The continuity claim
What makes the Yellowstone herd different from every other American bison herd alive today is genealogical continuity. By the late 1880s, the wild bison population across the Great Plains had been hunted to fewer than 1,000 animals. The pockets of survivors were small: a remnant herd in northern Alberta, the private Pablo-Allard herd in Montana, the Goodnight herd in Texas, and the Yellowstone herd of roughly 25 animals in the Lamar Valley. All other herds alive today descend in some way from captive-bred founders that were re-released to the wild from zoos and private preserves in the early 20th century.
The Yellowstone herd did not go through this captive-rebuilding cycle. The 25-or-so animals that survived the 1880s collapse remained wild, in the park's interior valleys, without any period of containment in zoos or private holding facilities. The herd was augmented in the early 20th century with animals from the Pablo-Allard line shipped in by Yellowstone superintendent Major John Pitcher and others, but those additions mixed with the surviving wild animals rather than replacing them. Modern genetic studies of the Yellowstone herd confirm that the wild founder genome is well represented in the current population.
The continuity matters for two reasons. First, the herd has preserved the cattle-introgression-free genome that most other conservation herds lack. Many non-Yellowstone conservation herds carry detectable cattle gene introgression from the early 20th century hybridisation experiments by Charles Goodnight and others. Yellowstone, Wind Cave, and a handful of other strict-management herds have been shown to be largely free of this introgression. Second, the continuity preserves wild behavioural and ecological adaptations that depend on uninterrupted residence in a real ecosystem: migration, predator response, wallow site use, and the full annual cycle of habitat use across thousands of square kilometres.
The current census
The Yellowstone bison herd is censused twice a year, in summer and again before winter, by aerial survey conducted by the National Park Service. The summer count picks up the year's new calves. The pre-winter count records the population that will overwinter in the park.
The 2024-2025 winter census recorded approximately 5,400 animals. The herd has fluctuated between roughly 3,500 and 5,900 over the past two decades. Variation reflects calf production (good in wet years, lower in drought years), winter mortality (modest in mild winters, higher in deep-snow winters that limit forage access), management harvest (the seasonal shipment to slaughter and the Tribal treaty hunt), and predation (wolves and grizzlies take some calves and weakened adults each year). The National Park Service's stated target population is in the range of 3,500 to 5,000 animals, with the upper number reflecting the park's estimated ecological carrying capacity.
The herd is divided into two largely distinct sub-populations: the Northern Range herd, which winters in the Lamar Valley and the lower-elevation areas around Gardiner, and the Central Range herd, which winters in the Hayden Valley and the geothermal areas around the central park. The two sub-populations mix during the summer rut but remain demographically separable on the winter range. Each sub-population is censused separately in the NPS reports.
Brucellosis and the Interagency Bison Management Plan
Brucellosis, a bacterial disease caused by Brucella abortus, is the central management issue for the Yellowstone bison herd. The disease is present in approximately 60 per cent of Yellowstone bison and can in principle be transmitted to cattle, where it causes abortions and where federal disease-management protocols require quarantine and herd-wide testing. Montana's status as a brucellosis-free state for cattle is commercially important to its cattle industry, and any case of brucellosis transmission to Montana cattle would trigger costly state-wide response measures.
The Interagency Bison Management Plan (IBMP), in effect since 2000, manages the interaction between the Yellowstone herd and the Montana state boundary. The plan involves the National Park Service, the Animal and Plant Health Inspection Service, the US Forest Service, the Montana Department of Livestock, the Montana Department of Fish, Wildlife and Parks, and several Tribal nations. Its principal tools are hazing (riding or otherwise moving bison back into the park boundary when they wander north into Montana), seasonal slaughter (live capture and shipment to slaughter facilities when the population exceeds management targets), Tribal treaty hunts (state-coordinated hunting on US Forest Service land adjacent to the park), and the more recent live-transfer programme to Tribal conservation herds.
The hazing component of the plan is contentious. Critics argue that the brucellosis transmission risk from bison to cattle is overstated (there is no documented case of bison-to-cattle transmission in the wild, with all known transmissions running the other way, from elk to cattle), that the cattle industry's interest does not justify the constraint on the wild bison population, and that the bison are being managed for cattle industry preference rather than for the ecological role of the species. The counterargument from the cattle industry and from Montana state officials is that even a low-probability transmission would carry high economic cost and that the IBMP framework is a reasonable balance of competing interests. The debate continues to be litigated in federal court and politically negotiated through periodic IBMP revisions.
Tribal treaty rights and the bison hunt
Eight Tribes hold treaty-protected rights to hunt bison on the unoccupied federal land adjacent to Yellowstone. The principal Tribes are the Confederated Salish and Kootenai Tribes of the Flathead Reservation, the Nez Perce Tribe, the Shoshone-Bannock Tribes of Fort Hall, the Confederated Tribes of the Umatilla, the Crow Tribe, the Northern Arapaho Tribe, the Yakama Nation, and the Eastern Shoshone Tribe. The treaties were negotiated in the mid-19th century and explicitly reserved hunting rights on "unoccupied" federal land outside the reservation boundaries.
The annual Tribal hunt typically takes several hundred bison per year, with the exact total dependent on the year's management framework and on Tribal participation. The hunt occurs in the winter months when bison move north out of the park onto the Custer Gallatin National Forest. The animals taken are processed and distributed within the participating Tribes; the meat, hides, and other materials support traditional cultural use and food security. The hunt is one of the most significant modern exercises of treaty-protected hunting rights and is a visible reaffirmation of the rights that the treaties guaranteed and that were largely dormant during the collapse and recovery of the species.
The Tribal transfer programme
Since the 2010s, Yellowstone has progressively expanded a programme of live bison transfer to Tribal conservation herds. The mechanism is the Bison Conservation Transfer Program, which puts brucellosis-tested negative animals through a multi-year quarantine to confirm freedom from disease and then transfers them to participating Tribes for establishment or expansion of their own bison herds. The InterTribal Buffalo Council, a coalition of more than 80 Tribal nations engaged in bison restoration, is the primary partner organisation.
Several thousand animals have been transferred under the programme since its expansion in the late 2010s. Recipient Tribes have used the animals to establish or grow conservation herds on Tribal land across the Plains and the northern Rockies. The genetic and cultural significance of the programme is substantial: the Yellowstone genome, with its continuity from the wild pre-collapse population, is being re-introduced to Tribal land that historically held wild bison and that has not held them in significant numbers for more than 130 years.
Visiting the herd
Yellowstone is the most accessible place in the world to see wild American bison. The Hayden Valley and the Lamar Valley both hold large herds that are visible from the road throughout the summer and into the autumn. Winter access to the park interior is restricted to snowcoach and ski tours, with reliable bison viewing in the geothermal areas around the central park where snow cover is lighter.
Peak rut viewing is late July and August (see the bison rut behaviour page). Peak calving viewing is May and early June. Winter wolf-bison interactions, including occasional kills, are documented from the Lamar Valley road by careful observers with spotting scopes. The 25-yard minimum distance rule is strictly enforced and is the most frequently violated park rule; bison-related visitor injuries occur annually and are nearly all the result of visitors approaching too close for photographs.
Yellowstone in the broader recovery picture
Yellowstone is one of approximately 60 conservation bison herds across North America, but it is unique in size, genetic continuity, and policy significance. The approximately 5,400 Yellowstone bison are roughly one-sixth of the total US conservation herd. The park is the de facto source population for the genetic re-introduction underway across the Tribal restoration network, and the brucellosis management framework constrains how that source can be used. Decisions about the Yellowstone herd ripple across the entire North American bison conservation network.
The next decade is likely to see continued expansion of the Tribal transfer programme, continuing debate over the brucellosis framework, and probably some adjustment to the IBMP target population as the park's ecological carrying capacity is re-evaluated. The herd is more secure than it was twenty years ago by most measures: the population is large, the calf production is consistent, the genetic base is well documented, and the political support for the species is broader. The constraints are also clearer than they were: the park boundary is the boundary, the brucellosis policy will not change quickly, and the management harvest will continue to be a significant fraction of annual mortality.
Frequently asked questions
What time of year is best to see Yellowstone bison?
The herd is present and visible year-round. May and June for calving and the red dog calves. Late July and August for the rut, with peak activity and the largest visible aggregations. September and October for autumn colour and slightly thinner visitor crowds. Winter via snowcoach for the most atmospheric viewing in the geothermal areas, where bison cluster near warm ground.
Why don't more parks have herds like Yellowstone?
Because no other US national park has the combination of ecological scale, the century-plus of continuous protection, and the historical accident of the small surviving wild herd that became the foundation of the modern population. Wind Cave, Theodore Roosevelt, the National Bison Range, and several others maintain substantial herds but none has the continuity claim that Yellowstone holds.
What happens to Yellowstone bison that wander outside the park?
Under the current IBMP framework: hazing back into the park where possible, capture and shipment to slaughter when capacity is exceeded, eligible for Tribal treaty hunting on adjacent US Forest Service land, and increasingly available for transfer to Tribal conservation herds after quarantine. The exact disposition depends on the time of year, the animal's health status, and the herd's current management target.
Is the brucellosis policy going to change?
Slowly if at all. The cattle industry's economic interest in the brucellosis-free state designation is well organised and has held the policy framework largely in place for two decades. Litigation has produced incremental changes (more transfer to Tribes, more tolerance for bison movement onto adjacent federal land in winter) but not fundamental restructuring. The underlying disease ecology (the documented absence of bison-to-cattle transmission, the ongoing elk-to-cattle transmission) is increasingly hard to reconcile with the bison-focused management approach, and that tension is likely to drive further change over the next decade.