Yak Bos grunniens
Definition
The yak is the bovine of the cold high plateau: more than 17.6 million animals, concentrated on the Qinghai-Tibetan Plateau and adjacent highlands above 3000 m, where the annual mean temperature sits below 5 C, the hottest month does not exceed 13 C and the animals tolerate -40 C. Herders take milk, meat, hair, hide and manure from them and also use them for draught, pack and riding. The adaptations are gross anatomical and easy to remember: lungs of 3.7 kg against 1.8 kg in local cattle, hearts of 1.3 against 0.87 kg, higher erythrocyte and haemoglobin concentrations, an unusually large rumen that lets them live on coarse forage, and a coat of long outer hair over dense down. The clinical corollary is that yaks cannot shed heat. Functional sweat glands are few and confined to the muzzle, so respiration climbs from 25 breaths a minute at 5 C to 49 at 10 C and 80 at 28 C, against a stable body temperature of 37.6 C in the morning and 38.5 C in the evening. Reproduction is seasonal and unhurried: first oestrus at 13 to 30 months but first mating usually at three or four years, a cycle of about 20 days confined to the warm months, gestation around 258 days, and most cows calving only once in two years.
Vital facts
- Age at first breeding
- 36 – 48 months sources disagree
- Gestation
- 250 – 260 days
First oestrus occurs at 13-30 months, but the source states yak are normally not mated until three years old and often not until four.
PURE-BRED yak calf carried by a yak cow. Both endpoints come from ONE study (Lei Huanzhang et al. 1964, 36 yak, Datong Yak Farm) and are the means for the two calf sexes: 260 days for male calves, 250 days for female calves - they are not stitched from different papers. FAO's own headline for the species sits inside this: 'Gestation length on average is around 258 days, shorter than in other cattle.' Other averages compiled in the same chapter: Denisov 1938 258; Dubrovin 1992 258 (800 cows, Caucasus); Katzina and Maturova 1989 259 (Tuva); Yu et al. 1993 254 (SD 2.7, Gansu); Joshi et al. 1994 258 (Nepal). Individual-animal extremes of 226-283 days appear in the literature, but the authors warn these ranges 'provide no clue to whether the extreme values are isolated cases and might include premature parturition', so they are not stored as the constant. CLINICALLY IMPORTANT AND SEPARATE: a yak cow carrying an F1 hybrid calf (mated to a bull of another cattle species) carries about 20 days longer - around 270 days (273.2 +/- 12.7 male, 268.6 +/- 10.2 female; Cai Li 1989a), and at Datong 277 +/- 6.2 days for male Hereford-sired hybrids - which is what drives dystocia.
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