A Close Call: The American Bison
A Natural History and Photos
By Eddie Mercado
I first saw an American bison as a child. My mother had taken me to visit Buffalo Bill Cody’s homestead in Scott County. Only later would I understand Cody’s complicated relationship with the American bison and the role he played in an era of commercial hunting that helped push the species toward near-extinction. That a small herd of bison was kept on the property is a striking historical irony.
From behind a large fence, I could easily see them in their enclosure. Through my young eyes, they were majestic—larger than life.
Recently, I spoke with David Weidt, herd manager of Jester Park’s Bison and Elk Exhibit in Polk County. He described a time when he misread the cues of a 1,800-pound male bison that was about to join Jester’s herd.
“I thought his cues were saying, ‘Hey, what do you have there in that bucket?’” Weidt recalled. “So naturally, I shook the bucket again to let him know I had corn in there. Turns out, his cue was saying, ‘If you shake that bucket again, I’m gonna chase you across the pasture.’ And he did. Well, I ran.”
Luckily for Weidt, the charge was a bluff. The bull broke off the chase and circled back once it became clear that Weidt was retreating.
My childhood encounter and Weidt’s close call reflect two sides of the same animal: the bison’s capacity to inspire awe from a distance and the physical power that commands respect up close. Those qualities are the products of a long evolutionary history on North America’s grasslands—and of a species whose ecological and cultural importance has endured despite an extraordinary near-collapse.
For more than 10,000 years, American bison (Bison bison) served as biological and cultural keystones of North America’s interior plains. Their history is one of evolutionary adaptation, ecological influence, catastrophic population decline, and ongoing recovery. In fewer than two decades during the late nineteenth century, market hunting, military policy, habitat conversion, and other forces reduced what had once been a population of millions to an estimated 1,000 individuals.
As the herd crosses the road at Neal Smith National Wildlife Refuge, two young bison pause for a playful sparring match.
Bison vs. Buffalo: What's the Difference?
Although often called "buffalo," true buffalo (such as the African buffalo or water buffalo) are native to Africa and Asia and lack the large shoulder hump and thick winter fur of the American bison. The name arose when early European settlers mistook bison for true buffalo.
Life Cycle
The American bison life cycle is closely tied to the seasons of the grasslands. These stages overlap, and timing varies with sex, health, nutrition, weather, and the conditions of a particular herd.
Birth and Infancy (months 0–6)
After a gestation period of about 9.5 months, calves are typically born in spring, as prairie grasses begin growing again. Newborns weigh 30 to 70 pounds and have reddish-orange coats, earning them the nickname “red dogs.” A calf can stand and walk within minutes of birth and is soon able to keep up with the herd. Calves nurse heavily during their first months but begin sampling grass within a few weeks. By two to three months, their coats begin darkening to deep brown, and small horn buds begin to appear.
Juvenile and Subadult Stage (years 1–3)
Calves usually nurse for seven to eight months and are fully weaned by the end of their first year. Juveniles remain near their mothers and within female-centered groups that include cows, calves, and young males. During these years, bison grow rapidly and develop the characteristic shoulder hump: a powerful mass of muscle supported by elongated vertebral spines. Together with the animal’s large head and strong neck, this anatomy helps bison move snow aside to reach winter forage.
Sexual Maturity and Social Divergence (years 2–5)
Cows generally begin reproducing at about two to three years of age. Under favorable conditions, an adult cow may produce one calf each year, although age, nutrition, weather, and other conditions can affect whether she calves in a given season. Adult females typically remain in female-centered social groups throughout their lives. Bulls generally reach reproductive maturity at about age three, but most do not successfully breed until they are older, larger, and better able to compete with mature males. As young bulls mature, they increasingly spend time in bachelor groups or alone, joining female-centered groups primarily during the summer breeding season.
Prime Adulthood (years 6–12)
By about age five or six, many bison have reached adult body size. Mature bulls may weigh 1,600 pounds or more, while adult females generally weigh substantially less; actual size varies by sex, age, habitat, and population. The breeding season, or rut, usually occurs from late July through August. During the rut, bulls use threat displays, bellows, wallowing, and sometimes head-to-head clashes as they compete for access to receptive cows. Dominant bulls may form “tending pairs” with receptive cows, staying close to guard them from competing males until mating occurs.
Later Life and Mortality (years 15+)
Wild bison commonly live about 12 to 15 years, though some survive to around 20; bison in captivity or closely managed settings may live as long as 25 years. As bison age, tooth wear from grazing can make it harder to process forage efficiently. In wild landscapes, calves and bison weakened by age, injury, poor nutrition, or severe winter conditions may be more vulnerable to predation. Drought and deep snow can further limit access to forage and increase mortality, particularly among calves and older animals.
Evolution and Ecological Adaptation
The American bison's dominance across North America’s interior—especially in the region that became Iowa—resulted from evolutionary adaptation, climate shifts after the last Ice Age, and active landscape management by Indigenous peoples.
Transition from Megafauna
During the Pleistocene epoch, larger ancestral species such as Bison latifrons (which possessed horns spanning up to 7 feet) and Bison antiquus inhabited North America. As the last glacial period ended roughly 10,000 to 12,000 years ago, megafauna like mammoths and ground sloths went extinct. Bison antiquus evolved into the smaller, faster, and more adaptable modern species, Bison bison.
Digestive Superiority
As a “ruminant” with a multi-chambered stomach, the bison was uniquely suited to digest low-quality, fibrous prairie grasses. Microbes in their digestive tract allowed them to extract maximum nutrients from coarse vegetation, giving them a competitive edge over other large herbivores.
The "Prairie Peninsula"
Following the retreat of the glaciers, a warmer and drier climatic period known as the Hypsithermal (roughly 8,000 to 4,000 years ago) allowed drought-tolerant grasses to expand eastward. This created a vast expanse of tallgrass prairie—often called the "Prairie Peninsula"—that stretched across modern-day Iowa, Illinois, and parts of Indiana.
Iowa’s Ideal Habitat
Prior to Euro-American settlement, over 80% of Iowa was covered in tallgrass prairie. Big bluestem, Indian grass, and switchgrass grew in nutrient-dense soil, providing an abundant food supply for grazing herds. Native American tribes across the Great Plains and the Upper Midwest routinely used fire as a land management tool. Controlled burns prevented woody shrubs and forests from encroaching on the grasslands, kept the ecosystem dominated by high-protein grasses, and cleared old dead vegetation. Bison preferentially grazed on the fresh, nutrient-rich shoots that sprouted immediately after a burn. This cyclical dynamic, where fire attracted bison and then the bison grazed, created natural firebreaks and maintained the structural stability of the tallgrass prairie for centuries.
Bison Dynamics in Pre-Settlement Iowa
Iowa served as a critical transitional zone between the shortgrass plains of the West and the wooded river valleys of the East. Herds moved fluidly across the region, spending warmer months grazing on the open tallgrass prairies of central and western Iowa, and retreating toward river corridors or wooded bluffs in the winter for shelter and water.
Keystone Species Impact
As bison moved across Iowa, their wallowing created depressions that collected rainwater, forming seasonal wetlands that supported amphibians and migratory birds. Their grazing patterns prevented any single grass species from dominating, maintaining high botanical biodiversity across the state. By the 17th and 18th centuries, an estimated 30 to 60 million bison roamed North America. This formed the largest mammalian biomass on the continent, with Iowa's rich tallgrass prairie situated right at the heart of their eastern range.
A solitary bull grazes among Indiangrass and sawtooth sunflowers at Neal Smith National Wildlife Refuge.
National Mammal of the United States
In May 2016, the National Bison Legacy Act officially designated the American bison as the National Mammal of the United States, recognizing its historical, cultural, and ecological significance alongside the bald eagle. Now it is up to us to cherish and protect this magnificent animal for future generations.
Numbers Decline
Climatic Cycles and Early Indigenous Hunting
Bison numbers were historically dynamic, fluctuating with centuries-long climate cycles. During severe multi-century droughts (e.g., 5000–2500 B.C. and A.D. 500–1300), bison virtually vanished from the Southern Plains, returning during cooler, wetter periods. Hunting also had an impact on the bison population. Archaeological sites like Ree Heights in South Dakota confirm that Indigenous hunters utilized sophisticated communal hunting strategies as early as A.D. 1180–1270 to harvest bison for food, hides, tools, and pemmican.
The Equestrian Revolution
The reintroduction of Spanish horses following the Pueblo Revolt of 1680 triggered a profound technological and cultural revolution. Indigenous groups such as the Comanche, Kiowa, Cheyenne, and Arapaho transitioned onto the High Plains, adopting mobile equestrian bison-hunting lifestyles.
Pre-Slaughter Cracks in the Ecosystem (1840s – 1860s)
Well before the arrival of commercial hide hunters in the 1870s, the bison population began experiencing cumulative environmental and human pressures:
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Severe Drought: Between 1846 and 1856, rainfall across the southern plains dropped 30% below average, severely reducing grassland carrying capacity.
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Resource Competition: Pastoral horse herds exploded, with native groups maintaining 250,000 to 500,000 domestic horses while an estimated 2 million wild mustangs competed directly with bison for grass and water holes.
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Exotic Livestock Diseases: Diseases introduced by Euro-American cattle, including anthrax, tuberculosis, and brucellosis, weakened herd reproduction rates.
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Market Hunting & Cow Selectivity: Expanding trade posts (such as Bent’s Fort) incentivized Indigenous hunters to harvest bison robes for export to St. Louis and Euro-American markets. Hunters preferentially targeted 2-to-5-year-old female cows for their tender meat and supple hides, suppressing herd replenishment.
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Dissolution of Buffer Zones: The intertribal peace agreement of 1840 between the Comanche, Kiowa, Cheyenne, and Arapaho eliminated formerly unhunted boundary zones, opening key river valleys to intensive hunting.
By 1850, native leaders reported severe bison scarcity, and Kiowa winter counts recorded "few or no bison" for four consecutive years between 1849 and 1852. Historical and ecological studies attribute this near-extinction to a multi-causal convergence involving:
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commercial market exploitation
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military and government eradication policies aimed at subjugating Indigenous nations
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habitat fragmentation by transcontinental railroads
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competition for resources with introduced cattle and horses
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period-specific environmental stresses like severe droughts
Three bison graze beneath the summer sun at Neal Smith National Wildlife Refuge,
Timeline of the Demise
By 1820, bison were effectively extirpated east of the Mississippi River. Between 1820 and the 1860’s, commercial robe trade expanded in the Northern and Southern Plains, with tens of thousands of hides processed annually. Concurrently, severe droughts beginning in the 1840s and grazing competition from large herds of domesticated horses and introduced cattle lowered the Great Plains' carrying capacity. Between 1867–1870, construction of transcontinental railroads (such as the Union Pacific) split the continent's bison into two major groups: the Southern Herd and the Northern Herd.
Collapse of the Southern Herd
From 1871–1874, the invention of industrial tanning techniques for bison hides triggered intense commercial hunting. Armed with long-range rifles, hunters decimated the southern herd in Kansas, Colorado, and Texas. Military officers and government officials actively encouraged hide hunting to undermine the subsistence base of Plains Indian tribes.
Collapse of the Northern Herd
Between 1880–1884, the Northern Pacific Railway reached Montana and the Dakotas, opening the last vast habitat to commercial hide hunters. Within four years, the Northern Herd was systematically destroyed. The last wild Sioux hunt occurred in 1882, and by 1883–1886, wild bison were virtually extinct, leaving fewer than 1000 surviving individuals in captive ranches.
In 1889, a landmark survey conducted for the Smithsonian Institution, zoologist William T. Hornaday counted only 1,091 surviving bison across all of North America, including small wild groups and private captive herds. Fewer than 100 individuals remained truly wild in the Great Plains. The loss of the bison was a transformative trauma that reshaped the very biological, cultural, and financial trajectories of most Great Plains peoples.
20th Century Recovery (1905–Present)
The founding of the American Bison Society in 1905 and the establishment of federal refuges (such as Wind Cave National Park and the National Bison Range) rescued the species from genetic extinction. Through concerted conservation efforts by tribes, governments, and private ranchers, modern populations have recovered to over 20,000 wild/protected bison (primarily on public lands like Yellowstone National Park) and over 400,000 commercial livestock bison.
The 19th-century population crash forced American bison through an extreme evolutionary bottleneck, reducing tens of millions of animals to roughly 1,000 individuals. Modern conservation geneticists and wildlife managers have to address three primary consequences of this bottleneck: loss of overall genetic diversity, inbreeding depression, and cattle gene introgression.
Three bison keep a watchful eye on the photographer at Neal Smith National Wildlife Refuge.
Modern Challenges and Management
The entirety of today’s wild and semi-managed bison population descends from fewer than 100 wild survivors and a handful of private, captive herds. This limited gene pool makes isolated herds vulnerable to reduced fertility, lower calf survival rates, and increased susceptibility to disease. Most modern conservation herds are small (under 1,000 animals) and enclosed within fenced reserves or national parks. Without physical corridors to migrate between ranges, natural gene flow is halted. Additionally, some of the cattle ranchers who saved bison from extinction crossbred them with domestic cattle (Bos taurus) in attempts to create hardier livestock. Modern genomic testing reveals that many public and private bison herds carry lingering domestic cattle DNA.
To offset these impacts, federal agencies (such as the U.S. National Park Service and U.S. Fish & Wildlife Service), tribal nations, and conservation organizations use a mix of genomic science and multi-agency coordination. Because individual fences prevent natural migration, managers must treat isolated herds across North America as a single, distributed "metapopulation." Biologists must regularly move select individuals—usually young bulls—between federal, state, and tribal herds. These deliberate translocations simulate natural gene flow and slow the rate of inbreeding depression. Today, intertribal groups like the InterTribal Buffalo Council (ITBC) work alongside federal initiatives (such as the U.S. Department of the Interior's Bison Shared Stewardship Strategy) to establish new, larger herds on extensive tribal sovereign lands. Expanding total herd sizes across millions of combined acres provides the physical space required for herds to scale beyond the threshold where random genetic drift strips diversity away.
Perched on a bison’s back, a starling takes advantage of a mobile feeding platform, searching for flies and other small pests.
Built for Speed, Survival, and the Prairie
Surprising Agility and Speed
Despite weighing up to 2,000 pounds, bison are remarkably athletic. They can sprint up to 35 mph, easily outrunning horses over short distances. Bison can also clear vertical fences up to six feet high and are strong swimmers, capable of crossing wide, fast-flowing rivers like the Missouri and Mississippi.
The Tail Communicates Mood
Bison body language is readable through their tails:
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Hanging flat down: Calm and relaxed.
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Slightly raised or switching: Alert or agitated.
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Standing straight up: Highly agitated or preparing to charge.
Wallowing Shapes Entire Ecosystems
Bison frequently roll in dry dirt or mud—a behavior called wallowing. This deters biting flies and mosquitoes, protects against sunburn, and sheds loose winter fur, which actually helps to distribute, insulate, moisten, and fertilize native seeds. Over decades, repeated wallowing creates permanent bowl-shaped depressions in the earth. These "bison wallows" catch rainwater, creating micro-wetlands that sustain rare prairie plants, tadpoles, and insects across otherwise dry plains.
Built for Extreme Cold
Rather than using their hooves to dig through winter snow, bison use sweeping side-to-side motions of their massive, muscular heads to clear heavy snowdrifts and reach buried grass. Their fur coat is so dense that snow can sit directly on a bison’s back without melting, because virtually no body heat escapes through the hide.
At Neal Smith National Wildlife Refuge, towering Indiangrass and big bluestem rise above a small group of bison—an echo of the tallgrass prairie that once stretched across much of Iowa and the Midwest.
Public and Private Herds in Iowa
Public
Broken Kettle Grasslands Preserve, Westfield, IA
Neal Smith National Wildlife Refuge, Prairie City, IA
Jester Park Nature Center, Granger, IA
Fontana Park, Hazleton, IA
Buffalo Bill Cody Homestead, Princeton, IA
Private
Meskwaki Nation Bison Herd, Tama County, IA
Hawkeye Buffalo & Cattle Ranch, Fredericksburg, IA
Heritage Bison Ranch, Hinton, IA
Tall Grass Bison, Promise City, IA
Native Prairie Bison, Story City, IA
Sunset Hills Bison Ranch, Malcolm, IA
Dreesman Buffalo Ranch, Tama, IA
Timber Ridge Bison, Waverly, IA
Bare Bison Farm, Van Meter, IA
Quarry Creek Elk & Bison Co., Fort Madison, IA
Eddie Mercado is an Iowa teacher with certifications in K-12 Art, 5-12 Biology, and K-12 TAG. He received a BS in Biology, a BFA in Photography, and a MS in STEM Education, all from the University of Iowa. Go Hawks! He is the co-founder of Iowa Natives.
Works Cited
"Ancient buffalo DNA lays out a roadmap to rehabilitating healthy herds." Anthropocene Magazine, 19 Aug. 2026. Gale Academic OneFile, link.gale.com/apps/doc/A899826001/GPS?u=eicc&sid=bookmark-GPS&xid=84c46467. Accessed 4 Oct. 2026.
Bison Shared Stewardship Coauthor Team. "Bison Shared Stewardship Strategy." DEPARTMENT OF THE INTERIOR, Oct. 2024, www.nps.gov/subjects/bison/upload/bison_stewardship_strategy_508.pdf.
Feir, D. L., Gillezeau, R., & Jones, M. E. C. (2024). The slaughter of the bison and reversal of fortunes on the Great Plains. Review of Economic Studies, 91(3), 1634–1670. https://doi.org/10.1093/restud/rdad060
"Interior Department Strengthens Conservation of American Bison." FWS.gov, 1 Nov. 2024, www.fws.gov/press-release/2024-11/interior-department-strengthens-conservation-americanbison#:~:text=The%20order%20formally%20established%20a%20Bison%20Working,restoration%2C%20including
%20strengthening%20long%2Dterm%20bison%20conservation%20partnerships.
Nixon, L. (2024). What the Bones Say: Ree Height “kill sites” reveal plains history. South Dakota Magazine, 40(3), 82–86, https://southdakotamagazine.com/2025/11/03/what-the-bones-say/.
Oppenheimer, Jonas, et al. "Paleogenomic insight into the collapse, recovery, and management of American bison." Science, 6 Aug. 2026, www.science.org/doi/10.1126/science.aee4205.
Weidt, David. Phone Interview. 10 Sept. 2026.