Ecology and Biology of Yak Living in Qinghai-Tibetan Plateau

Summary

The ability of yaks to live in ecological conditions in which other bovines will not survive, or at least, not thrive, suggests that yaks have developed special adaptations. Yaks can cope with cold temperatures by conserving heat, rather than by generating it, which would require food that may not be abundant. Heat conservation is achieved by a compact body conformation and a thick fleece of coarse outer hair and an undercoat of fine down. Adaptation of yaks to low oxygen content of the air involves their large chest, large lungs and large heart relative to their overall body size. The skin is highly pigmented and the predominant hair color is black. Both of these help to resist the damaging effects of solar radiation.

Introduction

Yak (Bos grunniens or poephagus gruniens) is the prominent livestock breed in the Qinghai-Tibetan plateau where other livestock species can hardly live but yaks can survive, reproduce and produce milk, meat, wool and other by-products. Yaks live extensively on Chinese plateaus in alpine and sub-alpine regions at altitudes ranging from 2000 to 4500 meters with a cold, semi-humide climate (Fig. 1).

Figure 1. A yak herd in Qinghai-Tibetan highlands.

The highest altitude at which yaks live normally is at about 5500 m (Tibetan Rongbushi region, located on the northern slope of Himalayas); yaks used as pack animals are quite capable of traveling terrains at 7200 m, playing an important role in the transportation of goods to Chinese Mount climbers. The distribution areas extend from the southern slope of Himalayas in the south to the Altai in the north and from the Pamir in the west to the Minshan Mountains in the east. There are about 13 million yaks (of which about 15% are hybrids mostly with Bos Taurus cattle) distributed in 210 counties in China. These form about 11% of the Chinese cattle population and 90% of World’s yak population. The available grassland for yaks in China is about 1.03 billions ha, which is about 25.6% of the Chinese natural grassland [1]. Most of the yaks in Mongolia are found in the Hangay Mountains of the western parts of the country and in the high altitude areas of Altai, with the reminder in the mountains of mid-north Mongolia. Yaks in countries of the Russian Federation are distributed on the narrow mountain areas on the borders with China and Mongolia from Pamir in the west to Lake Bakail in the east. Yaks were also introduced to the high alpine areas of the northern Caucasus in 1970, and to the Yakutsk Valley of Siberia as recently as 1971 [2], to exploit the potential for meat production for otherwise inhospitable alpine grasslands. The yaks of Nepal and Bhutan are on the southern slopes of the Himalayas [1,3] while those of India are distributed in the high altitude northern provinces and in the small territory of Sikkim [4]. Other yaks are found in alpine areas of Afghanistan and Pakistan, adjacent to the Qinghai-Tibetan Plateau (Fig. 2).

Figure 2. Distribution of the Yak population in China and neighboring countries.

The basic features common to the environment where yaks live are extreme cold, mountainous terrain, high altitudes with reduced oxygen content in the air, high solar radiation and short growing season for herbage and a variable assortment of herbage, sparse in some areas. In Qinghai-Tibetan plateau, the weather is rather cold and humid, the average relative humidity is 55 - 65%, the annual precipitation averages 350 - 650 mm, there are no absolute frost-free days in this area and the growth period of plants is only about 120 days a year. The annual average temperature is 0°C, in Maduo County of Qinghai Province, the extreme lowest temperature is -41.8°C.

In the yaks native regions, at present, the stocking density of yaks declines with the increase in average annual temperature. The greatest concentrations of yaks are found at average annual ambient temperatures between -3° and +3°C. In the Qinghai province yaks are concentrated in areas with annual temperatures between -3° and -4°C, those in Tibet are densest at the range of -3° to -5°C and in Sichuan Province between -1.6° and +3.4°C.

Yaks live in the high mountainous areas where, with increased altitude, the oxygen content and temperature decrease. At the elevation of 3500 m above sea level where most yaks live, the air oxygen content is around 35% lower than that at sea level. On even higher grazing areas at an altitude of 5000 m, the oxygen content is halved. Also, in most of the areas there are more than 2000 hours of sunshine and levels of solar radiation are between 130 and 165 Kcal/cm2(540 - 690 KJ/cm2). The ability of yaks to survive in such harsh conditions and the ability of people to derive sustenance from them are classic examples of adaptation by both animals and human beings.

Anatomical Adaptations

The yak’s body is compact with a short neck, short limbs, no dewlap, small ears and a short tail. The scrotum of the male is small, compact and cover with hair, and the udder of the female is small and also covered with hair. The skin has few wrinkles and the surface area of the yak is relatively small per unit bodyweight (0.016 m2/kg) [5]. Compared with common cattle, yaks have a shorter and wider trachea to allow a high rate of air intake. The length of trachea in Tianzhu White Yaks (Fig. 3) is 44 - 51 cm (65 cm for other cattle at the same age), but the diameter is appreciably greater [6]. The section of trachea in yaks is crescent compared to the round shape in common cattle [6].

Figure 3. Tianzhu White Yaks: a unique yak breed in Tianzhu area of the Gansu Province of China.

A large space exists between the two ends of cartilage, being 5 cm in the adult male and 2.2 - 3 cm in the female yak. The trachea in yaks is comparable to that of dogs and may be adaptive for respiration in low oxygen content environment. This also allows yaks to breathe rapidly and to quickly increase air intake into the lungs when conditions demand it.

Yaks have narrow and long ribs with a relatively larger space between ribs and a good development of muscle in between. Compared to local cattle, yaks have a large thorax and chest allowing the development of large lungs and a large heart, and these in turn, may be have come about as adaptive strategies to assist in the intake and circulation of adequate amounts of oxygen under conditions in which it is in low supply.

Yaks have 47 - 48 vertebrae, of which 14 - 15 are thoracic vertebrae and 14 pairs of ribs, one or two more than in other cattle. This gives the yak a larger chest capacity. The heart and lungs indexes (the ratio of heart and lung weight to body weight) are higher in yaks than in common cattle. The heart index is 0.52% in female adult yaks and 0.42% in male adult yaks, the lung index is 0.98% and 0.75%, respectively. Alveolar areas represent about 59% of cross-section areas of the lung section, compared to 40% in Jargas cattle [7]. Thus, the heart and lungs are exceptionally well developed in the yak, and the lungs of yaks have a relatively large surface area from which to absorb air in order to compensate for the lower oxygen content of the air.

There are five lumbar vertebrae, one less than in cattle. The number of coccygeal vertebrae is variable ranging from 12 to 16 (other cattle have 16). There are five sacral vertebrae and there are seven cervical vertebrae, the same as in cattle. Total numbers of vertebrae are thus fewer than for other species of cattle.

Yaks can walk freely in precipitous places at high altitudes which cannot be reached by horse or sheep and they can cope well with marshy grounds. Yaks have strong limbs and small hooves of compact texture, with a narrow and sharp hoof tip, hard hoof edges and a close hoof fork. These hoof characteristics make deep imprints on the ground, and may be an adaptation to allow the yak to control its movement when going downhill.

Physiological Adaptations

The capacity to intake sufficient air, by virtue of anatomic features, respiration rate and physiological response, is clearly an important aspect of the yak’s adaptation to high altitude. It is also important that absorption of air oxygen be adequate for their needs. The yak respiratory rate is 9 - 77 times per minute and the heart rate is 45 - 79 times per minute, both of which are faster than in common cattle. The concentration of red blood cells in yaks is higher than in other cattle, being 6.60x1012/mm3 in Tianzhu White yaks and 10.38x1012/mm3 in Noregai yaks of Sichuan [6,10]. The diameter of erythrocytes in yaks is larger than in common cattle. The diameter of red blood cells in adult female yaks was reported to be 4.83 μm, while that of local cattle was 4.38 μm. This suggests that the oxygen content per unit of blood is higher in yaks than in local cattle breeds.

Cai et al., [8] observed changes in respiration rates in 48 adult yaks on pasture at an altitude of 3450 m in July and August. The respiration rate was between 20 and 30 per minute when the ambient temperature was below 13°C, but the respiration rate rose rapidly when the temperature increased. Respiration rate was significantly higher in the evening than in the morning, however, respiration rate was not significantly correlated with humidity, wind speed or the prevailing weather [4].

Zhao [9] examined the seasonal changes in respiration rates of 5 adult female yaks at an altitude of 3400 m on the cold grassland. Over a one year period, the animals were observed each day between 06:00 and 08:00 hr and again between 18:00 and 20:00 hr. Respiration rate was 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. These results suggested that yaks alter their respiration rate not only in response to a changing need for oxygen, but also in regulating body temperature. The yak, with its thick skin, absence of sweating and a heavy coat has few means, other than respiration rate, at its disposal for heat dissipation. The lowest pulse rate in March corresponds to the time of year when yaks are in the poorest body condition, often close to exhaustion, and with low metabolic rate following a shortage of feed over winter, leading to near-starvation.

The hemoglobin content in yaks is higher than in other cattle breeds, being 8.56 g/dl in Tianzhu White yak and 12.71 g/dl in Nouregai yak of Sichuan, values which are significantly higher than the 7.86 g/dl for local cattle [6]. It has also been reported that the hemoglobin content in adult female yaks changes with the season, being 12.3 g/dl in winter and 10.2 g/dl in spring. Generally, hemoglobin concentrations in blood increase with an increase in altitude [7].

These anatomical and physiological characteristics appear to be adaptations of yaks to ensure survival in the harsh ecological conditions of yaks native environment. The well developed chest, heart and lungs, short trachea, cardiac and respiration rates, and high hemoglobin content in blood, contribute to provide oxygen to tissues and allow yaks to survive at altitudes greater than 3000 meters above sea level.

Reproduction Adaptation

Yaks in the Qinghai-Tibetan Plateau show many reproductive adaptations. The seasonal onset of estrus in yaks appears to be delayed at increasing altitude. For example, at an altitude of 2100 to 2400 m, estrus starts on 10 - 15 June; at 2700 m, on 19 - 22 June; and at 3000 - 3800 m, females who did not calve in the current year will show estrus on June 25th. In Naqu district only, individuals will start to show heat at the beginning of July. This allows for calves to be born in warmer weather or closer to the onset of such weather, and during, rather than before, the start of significant growth of grass in the following warm season. This suggest that a delay in the ability to breed is an adaptive response. However, yaks that are mated late in the season have a lesser chance of being re-mated that same year (should conception fail), than females mated earlier in the season. Thus, the adaptation is not ideal by any means. Also, calves that are born late in the year may have insufficient time to achieve a good body condition to improve their chances of surviving the rigors of their first winter [10]. Thus, conception at first estrus is important in this species.

Compared with common cattle breeds, yaks have a shorter gestation period, averaging 255 days, which is about 30 days shorter than that of common cattle. The shorter gestation period may benefit the female by reducing the load on the heart and respiratory system, and might also be the reason why neonates maintain the fetal type of hemoglobin (HbF2). Shorter gestation, with consequently smaller calves, also leads to a less stressful and quicker parturition, which may be a matter of some importance in the yak environment, especially in the face of danger from wolves. Whether the resulting relatively-low birth weight of the calf is or is not a disadvantage to the calf itself is a matter for debate [10].

Another aspect of reproduction in the yak, which might be regarded as adaptation to its environment, is the fact that some female yaks show only one estrus during the breeding season, and, if not pregnant, the next occurrence of estrus will be delayed until the following year. Perhaps, in such instances, late in the breeding season, priority is given not to conception but to the deposition of internal fat reserves.

Newborn yak calves contain four types of hemoglobin in their blood (Table 1) . Adult types of hemoglobin (HbI and HbII) represent about 51% of total hemoglobin, and fetal hemoglobin (HbF1 and HbF2) represents 49%. The early fetal hemoglobin HbF2 is not totally replaced by HbF1. Even though HbF2 represents only about 8.9% of the total hemoglobin, because of its higher affinity for oxygen, it plays an important role in the survival of yak neonates in low oxygen environments. In newborn calves in common cattle, there is reportedly only one fetal type of hemoglobin. However, in a fetus collected from the abattoir (body weight 12 kg), there were two types of fetal hemoglobin [7]. Changes of HbF1 and HbF2 in young yaks are shown in Table 2. As indicated in Table 2, HbF2 disappeared at 45 days after birth and HbF1 at 90 days after birth.

Skin and Coat Adaptive Characteristics

There are few skin wrinkles on the body surface of the yak giving the animal a relatively small radiation surface. The skin in yaks is thick, there are well-developed dermal and fat layers, and fat is readily deposited during the warm season. Li et al., [5] reported that the average thickness of skin is 6.2 mm in yak, 6.12 mm in Bos taurus, 5.62 mm in Bos indicus. Skin thicker in the yak's back than in other parts of the body, perhaps because the back is the part of the animal that is most exposed to wind, rain and snow. Li et al., [5] also measured the skin thickness on the shoulder blade, the back and the knee on 7 live female yaks and found the average thickness at the three anatomical regions to be 5.61+/-0.36, 7.5+/-0.83 and 5.6+/-0.40 mm, respectively. Sweat glands are not well developed in yaks. They are apocrine glands with long and tortuous ductules distributed in the skin over the entire body. Sweat secretion does not occur readily, reducing the heat radiation surface. This appears to force the animal to retain heat in the body and to increase its tolerance to cold [10]. Density of sweat glands per square cm was found to be greatest on the forehead (89/cm2) and lowest on the rump (138/cm2), with an overall average of 399+/-251/cm2 [5,11].

The coat of yaks seems well suited to insulate animals from cold, protecting them from heat and repelling moisture. All these factors are important to survive in the prevailing climatic conditions. 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 generation of additional heat. To generate extra heat would ultimately require additional feed that is in short supply over winter. Interestingly, one of the most successful of all yak breeds, the Jiulong yak of Sichuan Province, has a fiber strain which produces between 3 and 5 times as much fleece as that of other types of yaks. This breed also inhabits one of the coldest, dampest and most fogbound areas of all yak territory. It is possible that the dense, heavy coat has allowed the Jiulong yak to survive under these prevaling conditions.

The coat of yaks consists of three types of fiber: coarse, long fibers with a diameter over 52 μ; down fiber with a diameter smaller than 25 μ; and, mid-type hairs with diameters between these two values. The down fiber is a particular attribute of the winter coat of yaks and provides the additional insulation required. Coats with a mixture of fiber types have been shown to maintain a stable air temperature within the coat. Ouyang et al., [12] reported that the gradient in temperature between the skin surface and the top of the hair surface is far greater in winter than in summer for parts of the body trunk like shoulder, rump and belly, but that the seasonal difference in the temperature gradient from skin surface to top of the hair surface is much less in the extremities of the body, such as the ears, where vasoconstriction occurs during cold [9].

The function of the coat in helping yaks survive in very cold and wet conditions is enhanced by the coat's low water absorption [12]. The erectores pilorum muscles are well developed in the dermis of the yak [13] and their contraction makes the fibers stand up and effectively increase the depth of the coat and reduces heat loss under stress from cold.

Hair growth and the composition of the coat changes with season in the yak. As ambient temperature falls with the approach of winter, down fibers grow densely among the coarse hairs, especially on the shoulder, back and rump. Ouyang et al., [12] found that, in winter, the proportion of down fiber increased between 17.5% and 30.0% in winter, through the activation of down follicles which had lain dormant [14]. The proportion of coarse hairs, therefore, correspondingly decreased. As air temperature rises with the onset of the warm season, down fibers began to shed from the fleece.

As a consequence of the abundant grazing in summer and early autumn, yaks are able, normally, to deposit a layer of subcutaneous fat which then helps to insulate them from cold and also provides an energy reserve to be used to withstand nutritional deprivation over winter and early spring.

Energy Metabolism Adaptation

Hu [15] studied the energy metabolism of growing yaks at three different ages (1, 2, and 3 years old) and compared it with that of the local yellow cattle, at three different altitudes (2261, 3250 and 4271 m). The author reported that heat production of the fasting yak remained fairly constant irrespective of altitude, whereas that of yellow cattle rose markedly. This could well point to an adaptive response of yaks to life at high altitude and to the nutritional deprivation which yaks experience in winter [15]. Heat production of fasting yaks was higher than that of the yellow cattle at the lowest of the three elevations, but not at the higher altitudes. In another experiment by the same author, yaks generated a little more heat in the course of walking than did the somewhat larger yellow cattle. The author attributed the differences in heat production to the difference in body size, with smaller animals being expected to generate more heat.