American Bison - Buffalo Field Campaign

Chapter 6 General Biology, Ecology and
Demographics
6.1 General Biology
An understanding of the ecology and biology of bison is
fundamental to their successful management, conservation,
and restoration. Bison have the broadest original range of
any indigenous ungulate species in North America, reflecting
physiological, morphological, and behavioural adaptations that
permit them to thrive in diverse ecosystems that provide their
diet of grasses and sedges. Successful population management,
conservation of genetic diversity and natural selection, modelling
and predicting population level responses to human activities,
and managing population structure all depend on understanding
the biological characteristics and ecological roles of bison. The
purpose of this chapter is to summarise what is currently known
about the biology of bison; for an earlier comprehensive review,
see Reynolds et al. (2003).
6.1.1 Physiology
6.1.1.1 Metabolism
Bison exhibit seasonal variation in energy metabolism.
Christopherson et al. (1979) and Rutley and Hudson (2000)
observed that metabolisable energy intake and requirements of
yearling male bison were markedly lower in winter than summer.
This was attributed to a reduction in activity and acclimation.
Bison are better adapted to temperature extremes than most
breeds of cattle. They expend less energy under extreme
cold than do cattle because of the greater insulating
capacity of their pelage (Peters and Slen 1964).
Cold tolerance of hybrids between bison and cattle
is intermediate between the two species (Smoliak
and Peters 1955). Tolerance of bison to heat has not
been studied, but the original continental range of
the species included the dry, hot desert grasslands of
northern Mexico, where a small population of plains
bison still exists today (List et al. 2007).
Figure 6.1 Age-specific live-weights of male and female plains
bison at Wind Cave National Park, South Dakota, obtained at
fall roundups 1986–1989 and 1991–1999. Data courtesy D.
Roddy and B. Meunchau, Wind Cave National Park.
Lead Authors: Peter J.P. Gogan, Nicholas C. Larter, James H. Shaw, and John E. Gross
6.1.1.2 Growth
Contributors: C. Cormack Gates and Joe Truett
Birth weights of intensively managed plains bison have been
reported as 25 kg for females and 30 kg for males (Agabriel et al.
1998; Agabriel and Petit 1996; Rutley et al. 1997). Birth weights
(near-term foetuses) of free-ranging plains bison range from 14
to 32 kg (McHugh 1958; Meagher 1986; Park 1969). Gogan et
al. (2005) estimated that the birth weight of free-ranging bison
calves is on average 10% less than that of captive bison. Growth
from calfhood to adulthood followed a similar pattern to that of
adults, with weight gain during the summer and loss during the
winter (P.J. Gogan, unpublished data). Weight gain among calf
and yearling plains bison was affected by the influence of the
timing and magnitude of summer precipitation on graminoid
physical structure (Craine et al. 2009).
Differences in weights of plains bison in geographically separate
herds have been attributed to differences in climate, nutritional
plane, and genetic lineages (Berger and Peacock 1988; Lott and
Galland 1987). At Elk Island National Park (EINP), female plains
and wood bison achieved asymptotic body weight by six years
and maximum body weight at 10 years (Olson 2002; Reynolds
et al. 2003). Female plains bison at Wind Cave National Park
(WCNP) reached an asymptotic body and maximum body weight
at five years (Figure 6.1). Male plains and wood bison at EINP
reached an asymptotic body weight at eight to nine years and
maximum body weight by 13 years (Reynolds et al. 2003). Male
plains bison at WCNP continued to gain weight through the
American Bison: Status Survey and Conservation Guidelines 2010 39