For most of the year, male and female wild yaks spend their time in separate herds. While females and young (occasionally including some young males) usually stay in large herds, males tend to either spend their time alone or in small groups. During the mating season, males leave their groups and join with the female herds. Males compete for access to receptive females, often violently. (Nowak, 1999; Buchholtz, 1990)

Mating System: polygynous

The mating season for B. grunniens starts in September, with births usually occuring in June (Buchholtz, 1990).

In the wild, the female B. grunniens have one calf every other year. Gestation is about 9 months, weaning occurs at one year, and full size is obtained in 6-8 years. At this age sexual maturity is also attained. For the domesticated yak the reproductive cycle is more varied, with the cow sometimes giving birth to more than one calf per year (Nowak, 1999).

Breeding season: In wild yak the mating season begins in September, with births occurring in June (Nowak, 1999).

Range number of offspring: 1 (low) .

Average number of offspring: 1.

Average gestation period: 9.33 months.

Range weaning age: 5 to 9 months.

Range age at sexual or reproductive maturity (female): 6 to 8 years.

Range age at sexual or reproductive maturity (male): 6 to 8 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); viviparous

Average birth mass: 18000 g.

Average gestation period: 274 days.

Average number of offspring: 1.

Most of the parental care of young is done by the female due to the nature of wild female and male B. grunniens to spend the majority of the year in separate groups (Buchholtz, 1990). Young are born able to stand and walk within several hours after birth.

Parental Investment: altricial ; female parental care