianzhu White yak is about 9.1 - 12.5 (106 per litre), higher than that of local cattle, 7.6 (106 per litre), according to Zhang Rongchang's report (1989)

Cai Li et al. (1975) showed (Table 4.2) that, as expected, lactating females had significantly lower blood glucose levels than dry females (61.5 vs. 66.5 mg per dl). Blood calcium and phosphorus concentrations (mg per dl) for the different groups of females in this study ranged from 8.7 for dry adult cows to 9.5 for calves (for Ca) and from 5.5 for the dry cows to 7.4 for the calves (for P). The blood pH was 7.0, on average, in these data.

Colour of coat and skin

The predominantly dark coat colours of the yak (see Chapter 2) help to protect it against the effects of solar radiation, which is particularly intense at the southern latitudes. The lighter colours and white yak are found farther north and at lower altitudes where solar radiation is less intense. However, were it not for selection and colour preferences by man, it is unlikely that the lighter shades, and white yak in particular, would exist. These light shades are not generally found among wild yak where these would be expected to be at a disadvantage in terms of natural selection. Hair colours of yak, in different areas at different elevations, are shown in Table 4.2.

The cells of the epidermis of the yak contain many pigmented granules, especially in the cells of the stratum basale. These pigmented granules can help to prevent injury from ultraviolet light in the deeper layers of the skin. Yak with white faces generally have eyes surrounded by black hair. The black eye sockets of Maiwa yak of Sichuan province are accepted as a breed characteristic. Also, the hair on the yak's forehead is well developed and can cover the eyes. No specific research appears to have been done on the possible protection against solar radiation afforded to the eyes and face by this forehead hair, but it is reasonable to assume that the hair has such a function.

Adaptive characteristics related to grazing conditions

The cold pastures on which yak graze have predominantly short grass in some areas and rough grazing conditions with sedges and shrubby plants in others. Yak have developed organs for food intake and a grazing behaviour peculiarly suited to this environment.

Grazing procedure and grazing behaviour

Mouth. Yak have broad mouths, small muzzles and thin flexible lips. The front (incisor) teeth are hard and broad and have a flat grinding surface. The tip of the tongue is also broad and blunt and the filamentous papillae on the surface of the tongue are highly developed and cutinized. The surface of the tongue feels rough and "thorny".

Grazing habit. Yak can graze long grass, using their tongue as do other cattle, but they can also graze in the manner of sheep, using incisor teeth and lips to graze short grass and creeping stems, and roots of grass. Yak will also take tender branches of shrubs in alpine bush meadow. Under most normal conditions, yak have learned to avoid poisonous or thorny plants as recorded instances of poisoning are very rare. However, there are reports of extensive pyrrolizidine alkaloid poisoning in Merak Sakten, a part of Bhutan (Winter et al., 1992). This was thought to be due principally to grazing of Senecio raphanifolius (although other plants may also have been involved) (Winter et al., 1994). This plant, as pointed out by the authors, was almost certainly eaten by the yak because of overstocking of the pastures concerned, leading to overgrazing. Otherwise, with a more plentiful supply of feed, the yak would have avoided these plants.

Yak will also readily graze the rough stems and leaves of sedges in low-lying marshy areas. Zhou Shourong (1984) recorded more than 60 species of grasses in the diet of yak on alpine, subalpine marsh and semi-marsh meadow (see also Chapter 13). When the ground is covered with snow, as it typically is for long periods, yak will paw through quite thick snow layers, using both head and face to help them to gain access to the wilted vegetation underneath.

Yak will reduce grass with a height of 15 cm to between 2.6 cm and 5.2 cm (Ren Jizhou and Jing Juhe, 1956). In the spring, yak will graze green shoots no more than 2-3 cm above the ground - though it would be surprising if they could not graze more closely than that if necessary (as sheep in Scotland, for example, are known to do). In addition, the yak take stems and leaves from the residual wilted grass still available in the spring.

The grazing time of yak is affected by season, weather, type and quality of grazing and the structure of the herd in terms of age and sex. This has been studied by many researchers (e.g. Ren Jizhou and Jin Juhe, 1956; Cai Li, et al. 1960; Zhang Rongchang et al., 1982; Qi Guangyong, 1984; Lei Huanzhang et al., 1985; and Zhang Hongwu, et al., 1985).

The general conclusions are that the intake time varies between 34 percent and 80 percent of the total time available for grazing. The rest of the time is used for walking, resting and drinking. Normally, lactating yak herds spend more time grazing than do mixed herds - as also found in other grazing cattle. However, in herds of mixed age, where females have young calves at foot, the grazing rhythm is disturbed as the calves suckle and also learn to graze. Under such conditions, the intake time may be curtailed.

The speed with which the yak moves over the pasture varies with season and pasture conditions, but it is usually faster at the start of the day than later on, and it is also more rapid in the cold season than in warm weather. With the approach of snow or hail storms, yak can be seen to run over the pasture, in bursts of speed up to 57 m per minute - up to four times the normal speed at the start of the day.

There is relatively little variation in the bite rate - around 0.8 - 1.1 mouthfuls per second, but intake varies with season, sward height and other factors. Intake ranges from around 28 kg to 38 kg herbage over a period of ten hours in the summer to only 13 kg or less in the same period when grazing wilted grass in the cold season. The energy and protein intakes are adequate to meet maintenance, work and production, but in the later parts of the winter and the early spring they fall below the requirements. Yak then lose weight and condition.

Rumen volume

Xiao Wangji et al. (1997) reported that the rumen volume of Zhongdian yak relative to their body size was larger than in the local cattle (P<0.01). And the report from Zhang Rongchang (1989), see Table 4.4, showed that the rumen of yak was better developed than that of the cattle, based on the proportion of the weight of different parts to the total stomach. A large rumen may be a useful adaptation to forage roughage achieved by natural selection under the particularly rough grazing conditions prevailing in the yak territories.

Table 4.4 Proportion of each part as a percentage of the total stomach of yak and cattle [Source: adapted from Zhang Rongchang, 1989) (n=6, ±SD]

Stomach part

Tianzhu white yak

Kirgizia yak

Kirghiz cattle


72.2 ± 4.2




5.6 ± 1.0




10.4 ± 1.3




11.8 ± 1.7




Under normal conditions, when grass is abundant in summer, yak have four periods of rumination each day. The first of these is generally two hours after the start of morning grazing. A second period is around noon, when the ambient temperature is high and yak stop grazing. A third period of rumination occurs about two hours before the animals are driven from the pasture back to the campsite. A fourth period is in the evening. Rumination periods generally last between 0.8 and 1.9 hours.

If yak are allowed to graze at night, as well as during the day, the periods of rumination are different. In yak used for work, the periods of rumination fit in with the timing and intensity of the work being performed. Occasionally yak will ruminate at night, but usually they lie in a state of light sleep.

Contractions of the rumen reticulum was studied in 48 yak over a three-day period (in Rouergai county, Sichuan) and showed contractions at the rate of 8.7 ± 1.6 (mean and SD) per five minutes immediately before grazing, and almost the same (9.0 ± 1.2) after grazing. Similar results were obtained at other locations.

In other types of cattle, the frequency of contractions when the animal is resting or ruminating may be only half the rate during feeding (Phillipson, 1970). It is possible therefore that the results on yak suggest a different behaviour, which could reflect an adaptation to the grazing conditions.


Yak can walk freely in precipitous places at high altitudes, which cannot be reached by horse or sheep (very few domestic goats are found in these areas) and they can cope well with marshy ground. As described by Phillips et al. (1946), yak, if in danger of sinking in a marsh, will spread out their legs and use the underside of their bodies to prevent themselves from sinking. They will plod on with a swimming-kind of motion rather than panic and thrash around as a horse might. Yak can swim across rapids and are at ease trekking through snow. They can even be used to make tracks through the snow to clear paths for people - a sort of "biological snow plough".

To help in meeting the challenges of difficult terrain and inclement climate, yak have strong limbs and small hooves of compact texture, with a narrow and sharp hoof tip, hard hoof edges and a close hoof fork. There is an area of soft cutis on the sole.

As noted by Zhang Rongchang (1985), the characteristics of the yak hoof make deep imprints in the ground that allow the yak to control its momentum when going downhill - an important component of its aptitude to move freely in difficult, precipitous terrain.

Adaptation of reproduction

Zhang Rongchang et al. (1994) argued that two aspects of yak reproductive characteristics (see Chapter 5) are also adaptive responses to the environment. First, the higher the altitude where the yak live, the more delayed is the breeding season. (For example, according to these authors, it begins 29 May at 1 400 m a.s.l., 10 - 15 June at 2 100 - 2 400 m a.s.l., 19 - 22 June at 2 700 m a.s.l., 25 June at 3 000 - 3 800 m a.s.l. and the beginning of July at 4 570 m a.s.l.). This allows calves to be born in somewhat warmer weather or closer to the onset of such weather and during, rather than before, the start of significant grass growth in the following warm season. From that point of view, a delay in the ability to breed must be regarded as a sensible adaptive response. Nonetheless, one must question whether nature, in this case, has not chosen a second-best strategy because yak that are mated late in the season have less chance of being re-mated that year, should conception have failed, than females mated earlier in the season. Also, calves that are born late in the year have insufficient time to get into good body condition to improve their chances of surviving the rigours of their first winter.

A second adaptive response claimed by these authors is the short gestation length of yak females (258 days on average; see Chapter 5) relative to other species of cattle. Short gestation, with consequently smaller calves, leads to a less stressful and quicker parturition, a lower oxygen requirement by the calf and less body-weight loss by the cow. These factors must be of some importance in the yak environment, especially in the face of danger from wolves. However, it could be debated that the consequent, relatively low birth weight of the calf may be a disadvantage to the calf.

Another aspect of reproduction in the yak that might be regarded as an adaptation to its environment is that many yak females show only one oestrus in a breeding season and, if not then pregnant, the next occurrence of oestrus will be delayed to the following year (see Chapter 5). It is thought, though hard evidence is not presented, that priority is given by the yak to the deposition of internal fat reserves late in the breeding season rather than to conception. This helps the animal survive the ensuing harsh winter and spring when feed becomes in such short supply as to leave the yak starving. (Under more favourable feeding conditions and in some countries, however, as discussed in Chapter 5, yak regularly come into oestrus several times in one season).

General behaviour in relation to adaptation

Behaviourally, the yak is active and easily excited and can have a ferocious temper. Its conditioned reflexes make it respond rapidly to danger and external forces. In the grazing situation, yak will often jump and run, pursuing each other with tail in the air. Yak can gallop like a horse - an attribute that is enjoyed by herdsmen who organize annual races. Yak can also roll over on pastures as horses do - but unlike other bovines.

Yak have the ability to be readily trained. This ability helps the herdsmen in the feeding and management of the yak, especially as most yak graze year round and are not housed and not usually fenced in or tethered, except for milking. In some pastoral/agricultural areas of Mongolia, housing is sometimes practised (see Chapter 11, part 2). The yak are trained by the herdsmen to return to the campsite by the call of their names, or by special cries or singing. Yak are also readily tamed and trained for use as pack animals or for riding. Once trained, yak retain their acquired behaviour.

Yak are easily frightened and vigilant to attack by wild animals, wolves in particular. The yak will form a defensive position and fight off aggressors. It is often reported, for example from the grasslands of western Sichuan province, that male yak have killed wolves with their horns.

Yak are very gregarious. In herds of 100 or more, it is highly unusual to lose an animal from attack by wild animals, as the yak protect each other. When grazing, they are never far apart from each other. Yak can get very frightened if attacked suddenly while resting and not on guard. In such a case, they will flee from the herd and may then be killed by wild predators. Also, when startled on a hillside from above, by either human disturbance or by cries from wild beasts, they can panic and slip or roll down a hill and die in the fall. Dong Baosen (1985) reported that 312 yak died after rolling downhill in four separate instances in Gen county of Xinjiang - three of these due to incorrect driving of the yak by the people involved.

In ones or twos, yak are difficult to drive or manage, although some yak steers trained as pack animals will work individually, as will yak trained for ploughing. Yak will even find their way back to the campsite without a herder - if it is not too far away. Groups of ten or more yak steers trained as pack animals, however, can be easily managed as a group.


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