American Bison - Buffalo Field Campaign

Plate 6.1 Plains bison bull tending a cow, Jackson Valley,
first eight years (Figure 6.1). While differences among
populations in body size and weight may be apparent
to an observer, comparisons must take in to account
the annual cycle of weight gain and loss.
6.1.2 Behaviour
6.1.2.1 Social structure
There are many historical observations of huge plains
bison herds roaming the Great Plains (Dary 1989;
Hornaday 1889; Isenberg 2000; Roe 1970). Observers
of both plains and wood bison consistently report
a definable herd structure where cows, calves, and
immature males form unstable mixed-sex and age groups, and
large bulls form separate, smaller groups throughout much of
the year (Allen 1876; Berger and Cunningham 1994; Komers
et al. 1993; Meagher 1973; Melton et al. 1989; Schuler et al.
2006). Seasonal variations in group sizes are associated with
abundance or dispersion of forage (Jarman 1974; Schuler 2006),
landscape features (Berger and Cunningham 1994), breeding
behaviour (Berger and Cunningham 1994; Meagher 1973; Melton
et al. 1989; Komers et al. 1993) and population size (Schuler et
al. 2006). The largest aggregations occur during the breeding
season when mature bulls join the mixed-sex and age groups.
Mean group sizes during the August rut at Badlands National
Park range from a mean of 157 in flat terrain to 79 in broken
terrain (Berger and Cunningham 1994). Mean maximum group
sizes at Yellowstone National Park (YNP) increased from 140
in May to more than 250 in September (Hess 2002). Groups of
more than 1,000 bison have been observed during the rut in
contemporary Oklahoma (Schuler et al. 2006). Group size rapidly
diminishes during autumn in plains bison (Hornaday 1889) to
fewer than 30 (Berger and Cunningham 1994; Schuler et al.
2006). Similarly, in wood bison, typical group size is greatest
during the pre-rut and rut, then declines during the fall (Komers
et al. 1992). Mean maximum group sizes at YNP declined
throughout winter from more than 250 in December to 16 in April
as the area occupied by bison increased from 1,000 to more
than 1,200 km 2 (Hess 2002).
Male bison form temporary, unstable groups, and exhibit a linear
dominance hierarchy, with older, heavier animals dominant over
younger smaller males (Komers et al. 1994; Roden et al. 2005).
Dominance is also related to age in female bison (Rutberg 1983).
Groups of adult or subadult males rarely exceed 10 individuals
(Berger and Cunningham 1994).
Wyoming. Photo: Cormack Gates.
Plains and wood bison population substructure occurs at a
broad geographical scale due to traditional use of particular
parts of a range by segments of a population (Joly and Messier
40 American Bison: Status Survey and Conservation Guidelines 2010
2001; Olexa and Gogan 2007). Plains bison within the Greater
Yellowstone Area show strong fidelity to subpopulations
(Christianson et al. 2005; Gogan et al. 2005; Olexa and Gogan
2007) as do wood bison in the Greater Wood Buffalo Ecosystem
(GWBE) (Carbyn et al. 1998; 2004; Chen and Morley 2005; Joly
and Messier 2004). Bison within subpopulations show stronger
cohesion and coordinated movements during summer than in
winter (Chen and Morley 2005; Olexa and Gogan 2007).
6.1.2.2 Reproductive behaviour
Sexually mature male plains bison join mixed-sex and age
aggregations during the rut. Dominant bulls form so-called
“tending bonds” with individual cows just prior to, or during,
oestrus (Fuller 1960; McHugh 1958; Meagher 1973). The bull will
typically attempt to keep other bulls away and to keep the cow
near the edge of a mixed-sex and age group until she accepts
copulation (Berger and Cunningham 1994; Lott 2002; McHugh
1958). Mature males move away from mixed-sex and age groups
at the end of the rut (Berger and Cunningham 1994; Lott 2002).
Wood bison also aggregate during the summer (Joly and Messier
2001; Komers et al. 1992). Male wood bison become more
solitary with increasing age, are more frequently aggressive, and
test females for oestrus more frequently than do younger bulls
(Komers et al. 1992). During the rut, mature males join mixed sex
and groups to compete for mating opportunities and temporarily
leave these groups to recover from high cost breeding activities
(Komers et al. 1992). In the experimental absence of mature
males during the rut, subadult males fed less and interacted
more aggressively than when mature males were present
(Komers et al. 1994).
6.1.2.3 Cow-calf behaviour
Female plains bison close to parturition have been described
as restless and excitable (McHugh 1958). A pregnant cow may