American Bison (Bison bison) Fact Sheet: Reproduction & Development

• June - September for breeding season
  • Males, both solitary and ones from male groups, join females in breeding season (Buchholtz & Sambraus 1990).
•  Males are in rut approximately four weeks July-August (Mooring et al 2006)
• A female about to be receptive may run through the herd, attracting the attention of males who compete for rights to tend (Lott 2002)
• Female estrous cycle is 21 days; estrus lasts 9-28 hours (Nowak 1999)
• Cows may select a suitor that is not the highest ranking male. (Buchholtz & Sambraus 1990).
• Courting bulls are not very aggressive towards females. (Buchholtz & Sambraus 1990).

•  Dominant bulls tend (stay with) females that are in estrus.
  • Less experienced males also may tend females, but the females may not be in estrus.(Komers et al 1994)
  • Tending bonds may may exist a few seconds or up to a few days and may or may not result in insemination. (McDonald 1981)
• Copulation occurs about 3 days after rut begins and may be repeated up to four times in a single day. (Buchholtz & Sambraus 1990).

Gestation

• 285 days (Haugen, 1974) but may vary for pregnant cows in good condition in the wild in order to allow calves to be born at the same time as other calves in the herd. ( Berger 1992)

Birth

• Calves are precocial - can walk 10 minutes after birth
• 99% of all births are of a single calf
• Calves weigh 15 - 25 kg (33- 55 lb)
• At birth bison have all their deciduous teeth in place 

Infant (< 1 year old)

• Nursing continues for 7 to 9 months (Rutberg 1986)
• Bull calves are weaned at about 6 months. (Lott 2002)

Juvenile

• Both sexes generally reach sexual maturity between 2 - 4 years. (Nowak 1999)
• Females can conceive as yearlings; more commonly at 2 to 4 years

Adult  

• Physical maturity for males around 6 years; for females 3 years. (Nowak 1999)

Wild populations

• Typically about 20 years (Nowak 1990)

Managed care

• No AZA estimates available

• Wolves commonly prey on calves.
• Humans
• Harsh winter weather commonly kills many bison.

Image location: Custer State Park, South Dakota.

Image credit: © Lee Kinkade. Made publically available via Wikimedia under Creative Commons SA license 2.5.

Berger (1992)
Buchholtz & Sambraus (1990)
Rutberg (1986)
Komers et al. (1994)
Lott (2002)
Meagher (1986)
McDonald (1981)
Mooring et al. (2006)
Nowak (1999)
Weigl (2005)