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American Bison

Bison bison

The largest existing North American land mammal, American bison are members of the mammalian order Artiodactyla, family Bovidae. Bison bulls can weigh as much as 2,000 lbs and have heavy horns and a large hump of muscle which supports their enormous head and thick skull. They have a thick mass of fur on their heads and a heavy cape of fur even in the summer. This enhances their size and protects them when fighting. 

Cows weigh about half as much as bulls. Their horns are narrower and are slightly curved. They have smaller humps and a smoother summer coat. Both sexes possess true horns which are not shed. Mating occurs in June and July, with females giving birth to a single calf after a nine-and-one-half month gestation period. Calves are born from late March through May and are a bright rust red color for the first month or so. Cows are very protective of their young and can be even more dangerous than a bull when they have a calf at their side.

Bison calves change color, and, as they grow older, become dark brown like their parents. Normal bison life span is 10 to 20 years, but a few live to be older. Two bison cows are known to have had calves at 28 years of age. In most cases, the older bulls become solitary at about 12 years of age due to their inability to out compete younger bulls.

Bison were a keystone species of the Great Plains. Their gregarious grazing habits have had significant impacts on grassland plant species, plant communities and over time helped establish the distribution of grasslands within the Great Plains.

The current bison herd is maintained at approximately 650 animals. Excess animals are sold live at public auction, which is held the fourth Thursday of every October. Since 1907, more than 10,000 animals have been removed from the refuge by sale or donation in order to preserve the high quality of Wichita grasslands. Most animals sold at auction are purchased by private bison breeders.

Current management consists of habitat monitoring, herd productivity counts to determine the number of calves born each year, roundups in preparation for the annual auction, and various studies including mitochondrial and nuclear DNA evaluations.

Bison belong to the cattle family and are subject to some of their diseases. A brucellosis testing program has proven the Wichita herd healthy and free of this viral disease. All sale animals are tested.

These animals are not tame. You are a visitor in their pasture. Do not venture afoot into open pastures close to bison. Although they may look clumsy, they are extremely agile and fast.   They can and will charge with little or no warning!  
Nuclear DNA
Bison are no longer in danger of becoming physically extinct; however they are still in danger of becoming genetically extinct through genetic drift (small, isolated populations of wildlife have a small enough gene pool that chance events can change the genetic makeup of a species). Understanding this, the refuge has taken a new approach in managing the bison herd to maximize gene diversity and minimize gene loss.

Each animal that is brought in during roundup is scanned to see if that animal is in the existing herd database. If that animal does not have a pit tag (micro chip), one will be inserted and DNA samples will pulled by way of tail hair follicles and blood drawn for disease and herd health management. These samples are sent to the lab for analysis and the resulting information is entered into the database.

Animals that are excess to refuge needs are sold at auction and will be selected from this database. Many factors such as sex ratio, age structure along with gene frequency are taken into consideration when deciding how many and what type (age and sex) of bison to excess from the herd.

Learn more: 
History of the Bison Herd 
Annual Public Auctions 

Facts About American Bison

Diet 
Herbivore

Average Lifespan 
10-20 years

Size 
Bulls = 2000 lbs 
Cows = 1000 lbs

Mating Season 

June/July
9 1/2 month gestation

Page Photo Credits — American Bison. USFWS, USFWS, Historic Bison transport / USFWS
Last Updated: Jun 28, 2012
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