Bos mutus (Przewalski, 1883)

Don E. Wilson & Russell A. Mittermeier, 2011, Bovidae, Handbook of the Mammals of the World – Volume 2 Hoofed Mammals, Barcelona: Lynx Edicions, pp. 444-779 : 579-580

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Yak

Bos mutus

French: Yack / German: Wildyak / Spanish: Yak

Taxonomy. Poephagus mutus Przewalski, 1883 ,

W Nan Shan (approx. 39° 20’ N, 95° E), China.

Opinion 2027 of the International Commission for Zoological Nomenclature in 2003 retained usage of both Przewalski’s mutus for the wild Yak and Linnaeus’s grunniens for the domestic Yak. Monotypic.

Distribution. Fragmented distribution in the Tibetan Plateau in China (Xinjiang, Xizang & Qinghai); one population drifts from S Xinjiang into NW India (Ladakh) and another from Qinghai to Gansu, China.

Descriptive notes. Head-body up to 306-380 cm,tail up to 100 cm, shoulder height 170-203 cm (males) and 137-156 cm (females); weight 535-1000 kg (males) and 300-350 kg (females). Male Yaks have massive but compact bodies on sturdy short legs; they are reported to be the third largest mammal in Asia after the Asian Elephant (Elephas maximus) and Greater One-horned Rhinoceros (Rhinoceros unicornis). Female Yaks can be one-third the size of males. Both sexes have nearly smooth, cylindrical, gray-to-black horns, but those of males are larger and longer and sweep outward and forward more than the upright smaller horns of females; from a recent sample in the Chang Tang Nature Reserve, Xizang, length of outside curve, 47.5-99 cm (males), 37-64. 5 cm (females); basal circumference, 26-42 cm (males), 15-23 cm (females); tip-to-tip, 26-83 cm (males), 18-67 cm (females). The Yak forehead is short, wide, and slightly convex. The skull is heavy, with broad nasals with tapering ends; narrow lachrymal; upper edge of maxillary is in contact with middle of nasals; outer sides of premaxillaries nearly parallel, not tapering. Other characters include small ears; no dewlap; large and rounded hooves; conspicuous hump, most pronounced in males, arising abruptly behind the short neck as a result of elongated neural spines of cervical and dorsal vertebrae tapering level at the mid-back and notfalling away above the hips; black pelage with rust-brown hues and sometimes peppered with gray on the head and neck of older adults (a rare pale golden-brown mutation in about 2% of animals occurs in the Aru Basin, Xizang); muzzle grayish; young dark brown; pelage dense with an undercoat of wool and long coarse guard hairs; long draping hair on chest, flanks, and thighs almost reaches the ground in mature males; tail long and bushy on the lower one-half; few functional sweat glands; no preorbital glands or associated lachrymal fossa. The pelage consists of three types of hairs: long coarse guard hairs, intermediate down fiber, and dense, fine down fiber. Down fibers grow dense in winter, particularly on the neck, shoulders, and back. Dental formulais10/3,C0/1,P3/3,M 3/3 (x2) = 32. Yak neonates are not born with their deciduous incisors; the first pair erupts after about one week, with successive pairs erupting weekly thereafter until the calf is four weeks old; at about two years of age, the first pair of deciduous incisorsis replaced by permanent incisors, and that process continues until aboutfive years of age.

Habitat. Yaks are highly adapted for existence under extreme conditions of low temperature, high elevation and associated low oxygen availability, extreme solar radiation at southern latitudes, and relatively arid conditions. They are now restricted to very high-elevation and remote uplands at 4000-6100 m, usually free of human harassment, and are undaunted by mountainous terrain because of their stout and strong limbs and their relatively small compact hooves with hard edges and sharp tips. They occur in greatest abundance on alpine meadows, less so in alpine steppe, and are scarce in desert steppe. Preferred habitats in Chang Tang Nature Reserve include partially glaciated mountains with slopes of alpine meadows, seasonally lush alpine steppe that may green-up 2-3 weeks before the plains, and edges of streams. Male Yaks occur often on gentle slopes, and female herds occur more often on high hills and upper slopes. Densities of Yak are difficult to estimate given their rarity and their clumped and fragmented distribution in their expansive and remote range on the Tibetan Plateau.

Food and Feeding. Yaks are grazers, seasonally eating grasses, sedges, lichens, and mosses. The Yak’s mouth is broad and lips flexible; the incisors have flat grinding surfaces, and the tongue is broad and blunt with highly cutinized and developed papillae. Such adaptations allow Yaks to forage like cattle on long grasses or like sheep on grasses as short as 2-3 cm. In winter when sedges such as Kobresia are short and brittle, Yaks simply lick them up with their rough tongue. In Chang Tang Nature Reserve, Yak feces contain grasses and sedges (Stipa, 52%; Kobresia, 4%; Carex mooreroftit, 14%; other grasses, 4%), followed by herbaceous plants (12%) and the dwarf shrub Ceratoides compacta (10%). In Yeniugou, Qinghai, Yak feces in summer contain 85-5% sedges and grass (sedges: 67-1 % Kobresia and 5-3% Carex; grasses: 13-1%) and almost 4% mosses; in autumn, grasses dominate (68:8%), and sedges become less important (25-3%). Little is known about their specific water requirements, but early chroniclers noted frequent visits to mineral-rich warm springs and rivers and consumption of snow.

Breeding. The breeding and birthing seasons are short, generally mid-August through September and mid-May through June, respectively. Some male Yaks stay with female herds throughout the year, but others live alone or in bachelor groups until rut. At some point, older males may become less competitive in obtaining mates than prime males and may live alone or in small groups apart from other groups. Males do not establish a territory or harem apart from a herd or each other. Rather, they wander day and night and attempt to tend and breed receptive females within existing female herds. As rut commences, males maintain a dominance hierarchy by fighting each other and likely attain maximum breeding performance at 5-10 years. Aggressive behavior among males in rut is intense; considerable scarring, wounds, and horn damage on male Yaks incurred during rut-induced fights have been noted. Almost every village close to areas frequented by wild Yaks in the Chang Tang Nature Reserve has experienced attacks, injuries, and even deaths caused by male Yaks, particularly during rut. Indirect threats involve lateral displays that emphasize the shoulder hump and mantle of hair; competing males will stand head-to-head or head-to-tail 3-6 m apart for more than five minutes. Direct threats include charges with lowered heads and head-to-head sparring that can last 15 minutes. Rutting wild males frequently wallow on dry ground, sometimes while defecating and urinating, which distinguishes them from most other Bos but parallels behavior of American Bison . Other rutting behaviors of males include grunting, bellowing, horning the ground and vegetation, rubbing the face and neck on the ground, and teeth grinding. Copulation is abrupt and of short duration. Females breed for the first time at 3-4 years of age, but this, and annual timing of estrus, likely varies depending on climate, latitude, elevation, and availability of nutritious forage. Yaks generally produce one calf every other year, or longer; twinning is rare. Gestation is 258-270 days. If wild Yaks are comparable to domestic Yaks, parturition occurs during the day, rarely at night, in a sheltered location away from the herd; birth is often from a standing position although the female may spend considerable time lying on her side. Offspring are precocial and attempt to stand within about ten minutes postpartum; nursing first occurs 11-30 minutes postpartum and may last 3-5 minutes. Females and their offspring rejoin the herd shortly thereafter, and herds will protect offspring by encircling them. Postpartum anestrus is about 125 days.

Activity patterns. Daily and seasonal activity patterns are no doubt related to heat conservation during lengthy winters and avoiding heatloading in summer. Consistently low temperatures and low primary productivity throughout most of the year in Yak range result in a strategy of heat conservation rather than heat production. Adaptations to the cold are so developed that signs of heat exhaustion are apparent when ambient temperatures exceed 13°C; heart rate and respiration increase and most activity ceases when ambient temperatures approach 20°C. Adaptations for heat conservation include a compact body, despite a large mass, with relatively short legs, neck, and ears and a low surface-to-volume ratio; thick pelage particularly on the neck, back, and rump; dark pelage and skin pigmentation to minimize effects of intense solar radiation but maximize heat absorption; thick unwrinkled skin with non-functional apocrine sweat glands, except on the muzzle, but with highly developed piloerection muscles; and a thick, but seasonal, subcutaneous fat layer. Yaks maximize heat dissipation and minimize heat production by seeking the coldest spots and shade, bedding in snow, and standing in icy water even during inclement weather.

Movements, Home range and Social organization. Yaks are non-migratory but are capable of moving considerable distances, both linearly and elevationally if, for example, harassment or seasonal food availability dictate. Because of persecution, wild Yaks today are very wary of humans, and disturbance may cause long and unpredictable movements; e.g. one observer recalled a Yak group that fled 20 km after spotting him! No home range information exists for the Yak. Groups of Yaks vary from single males to aggregations of more than 200 and even 400 individuals, but aggregations of a thousand individuals were reported in the 1800s. Groups are not stable through time. Grouping behavior depends on sex, time of year, and location. Group types include solitary adult males, mixed-aged males, females and offspring, and mixed sexes. Females rarely occur alone and typically in groups of about five, sometimes considerably larger. Most adult males segregate from females and subadults during non-breeding. Of 507 male Yaks observed in the Aru Basin, Xizang, 36% were alone, 43% in groups of 2-5, 13% in groups of 6-10, and the rest in groups up to 19. Observations from Chang Tang Nature Reserve suggest a sex ratio of 67-75 males:100 females. In Yeniugou, Qinghai, groups of 10-245 female Yaks with their young-of-the-year and yearling offspring, juveniles, barren females, and adult males (about 4% of the herd) were observed in summer; male only groups averaged 6-2 individuals. Maximum life span of the Yak in captivity is about 20 years; one wild Yak lived 22 years and nine months in the Beijing Zoo, China. Longevity probably is comparable, or less, in the wild. The Latin name “ mutus ,” meaning mute, is misleading because Yaks are quite vocal, particularly males in rut.

Status and Conservation. CITES Appendix I. Classified as Vulnerable on The IUCN Red List. Historically, the wild Yak occurred more widely than it does today, beyond the Tibetan Plateau in the mountains of Kazakhstan, Mongolia, and southern Russia until the 137-18" centuries (dates of extinction uncertain). It is considered extinct in Nepal, Bhutan, and western Sichuan, China. Numbers of Yak now total only about 10,000 -15,000 in Xizang and Qinghai, China. The wild Yak is now one of the most endangered species of the Tibetan Plateau and has been persecuted to the point that it only finds refuge in the most remote relatively human-free areas. In recent decades, construction of roads into remote areas and the availability of more sophisticated firearms and motorized vehicles have led to excessive harvest, even for markets beyond the Tibetan Plateau. The human population of the Tibetan Plateau has increased to such an extent in the past 50 years that all good grazing lands are now occupied and remnant populations of wild Yaks are seriously affected. Competition and interbreeding between wild and domestic Yaks (the latter numbering in the millions) and possible disease transmission are problematic. To address the conservation of Yaks and other Tibetan ecosystem components, the Chinese government has established an admirable network of reserves. The 300,000 km * Chang Tang Nature Reserve in northcentral Xizang is roughly the size of New Mexico ( USA) or Germany, making it the second largest reserve in the world. Important contiguous reserves to the north in Xinjiang include West Kunlun Reserve (30,000 km?*), Mid-Kunlun Reserve (32,000 km?), and Arjin Shan Reserve (45,000 km ®). Kekexili Reserve (45,000 km?) and Sanjiangyuan Reserve (150,000 km?) occur east of Chang Tang in Qinghai. This impressive network of reserves is important to the Yak, the Chiru ( Pantholops hodgsonii ), the Tibetan Gazelle ( Procapra picticaudata), and other Tibetan species. Nevertheless, remaining populations of wild Yak and others species are still threatened by human activities and difficulties with enforcement of protective laws because of limited personnel and the expansive and very remote nature of the western China.

Bibliography. Allen (1940), Bailey et al. (2002), Blanford (1888), Brantingham et al. (2007), Buchholtz & Sambraus (1990), Flad et al. (2007), Gentry et al. (2004), Harper (1945), Harris (1993, 2007), Harris & Leslie (2009), Harris & Miller (1995), Harris et al. (1999), Hedin (1934), Heptner et al. (1989), Huffman (2004f), International Commission on Zoological Nomenclature (2003), Jianlin et al. (2002), Leslie & Schaller (2008, 2009), Lydekker (1898, 1913), MacKinnon (2008), Miller (1992), Miller et al. (1994), Olsen (1986), Pal et al. (2002), de Pousargues (1898), Przewalski (1876), Rhode et al. (2007), Rockhill (1894), Schaller (1977a, 1997, 1998), Schaller & Liu (1996), Schaller, Kang et al. (2007), Schaller, Lu Zhi et al. (2005), Weigl (2005), Wiener et al. (2003).

Figures

1. Gaur (Bos gaurus), 2. Banteng (Bos javanicus), 3. Kouprey (Bos sauveli)

Distribution. Fragmented distribution in the Tibetan Plateau in China (Xinjiang, Xizang & Qinghai); one population drifts from S Xinjiang into NW India (Ladakh) and another from Qinghai to Gansu, China .