American bison are sexually dimorphic (there are two distinct gender forms). The male bull is larger than the female cow and has a massive head and shoulders. Bison have long, shaggy, coarse hair on their head, shoulders, and front legs. The hair on the back half is significantly shorter, becomes thick in winter, and is shed in spring and summer. Their coloration is shades of reddish-brown to black. Newborn calves are bright reddish-brown. They develop a chocolate-brown to black coat during maturity. The coat fades to a lighter tan with age. The pelt often turns a light yellow-brown in spring. Both males and females have horns that are relatively short and thick at the base. The horns are hollow and permanent, and they taper and curve to a sharp point. Horns begin to grow at two months of age on both sexes; males horns are larger. Both sexes have a distinctive hump that makes raising their head above shoulder level impossible; calves have no hump.
 

Association of Zoos & Aquariums (AZA) SAFE North American Bison Program 

Bison used to freely roam throughout North America plains until colonization and intentional killings nearly wiped them out entirely. There are now an estimated 20,500 bison in conservation herds and 400,000 in commercial herds. 

Brookfield Zoo Chicago participates in the AZA SAFE North American Bison Program to help identify survival threats, support recovery, and offer you opportunities to learn more about at-risk species. 
 

Threats 

• • Ongoing loss of genetic diversity 

• • Habitat loss 

How We Care 

• • Brookfield Zoo Chicago cares for four bison (two male, two female). 

• • The species has been a symbol of the Zoo since its opening in 1934 and has resided here since 1940! 

• • The Zoo helps promote and build understanding of the needed conservation measures to keep both wild and zoo bison populations sustainable and healthy.