Female bison live in maternal herds which include other females and their offspring.
Male offspring leave their maternal herd when around three years old and either live alone or join other males in bachelor herds.
Male and female herds usually do not mingle until the breeding season, which can occur from July through September.
However, female herds may also contain a few older males.
During the breeding season, dominant bulls maintain a small harem of females for mating.
Individual bulls "tend" cows until allowed to mate, by following them around and chasing away rival males.
The tending bull shields the female's vision with his body so she will not see any other challenging males.
A challenging bull may bellow or roar to get a female's attention and the tending bull has to bellow/roar back.
The most dominant bulls mate in the first 2–3 weeks of the season.
More subordinate bulls mate with any remaining estrous cow that has not mated yet.
Male bison play no part in raising the young.

Bison herds have dominance hierarchies that exist for both males and females.
A bison's dominance is related to its birth date.
Bison born earlier in the breeding season are more likely to be larger and more dominant as adults.
Thus, bison are able to pass on their dominance to their offspring as dominant bison breed earlier in the season.
In addition to dominance, the older bison of a generation also have a higher fertility rate than the younger ones.

Bison mate in August and September; gestation is 285 days.
A single reddish-brown calf nurses until the next calf is born.
If the cow is not pregnant, a calf will nurse for 18 months.
Cows nurse their calves for at least 7 or 8 months, but most calves seem to be weaned before the end of their first year.
At three years of age, bison cows are mature enough to produce a calf.
Bison have a life expectancy around 15 years in the wild and up to 25 years in captivity.

Bison have been observed to display homosexual behaviors, males much more so than females.
In the case of males, it is unlikely to be related to dominance, but rather to social bonding or gaining sexual experience.
Female bison live in maternal herds which include other females and their offspring. Male offspring leave their maternal herd when around three years old and either live alone or join other males in bachelor herds. Male and female herds usually do not mingle until the breeding season, which can occur from July through September. However, female herds may also contain a few older males. During the breeding season, dominant bulls maintain a small harem of females for mating. Individual bulls "tend" cows until allowed to mate, by following them around and chasing away rival males. The tending bull shields the female's vision with his body so she will not see any other challenging males. A challenging bull may bellow or roar to get a female's attention and the tending bull has to bellow/roar back. The most dominant bulls mate in the first 2–3 weeks of the season. More subordinate bulls mate with any remaining estrous cow that has not mated yet. Male bison play no part in raising the young.
Bison herds have dominance hierarchies that exist for both males and females. A bison's dominance is related to its birth date. Bison born earlier in the breeding season are more likely to be larger and more dominant as adults. Thus, bison are able to pass on their dominance to their offspring as dominant bison breed earlier in the season. In addition to dominance, the older bison of a generation also have a higher fertility rate than the younger ones.
Bison mate in August and September; gestation is 285 days. A single reddish-brown calf nurses until the next calf is born. If the cow is not pregnant, a calf will nurse for 18 months. Cows nurse their calves for at least 7 or 8 months, but most calves seem to be weaned before the end of their first year. At three years of age, bison cows are mature enough to produce a calf. Bison have a life expectancy around 15 years in the wild and up to 25 years in captivity.
Bison have been observed to display homosexual behaviors, males much more so than females. In the case of males, it is unlikely to be related to dominance, but rather to social bonding or gaining sexual experience.
American bison