American Bison - Buffalo Field Campaign

Plate 4.1 Hereford x bison hybrid; cattle gene
introgression is morphologically evident. Photo:
Bob Heinonen.
that the introduction site was isolated from,
and unused by, the wood bison population,
and suggesting that the introduced animals
were too young to carry BTB (Fuller 2002;
Graham 1924). These arguments did not
consider the future habitat needs of the
growing wood or plains bison populations,
nor the likelihood that the two subspecies
would not remain isolated. As well, a
recommendation that only yearlings that
passed a tuberculin test be shipped to
WBNP was rejected (Fuller 2002).
It was not until 1957 that the discovery of
a seemingly isolated herd of 200 animals
near the Nyarling River and Buffalo Lake
alleviated fears that wood bison was lost
to hybridisation (van Camp 1989). Canadian Wildlife Service
researchers determined that these animals were morphologically
representative of wood bison (Banfield and Novakowski 1960).
To salvage the wood bison subspecies, bison from the Nyarling
herd were captured and relocated to establish two new herds.
Sixteen animals were moved to the MBS north of Great Slave
Lake in 1963 (Fuller 2002; Gates et al. 2001c), and 22 animals
were successfully transferred to Elk Island National Park (EINP)
east of Edmonton, Alberta in 1965 (Blyth and Hudson 1987).
Two additional calves were transferred to EINP between 1966
and 1968 (Blyth and Hudson 1987; Gates et al. 2001c). Of those
bison transferred, 11 neonates formed the founding herd.
Subsequent studies revealed that there was contact between
the Nyarling herd and the introduced plains bison (van Zyll de
Jong 1986). Although hybridisation within WBNP did not result
in a phenotypically homogenous population (van Zyll de Jong
et al. 1995), genetic distances among subpopulations in the
park are small, indicating that there is gene flow and influence
of the plains bison genome throughout all regions of the park
(Wilson 2001; Wilson and Strobeck 1999). Despite hybridization,
genetic distances between plains and wood bison are generally
greater than those observed within subspecies. Moreover, wood
bison form a genetic grouping on a Nei’s minimum unrooted
tree, suggesting genetic uniqueness (Wilson 2001; Wilson and
Strobeck 1999).
Morphological and genetic evidence suggest that care should
now be taken to maintain separation between these historically
differentiated subspecies. Efforts are in place to ensure
representative wood bison and plains bison herds are isolated
from each other to prevent future hybridisation between these
important conservation herds (Harper et al. 2000).
22 American Bison: Status Survey and Conservation Guidelines 2010
4.2.2 Domestic cattle x bison
The concept of crossing bison with domestic cattle dates back
to Spanish colonisers of the 16th Century (Dary 1989). There
are many accounts of historical attempts to hybridise bison
and cattle (Coder 1975; Dary 1989; Ogilvie 1979; McHugh
1972; Ward 2000). Private ranchers involved with salvaging
bison had aspirations to combine, through hybridisation, the
hardiness and winter foraging ability of bison with the meat
production traits of cattle (Dary 1989; Ogilvie 1979; Ward 2000).
The Canadian government actively pursued the experimental
production of crossbred animals from 1916-1964 (Ogilvie 1979;
Polziehn et al. 1995).
Historical crossbreeding attempts have created a legacy
of genetic issues related to the introgression of cattle DNA
into bison herds. Introgression refers to gene flow between
populations caused by hybridisation followed by breeding of
the hybrid offspring to at least one of their respective parental
populations (Rhymer and Simberloff 1996). The introgressed
DNA replaces sections of the original genome, thereby
affecting the genetic integrity of a species, and hampering the
maintenance of natural genetic diversity. Many contemporary
bison herds are founded on, and supplemented with, animals
from herds with a history of hybridisation (Halbert 2003; Halbert
et al. 2005a; 2006; Ward et al. 1999; 2000). This extensive history
of hybridisation between these two species raises questions
about the integrity of the bison genome and the biological
effects of cattle DNA introgression.
Fertility problems thwarted many of the original crossbreeding
attempts because crosses result in high mortality for offspring
and mother (Ward 2000). Experimentation has revealed that
crosses of bison females with domestic cattle males produce
less mortality in the offspring than the more deadly reverse