The Bison of Yellowstone National Park
NPS Scientific Monograph No. 1
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Internal parasites

YELLOWSTONE BISON appear comparatively free from internal parasitism. During the present study, parasite investigation centered on the occurrence of lungworm (Dictyocaulus sp.), as reported by Meagher (1966). Rate of infection for 185 animals examined was 11.4%. Aged animals, with an incidence of 35%, showed the highest rate of infection, contrary to the situation in cattle, where calves show both higher incidence and intensity of infections. Dictyocaulus sp. has been reported from a few animals in Wood Buffalo National Park (Fuller 1961), from the National Bison Range (Locker 1953), from Elk Island National Park (Conner and Cornell 1958), and from a fenced herd in Kansas (Frick 1951).

No other internal parasites were found in the brief examinations made of bison during the present study. Locker (1953) recorded only tapeworms (Moniezia benedeni) in Yellowstone bison, occurring randomly among all age groups, at an apparently low intensity of infection. A variety of internal parasites which have been recorded from other bison herds have not been found here.

External parasites

External parasites were not collected from animals during the study period. Burger (1967a, 1967b, 1969 pers. comm.), who studied biting insects in Yellowstone during 1966 and 1967, reported that mosquitoes (Aedes sp.), blackflies (or buffalo gnats), particularly Simulium venustum, horse and deer flies (Tabanidae), snipe flies (Rhagionidae), and members of the Muscidae may all have had some association with bison. The last two families were most important.

The genus Symphoromyia of the Rhagionidae apparently influenced bison distribution during the summer months (see Summer Range Movements). These small gray flies cling tenaciously, inflict a painful bite, and locally may be very abundant. During the summer, bison are nearly naked, as the newly growing hair is very short, resembling fine, black plush. They are particularly vulnerable to biting insects.

Three genera of Muscidae were associated with bison in Yellowstone. The stable fly (Stomoxys calcitrans) and the horn fly (Haematobia irritans) were observed feeding on bison with no noticeable effect. The face fly (Musca autumnalis) was first collected by Burger in 1967. He suggested that this exotic species, if it becomes established in Yellowstone, could cause eye disorders in bison. He noted evidence of severe conjunctivitis associated with the presence of this fly among bison at the National Bison Range.


Disease-caused mortality was not identified in the present wild bison population, although outbreaks of hemorrhagic septicemia in 1912, 1919, and 1922 caused considerable mortality in the introduced herd in Lamar Valley. In two instances during the study, young animals died from causes which were not apparent when ample food was available. One was a yearling female, observed by park personnel for some time, unable to keep up with the herd animals, moving very little, and becoming gradually weaker. The other was a young cow, seen for several days at Old Faithful before she died. Both carcasses were nearly consumed by scavengers before examination was possible, but the fat-depleted bone marrow in both cases indicated very malnourished animals. Neither tuberculosis nor anthrax, which have been important causes of mortality in Wood Buffalo National Park (Fuller 1961; Choquette et al., 1966), have ever been detected in Yellowstone.

Brucellosis (Bang's disease, undulant fever in humans), caused by the bacterium Brucella abortus, occurs in the present bison population. Whether the organism was introduced or was endemic among North American bovids is not known; it was first tested for and reported in Yellowstone in 1917. The rate of infection has varied considerably among tests made in different years during reduction operations, and also among the wintering populations of a given year. In 1964-65, 129 animals tested in Lamar, 33 tested in Pelican, and 302 tested at the Nez Perce Creek trap showed rates of 59, 42, and 28%, respectively (Barmore 1968).

Evidence suggests that brucellosis has little effect on the Yellowstone bison. Limited examination of reactors slaughtered during the study period indicated normal pregnancies. Rate of pregnancy was apparently not influenced, as discussed previously. Veterinarians who investigated brucellosis in Yellowstone before the study period agreed that there were no apparent effects on the population (Tunnicliff and Marsh 1935; Skinner 1941; Quortrup 1945). Quortrup mentioned that few abortions were observed and gross lesions were rarely seen at postmortem examinations. Dave Pierson, Buffalo Herder and Animal Keeper over a period of 30 years, believed that observed abortions occurred as a result of the handling of pregnant females in chutes, and their confinement in pens during the reductions held at the Buffalo Ranch (1968 pers. comm.). Quortrup believed that brucellosis had probably existed in the Yellowstone bison for a long time, and that they had acquired a natural immunity.

Investigations among bison have apparently concentrated on the incidence of brucellosis rather than its effects. Most bison herds in the United States are maintained in a brucellosis-free condition as part of the U.S. Department of Agriculture brucellosis control program. In Wood Buffalo National Park in Canada, where the presence of brucellosis was confirmed in 1956, Fuller (1962) considered it a possible influence on conception rate. Choquette et a!. (1966) also assumed that brucellosis influenced productivity. Among cattle, effects include abortion of calves, temporary sterility, and lowered milk production (Gilman and McAuliff 1956).

Further studies of brucellosis in bison may indicate that mutual adaptation or equilibrium exists, as between parasites and hosts that have long lived together (Allee et al. 1949). Physiological effects of brucellosis, if any, may contribute to maintenance of the bison population within levels which the habitat can support.

Brucellosis is of economic concern to cattlemen, and of health importance to the general public. It is presumed that bison can transmit brucellosis to cattle, because the causative organism is apparently the same in both species of bovids. Transmission tests have not been made to verify this, but on the basis of the assumption, the National Park Service has cooperated with the Department of Agriculture in brucellosis control among bison. In Yellowstone, cooperation has consisted of vaccination of calves and removal of reactors during reductions (held primarily to cut herd numbers). This cooperation resulted in reduction of animal numbers below the park's management objective at Lamar in 1964-65. No reductions have been held specifically for brucellosis control in Yellowstone.

Participation in brucellosis control in Yellowstone National Park has recently been reevaluated by the National Park Service (Barmore 1968). Present bison management objectives are to maintain a wild population under natural conditions. By order of the Superintendent of Yellowstone National Park, future reductions will be held only when research clearly indicates that compensation for a lack in natural controls is necessary. As an alternative to brucellosis control within the park, the objectives of the Department of Agriculture control program can be met by preventing contact between park bison and domestic livestock beyond the park boundaries (see Movements Beyond Park Boundaries).


There was no direct evidence for predation on the present herd. Circumstances suggested a grizzly kill of an apparently healthy, mature bull in mid-summer of 1967. Remains of bone and hair indicated the bull died at the edge of a small group of trees in a meadow, south of Hayden Valley. The lack of broken bones, the relatively young age of the animal, and the site of death all suggested a grizzly (Ursus horribiis) might have killed the bull. McHugh (1958) also mentioned indirect evidence of grizzly predation. Dave Pierson (1968 pers. comm.) believed both black (Ursus americanus) and grizzly bears would take calves in the spring, but calf counts during the study do not indicate much loss. However, such loss might have occurred at or immediately after birth, before the calves were observed with the cows.

Wolves (Canis lupus), which are known to prey on bison in Wood Buffalo National Park according to Fuller (1961), are the only other likely predators of bison in Yellowstone. Studies such as that made by Mech (1966) have indicated that effective wolf predation on large mammals results from the cooperative efforts of a pack. During the study period wolves were rare in Yellowstone, with no pack activity.

The observed survival, for a long time, of handicapped or weakened bison provided additional evidence that almost no predation was occurring. In one instance, a cow survived at least 2 years with a useless right foreleg. Although her mobility was impaired, she was usually with other bison, at times a mixed herd, more often with one or more bulls. Only once was she seen unaccompanied, except for her calf of that spring. Solitary animals were also usually free from predation. Old bulls, obviously weak, were often observed for several winter months in nearly the same location along the road in Lamar, until finally they died.

Historical information suggests that even during the early years of the park, when predators, particularly wolves, were more numerous, healthy bison of all ages were relatively safe from attack. Although predation becomes more difficult as the prey species decreases (MacArthur and Connell 1966), the predators in this case had alternate prey (elk) to sustain their numbers as bison decreased (from poaching). If bison had been easy or preferred prey, their increase, once they were protected from poaching, might have been prevented.

Other Causes

Fig. 25. Trench left in soft snow by traveling bison.

Winterkill, probably from the combined effects of climatic stress, forage availability, and physiological condition of individual animals, was the main cause of observed mortality (see Other Mortality). Death usually occurred after prolonged weakening, often in late winter (March and early April). A few animals died annually, but the number increased greatly with severe winters. This suggested that climatic influences (long winters, periods of prolonged cold, deep and sometimes crusted snow) acting directly and indirectly on the bison were the most important mortality factors.

The most apparent direct effect of the winter environment, above the energy levels required to maintain body temperature, was the effort required to travel between foraging areas. In deep snow the mixed herd groups usually traveled in line, plunging to create trenches several feet deep (Fig. 25), frequently for more than a mile. Variations in topography crossed added to their efforts.

Observations during the mild and average winters of the study period indicated that snow depth did not limit forage availability. Bison commonly cleared fairly deep snow (by swinging the head in a sideways motion) with apparent ease (Figs. 26, 27). McHugh (1958) observed bison feeding in snow up to 4 feet deep. At higher population levels, snow depth may limit access to forage. However, the size and location of the most used foraging areas suggested that this might happen only under the most extreme conditions (see Use of Forage Areas).

Susceptibility to winterkill varied according to physical condition, as would be expected. Aged animals found dead in mild and average winters indicated that they were most susceptible. Crippled and otherwise handicapped animals, and those weaker because of size, would also be affected more quickly. Differential and total mortality among subadults, as discussed previously, suggested that among otherwise healthy bison of all ages they were most vulnerable. Size, together with social standing in the population (see Suggested Mechanisms of Population Regulation), may explain this.

bison grass
Fig. 26 Bull bison foraging in snow approximately 2.5 feet deep. Fig. 27. Feeding site in snow approximately 2 feet deep.

A quick-acting type of winterkill occurred once during the study period, but old bison bones indicated similar occurrences in the past. During the severe winter of 1964-65, a group of 18 bison (including 3 calves, 2 mature and 1 spike bull, 7 cows of various ages, and 5 unidentified animals) died before the end of January at a small warm-slough area in the Bechler Meadows. The snow there, west of the Continental Divide, was too deep that winter to allow the animals to move away from the slough area, and presumably they starved to death when the small amount of feed was gone. The slough area apparently becomes a unique trap in severe winters, since 13 other bison survived in Bechler that year, near the slough but along the river banks where movement was possible in the open water.

Accidents accounted for the deaths of a few individuals of all ages nearly every year, as indicated by ranger reports. Most commonly animals drowned in bog holes or fell into hot pools. The accidental death of whole groups while crossing thin river ice has been reported twice. At Slough Creek in 1941, seven yearlings and 2-year-olds died. In 1946, near the Mud Volcano, in the Yellowstone River, 39 died (including 5 calves, 9 yearlings, 6 two-year-olds, 2 mature bulls, 16 mature cows, and 1 unidentified animal).

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Last Updated: 24-Jan-2005