BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Bison bison | Bison 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Breeding season - Female bison are seasonally polyestrous, with a cycle
of approximately 3 weeks' duration [2,11].  However, unseasonal estrus
and mating sometimes occur [2,11,51].  The breeding season for bison
generally occurs between July and October [2,11,52].  However, it varies
in length depending on herd location.  The breeding season has been
observed to last from June 15 to September 30 at Hayden Valley,
Yellowstone National Park [51]; from mid-July to mid-August in other
areas of Yellowstone National Park [52]; and from June 1 to July 30 at
the Wichita Mountains Wildlife Refuge [35].

Age at sexual maturity - Sexual maturity most commonly occurs at 2 to 4
years of age.  However, some females breed as yearlings [11,53].  In
Wood Buffalo National Park, a small proportion of the yearling bulls,
approximately one-third of the 2-year-olds, and virtually all bison 3
years of age and older were sexually mature [11].  Bulls attain sexual
maturity well in advance of becoming part of the active breeding
population [52].  In the Hayden Valley bison herd, Yellowstone National
Park, bulls 8 years old and older were the most active sexually [51].

Gestation and calving season - The gestation period for bison is 9 to
9.5 months [11,51,53].  In most regions, the calving season is generally
from mid-April to May with births concentrated from the end of April
through the first 2 weeks of May [53].  However, conception and,
therefore, parturition can occur at any time of year [11].  McHugh [51]
noted that a few calves were born from June through October in herds at
Yellowstone National Park, the Crow Reservation, Montana, the National
Bison Range, and Wind Caves National Park.

Number of young - Females generally give birth to one calf; twins are
rare [11,53].  Bison generally produce two calves every 3 years [11].

Development of young - Calves are able to stand and nurse within 30
minutes of birth.  They may try to graze by 5 days of age and will drink
water after the first week.  The cow nurses her calf for at least 7 to 8
months; most calves are weaned by the end of the first year [53].

Longevity - In the wild few bison survive more than 20 years, although
there are records of cows surviving at least 40 years [53].  In wild
populations, by the time a bison has reached age 15 it can be considered
old.  In captivity, lifespan increases [11].


PREFERRED HABITAT : 
The primary habitat requirements for bison are adequate forage, water,
and space [8].  Bison thrive in open grasslands, meadows, and parklands.
When available, bison use forested areas for shade and to escape
insects.  These areas also furnish forage when open areas are covered by
"deep" snow [8].  However, bison can survive in open valleys covered
with several feet of snow [94].  During severe weather, forested areas
and some topographical features furnish cover [8].  Scattered thermal
sites (particularly warm ground with less snow than the surrounding
area) provide favorable sites for wintering bison in Yellowstone
National Park [94].  In northern Canada forested and shrub covered
areas are often used as daily and seasonal travel corridors [11].

Most bison are seasonally migratory; movements are both directional and
altitudinal in some regions [11,52,70].  During historical times, large
herds of bison commonly moved southward 200 miles (322 km) or more to
winter range [2].  Directional and altitudinal movements between summer
and winter ranges still occur annually at Wood Buffalo National Park and
Yellowstone National Park [2,11,52,94].  During November and May, bison
at Wood Buffalo National Park migrate from wooded hills to the Peace
River Valley, a distance of as much as 150 miles (241 km) [2].  Factors
that may influence seasonal migrations include tradition, supply and
accessibility of forage, open water, shelter, insect harassment, spring
weather conditions and temperatures, and fall snowstorms at higher
elevations [53,58].

In mountainous areas, altitudinal movements to lowland winter range in
fall and to higher summer range in spring are quite common.  Snipe flies
(Symphoromyia spp.) may be responsible for some altitudinal movements by
Yellowstone National Park bison herds during the summer [52].  Large,
windswept prairies may also be chosen in summer for relief from
insects.  Bison, particularly cows, show strong affinity to traditional
winter range [52,70].  Shaw and Carter [70] found that older females
appear to be more prone than younger ones to seek new winter range and
return to the new range in subsequent winters.


COVER REQUIREMENTS : 
Bison thrive in open areas.  When available, shrub or forest cover may
be used for shade, escape from insects, or shelter from severe weather [8].


FOOD HABITS : 
Adult bison require approximately 30 pounds of forage every day [25].
Where grasses and sedges are available in the habitat, they are
selectively grazed by bison, and where they are sparse, browse may be
substituted [8,11,20,53,88].

Dietary shifts from grasses to sedges and back again within a habitat
type are usually associated with plant phenology [11].  In Yellowstone
National Park, sedges comprised the highest proportion of bison diets in
all seasons, while grasses were second in importance.  Minor quantities
of forbs (6%) and browse (2%) were consumed, mainly in summer [52].  In
northeastern Colorado, sedges were important to bison only during
spring.  Bison herds located at Wood Buffalo National Park and Elk
Island National Park were observed feeding on grasses in summer and
sedges in winter [11].

In the Slave River Lowlands, bison diets contained 29 different plant
species.  Slough sedge (Carex atherodes) was the most abundant plant in
the diet, varying from 42 percent in winter to 77 percent in spring.
The second most common food was reedgrass (Calamagrostis spp.), which
varied from 15 percent of the diet in spring to 35 percent in winter.
Together, these two forages comprised more than 70 percent of the bison
diet at all seasons [64].

Diet and habitat selection of wood bison were studied in the Mackenzie
Bison Sanctuary between February 1986 and April 1988.  Wood bison showed
pronounced seasonal changes in their diet.  Sedges constituted 96.1 to
98.8 percent of the winter diet.  During the summer, the diet became a
more diverse mix of sedge (Carex spp.), grasses (Poaceae), and willow
(Salix spp.).  Reindeer lichen (Cladonia spp.) became a major dietary
component in fall.  Summer browsing on willows increased when sedge
standing crops were reduced [45].

In some areas forbs are seasonally important to foraging bison [11,89].
In semidesert range in southwestern Colorado, forbs were common food
items during all seasons but never exceeded 17 percent in any one season
[89].  In Yellowstone National Park and in northern Canada, forbs
appeared to be important to bison only during summer; in northeastern
Colorado, forbs were important during fall and winter [11].

On the shortgrass plains in Colorado, where blue grama (Bouteloua
gracilis) is the dominant species, bison consumed 36 different plant
species.  However, only 11 contributed significantly to the total.  Blue
grama and buffalograss (Buchloe dactyloides) were the most abundant
plants in the habitat and also in the diet.  Where available, western
wheatgrass was preferred over blue grama.  Other commonly consumed
species were red threeawn (Aristida purpurea), sun sedge (Carex
heliophila), scarlet globemallow (Sphaeralcea coccinea), sand dropseed
(Sporobolus cryptandrus), needle-and-thread grass (Stipa comata),
fringed sagebrush (Artemisia frigida), buckwheat (Eriogonum spp.), and
sixweeks fescue (Vulpia octoflora) [61].

On semidesert range at Colorado National Monument, the most common plant
species in the bison diet during most seasons was fourwing saltbush
(Atriplex canescens), followed by needle-and-thread grass, which was
important during cooler months.  Sand dropseed and galleta (Hilaria
jamesii) were prominent in the diet in warmer seasons.  The only forbs
significantly utilized during all seasons except winter were mallows
(Sphaeralcea spp.).  Prickly pear (Opuntia spp.) was among the ten top
forage species during all seasons except summer.  Some of the most
common plants in the habitat, cheatgrass (Bromus tectorum), Utah juniper
(Juniperus osteosperma), and big sagebrush (Artemisia tridentata), were
the least preferred forages [89].


PREDATORS : 
The main predators of bison are gray wolf (Canis lupus), [10,11,38,65]
grizzly bear (Ursus arctos horribilis), and coyote (Canis latrans) [11].
On the Slave River Lowlands, bison were the most important of six major
prey types eaten by gray wolves and represented 88 percent of prey
weight during the period of snow cover (Nov. 8, 1976-April 15, 1977).
Gray wolf predation in 1976 and 1977 accounted for about 31 percent of
adult and subadult bison mortality and approximately 27 percent of calf
mortality [65].  Winter-killed bison may be important food sources to
grizzly bear in early spring after they emerge from their dens [11].


MANAGEMENT CONSIDERATIONS : 
Bison are attracted to grassland sites altered by black-tailed prairie
dog (Cynomys ludovicianus).  Bison often feed selectively near the
perimeters of colonies.  These areas are constantly clipped by
black-tailed prairie dog and, therefore, have more readily digestible
perennial grasses, with higher nitrogen concentration and greater
accessibility of green tissues, than vegetation from uncolonized areas.
Consequently, black-tailed prairie dog colonies may receive a
disproportionately high amount of bison use.  Prolonged grazing pressure
on colonies may result in changes in plant composition [18,19,12,87].
On a mixed-grass prairie, selective use of black-tailed prairie dog
colonies by bison resulted in considerably more biomass removed than by
prairie dog activity alone.  Additionally, selective use of plant
species (i.e., grasses) by bison may contribute to an increase in
forb:graminoid ratios [95].

The thick hair on the head and forequarters of bison is ideally suited
for dispersal of awned, barbed, or sticky seed-bearing structures.  For
example, the seeds of buffalograss, cocklebur (Xanthium perforatum), and
St. Johnswort goatweed (Hypericum perforatum) readily adhere to bison
hair.  The dissemination of the latter throughout the National Bison
Range is thought to have been accomplished by bison [51].  Bison also
aid in dispersal by ingesting seeds.  Samples from four buffalo chips at
Wichita Mountains Wildlife Refuge contained 219 seeds that germinated.
Of these, 195 (89%) were monocots, mostly Japanese brome (Bromus
japonicus).  Bison may accelerate seed dispersal to burned sites because
bison are attracted to recently burned areas [17].

Localized stands of timber may be considerably affected by bison horning
and thrashing during the rut and at other times.  McHugh [51] estimated
that 51 percent of lodgepole pine in some areas of Yellowstone National
Park has been horned by bison.  Such activity may inhibit succession of
grassland to forest [11].

Where bison trails or wallows (concave disturbances formed as bison paw
the ground and roll in the exposed soil) are cut into steep hillsides,
considerable water and wind erosion can occur.  Hillside trails can
serve as drainage channels, effectively lowering the water table in
upland areas and causing a change in the vegetation.  Where trails cut
near the top of steep, sandy hills, erosion and slippage may produce
barren areas.  However, by creating trails through different habitats,
bison help provide access corridors for many species of mammals,
including humans [11].

Bison wallows can serve as water catchments on flat terrain.  In
Oklahoma bison wallows have been observed to hold water for prolonged
periods during the spring rainy season.  Such small ponds become
available to both vertebrates and invertebrates.  These water-holding
wallows may also enhance growth of specific vegetation such as ruderal
species and species adapted to wet habitats [11,81].  On the Wichita
Mountains Wildlife Refuge, ruderal species such as Japanese brome and
false-pennyroyal (Hedeoma hispida) had highest cover values within bison
wallows.  Other common taxa within the wallows were Torrey rush (Juncus
torreyi), purple ammania (Ammannia coccinea), lythrum (Lythrum spp.),
and taperleaf flatsedge (Cyperus acuminatus), all of which are species
adapted to wet habitats [17].

Diseases - Anthrax (an infectious disease caused by the bacteria
Bacillus anthracis) outbreaks cause sporadic mortality in northern bison
herds.  In Wood Buffalo National Park, 50 percent of bison may be
infected by tuberculosis, a chronic infectious disease [53].
Tuberculosis in a herd of bison for more than 26 years did not appear to
interfere with herd productivity.  However, the importance of
tuberculosis as a mortality factor is difficult to determine for large
bison herds [11].  Brucellosis is an infectious disease caused by the
bacteria Brucella abortus.  Abortion caused by brucellosis has been
reported in bison.  It is assumed that infected bison shed brucella
organisms, thereby contaminating feed and water.  Dissemination of the
disease is enhanced due to the gregarious nature of bison [11].  The
role of brucellosis and its affect on reproductive activity in bison is
difficult to determine due to the lack of data on the incidence of
abortion in bison [11,52].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Bison bison | Bison