VARIATION IN HUMP MORPHOLOGY BETWEEN PLAINS BISON (Bison bison bison)
AND WOOD BISON (Bison bison athabascae).
Wes E. Olson and
_____________________________________________________
The variation in hump morphology between plains bison (Bison bison bison Linnaeus 1758) and
wood bison (Bison bison athabascae Rhoads 1897) was examined in 13 populations and subpopulations from across North America, to determine whether hump characteristics were
sufficiently different to support subspeciation. Plains bison from Elk Island National Park, AB,
the National Bison Range, MT, Yellowstone National Park, WY, Grand Tetons National Park,
WY, Alaska Delta Junction, AK, Badlands National Park, NE, Fort Niobrara Wildlife Refuge
NE, Wind Cave National Park and Custer State Park SD were examined, as were wood bison
from the sub-populations Lake Clair and Central in Wood Buffalo National Park, NWT,
Mackenzie Bison Sanctuary, NWT, and Elk Island National Park, AB. Data were collected from
photographs to produce size-independent ratios, measurements of surface area and degrees of
slope for four hump components. Hump characteristics were found to different enough to
consistently and reliably determine the subspecies of a bison of unknown origin.
Key words : allometry, bison, hump, morphology, subspecies,
INTRODUCTION
The classification of North American bison has long been a contentious topic (Geist 1971;
Krumbeigal and Sehm 1989; Reynolds et al 2003) with two opposing points of view. Many
authors support the view that two subspecies of bison currently inhabit North America – plains
bison (Bison bison bison Linnaeus 1758) and wood bison (Bison bison athabascae Rhoads 1897)
Attempts to justify subspeciation are numerous (Anthony 1928, Radford 1911, and
Richardson 1829 in Soper 1941; CITES; Frick 1937; Gates et al 2001; Geist and Karsten 1971;
Kurten and Anderson 1992; Mitchell and Gates 2002; Rhoads 1897; Seibert 1925; Seton 1929;
Scott 1913; Soper 1941; van Camp 1989; van Zyll de Jong 1986; van Zyll de Jong et al 1995;
Wilson and Strobeck 1999; Wilson, Zittlau and Nishi 2002; Wilson, Olson and Strobeck 2002;
Wilson and Zittlau 2004; and Olson 2005). Other authors present the argument that one
subspecies of bison resides in North America, and that any variation within or between
populations is related to environmental and habitat factors rather than true subspeciation
(Graham 1923, Mackenzie 1801, and Pike 1882 in Soper 1941; Geist 1991; Geist 1996; Halbert
2003).
The designation of bison as subspecies became an issue in 1990 when Agriculture Canada put
forth a proposal to slaughter all resident bison in around Wood Buffalo National Park in northern
Alberta (Connelly et al 1990, Environmental Assessment Panel 1990). The proposal was the first
step in an attempt to eradicate Mycobacterium bovis and Brucella abortus (Gates et al 1997)
from this population. The proposal met with considerable opposition (Geist 1991, McCormack
1992) and the proposal was subsequently abandoned. The work by Geist (1991) was a catalyst in
the debate about bison subspeciation. This paper was in direct contradiction to an earlier paper
(Geist and Karsten 1977) that described the physical characteristics of both plains and wood
bison, and which supported subspeciation of North American bison. Geist (1991) later suggested
that plains and wood bison were a single subspecies, and that the pelage and morphometric
differences he observed earlier were the result of ecotypic confinement effects. To validate this
assumption he presented photographs which showed similarities in phenotypic expression
between plains and wood bison. He then represented those photographs as simplified line
drawings to clarify the differences / similarities between 16 individual bison. These 16 bison
2
represented ages described as “young” to “very old”. The photographs were taken during early
summer through to late winter. This confused ontogeny and season with phenotype. The
combination of small sample size, wide geographic distribution (from Texas to Wood Buffalo
and the Hamburg Zoo), different ages sampled and different seasons of sampling, led to research
conducted and presented in van Zyll de Jong et al (1995). This research standardized the age of
bison sampled to breeding age males and females, and season to the period of the rut (mid-July
to mid-August). While that research supported the differences between plains and wood bison
and concluded that they were indeed two distinct subspecies, it did not examine the variation
within those populations. As Geist (1991) stated, “… subspecies are distinguished by at least one
consistent taxonomic difference.”
The issue of subspeciation received further attention in 2004 when the Committee on the
Status of Endangered Wildlife in Canada (COSEWIC) listed plains bison as “Threatened” in
Canada. As a result of this proposal, the Canadian Bison Association expressed concern
regarding the potential negative impacts of the Threatened designation on the bison ranching
industry in Canada. COSEWIC agreed to a moratorium on SARA listing, until January 1 2011.
Subsequent to that date, the CBA is to have identified at least 1000 breeding age bison of
conservation value (see Appendix 5.2 for a discussion on Conservation Herds) to satisfy the
minimum population requirements of COSEWIC. An ancillary result of this decision is the need
to be able to recognize wood bison from plains bison, and from their hybrid cross, the Parkland
bison.
We developed a methodology that can assign a adult breeding aged male bison, of unknown
subspecific origin, to the correct subspecies.
3
RESEARCH QUESTIONS
Geist (1991) postulated that plains and wood bison are ecotypes, rather than true subspecies.
He also stated that for a species to be a “true” subspecies, they must be differentiated by at least
one consistent taxonomic difference. How long, and under what circumstances, does it take for
an isolated ecotype of a species to evolve characteristics of behavior, taxonomy, and genetics
distinct enough for it to be scientifically accepted as a true subspecies? Where is the evolutionary
dividing line that says, this animal has attained the required characteristics that make it unique
from its parent type? Given mankind’s intervention in the evolutionary process of bison and the
interruptions it inadvertently caused, is it still possible to determine whether plains and wood
bison were evolving towards subspeciation or is it too late?
Limited research has been conducted on the differences in skeletal morphometry between
plains and wood bison with the specific goal of identifying post-cranial skeletal variation
between the two types. One of the largest studies was conducted by van Zyll de Jong in 1985. He
conducted a wide variety of measurements on the skull and post-cranial skeleton of plains bison
(B. b. bison), wood bison (B. b. athabascae) and the Wisent (B. . bonasus) and this research
helped to explain why bison are shaped the way they are. This anatomical exploration is further
enhanced by the work of Guthrie (1990) who describes the ecology of the hump, its role and
function as it relates to combat, locomotion and behavior.
All extinct and extant bison have 7 cervical, 14 thoracic, 5 lumbar vertebrae (Guthrie 1990)
and 8 caudal vertebrae (Figure 1). It is the placement and length of the vertebral neural spinal
processes which provide the foundation upon which the various hump muscles (the
rhomboideus, splenius and spinalis muscles for example) and tissues (such as the nucal ligament)
are laid and supported (Guthrie 1990, pg 138). These in turn are overlain with a layer of fat and
then the hide with its secondary sexual pelage characteristics. Together, these structural
elements produce the dorsal contour of the bison hump that is measurable on photographs.
For a cursorial species like the plains bison (Guthrie 1989, Geist 1991), the ability to travel
long distances during seasonal migrations (McHugh 1958, Meagher 1986, Schult and Haugen
1979) is enhanced when the centre of gravity is located over a central pivot point. Bison have an
exceptionally long scapula that permits long stride lengths (Guthrie 1989). With the highest point
4
and consequently greatest mass, placed over the front legs, the plains bison is superbly adapted
to traveling great distances,. The centrally placed hump aids in offensive and defensive agonistic
behavior between rival bulls during the rut, and permits fast and agile defence responses against
the large wolf populations which existed on the great plains (Bergon 2003, pg 116).
Wood bison living in the boreal forests of northern Canada did not make the long distance
movements typical of their southern cousins (Graham 1923, Soper 1941, van Camp 1975). While
wolves remain a significant predator on wood bison (Carbyn 1981, 1987, Fuller 1951, Gates and
Larter 1990, Joly and Messier 1990) they probably never existed in the numbers reported
historically for plains bison (Bergon 2003, MacEwan 1995, McHugh 1958), and perhaps the
frequency of attack by wolves was less for wood bison than for plains, due to lower wolf
densities. Bergmann’s Rule (1847), suggests that mammals increase in body size with increases
in latitude; an adaptation to colder climates. Olson (2006) found that wood bison in Elk Island
National Park were an average of 17 % heavier than plains bison in Elk Island. Van Zyll de Jong
(1986) measured skulls from extant, historical through to the Holocene and Pleistocene bison.
Using his data, wood bison skulls were an average of 13% to 14% larger than those of plains
bison from the great plains and the eastern US respectively. These factors, combined with long
winters, may have contributed to the development of a large, forwardly placed hump. The
evolution of a massive, forwardly placed hump may be more suited to carrying weight and
designed for efficient cratering / foraging through the long months of winter, than for agonistic
or defensive purposes.
Three general questions were posed to determine whether consistent taxonomic differences
exist between adult, breeding aged male plains and wood bison.
1. Are there inter- and intra-specific differences in the four angles of the hump?
2. Are there inter- and intra-specific differences in the area of each component of the hump?
3. Are there inter- and intra-specific differences in the relative proportions of the torso and legs
of the body?
5
METHODS
Sampled bison populations.
The following 13 populations were selected primarily because a photographic collection
acquired for the research of van Zyll de Jong et al. (1995) was in existence and could be
employed for this research. Additional photographs were acquired opportunistically and were
added to the original photographic database.
PLAINS BISON
1. Alaska, Delta Junction (ADJ), Alaska.
The plains bison populations in Alaska are from a 1928 translocation from the National
Bison Range, Montana in. At that time 23 bison were released along the Delta River, near the
mouth of Jarvis Creek (Dubois and Rogers 2000). Through emigration and translocations,
additional herds have been established at Copper River, Farewell and Chitna (Berger and
Cunningham 1994, Boyd 2003, Halbert 2003). Only photographs of bison from the Delta River
herd were used for morphometric analysis.
2. Badlands National Park (BLNP), Nebraska.
In 1963 three plains bison from Fort Niobrara, and 50 from Theodore Roosevelt National Park
were reintroduced to Badlands National Park. Another 20 bison were added to the herd in 1983
from the Colorado National Monument population (Berger and Cunningham 1994, Boyd 2003,
Halbert 2003).
3. Custer State Park (CSP), South Dakota.
The Custer State Park bison population was established in 1914 with the acquisition of 36
plains bison from the herds of Scotty Philips and Fred Dupree (Boyd 2003). The herd was
supplemented during the 1950’s with a movement of about 800 bison from Wind Cave National
Park into Custer State Park.
6
4. Elk Island National Park-Plains (EINP-P), Alberta.
The plains bison in EINP originated from the well documented Pablo – Allard herd
(American Bison Society 1913; Bork 1990; Coder 1975; Corner 1958; Douglas 1909; Gates et
al 1992; MacEwan 1995; Michiel 1990; Polziehn, Beech, Sheraton and Strobeck 1996;
Reynolds and Parkinson 1983; Reynolds 1989; Seton 1926; Shackleton 1968; van Camp 1975;
Wilson and Zittlau 2004). Between June and October 1907, 419 plains bison were transported
north to the newly established Buffalo National Park near present day Wainwright Alberta
(Brower 2002; Fuller 2002). The new park was not fenced and ready for the bison so they were
temporarily off-loaded into Elk Island National Park, where they were confined until October
1909 when Buffalo National Park was ready for them. Attempts were made to capture the entire
herd, but approximately 48 plains bison remained in Elk Island National Park and formed the
foundation of the present population. No additional plains bison have been added to this
population since the initial translocation (Boyd 2003). Population size has ranged from a high of
about 2500 in 1936 to the present when a base population of about 350 is maintained (Blyth
1987; Olson 2004).
The population has a fairly broad genetic base, despite its several genetic bottlenecks.
5. Fort Niobrara Wildlife Refuge, (FNWR), Nebraska.
This population was established in 1913 with 6 bison from Nebraska, and was supplemented
during latter years with bison from Custer State Park and the National Bison Range (Boyd 2003,
Halbert 2003).
6. Grand Tetons National Park (GTNP), Wyoming.
The first bison to repopulate the region that is now GTNP came from Yellowstone National
Park in 1948. A total of 20 bison were moved to the Jackson Hole Wildlife Park. These were
supplemented in 1964 with the addition of another 12 bison from Theodore Roosevelt National
Park. (Boyd 2003, Halbert 2003).
7
7. National Bison Range (NBR) Montana.
The National Bison Range population began with the arrival of 34 bison from the Conrad
herd, 3 from the Corbin herd, and 1 bull from the Goodnight herd in 1908. No further additions
to the population occurred until 1939 when 2 bulls were added from the 7-Up Ranch, in 1952
with 4 bulls from FNWR, 2 bulls from YNP in 1953 and 4 females in 1984 from the Maxwell
State Game Refuge (Halbert 2003). The Elk Island plains bison population and the Delta River
Alaska population were both established from the NBR herd, when it only contained animals of
1908 origin. (Boyd 2003, Halbert 2003).
8. Wind Cave National Park (WCNP), South Dakota.
The WCNP population was established in 1913 with 14 bison from the New York
Zoological Park. These were supplemented in 1916 with an additional 6 bison from YNP. No
further additions to the population have taken place. (Boyd 2003, Halbert 2003).
9. Yellowstone National Park (YNP), Wyoming.
By the end of the 19th century, the Yellowstone population contained the only freeroaming wild plains bison population in North America. By 1902 the population had been
reduced to about 30 wild bison. The herd was supplemented in 1902 with 18 cows from the
Pablo-Allard herd and with 3 bulls from the Texas Goodnight herd. (Meagher 1973, Boyd 2003,
Halbert 2003).
WOOD BISON
10. Elk Island National Park-Woods (EINP-W), Alberta.
EINP wood bison originated from the Needle Lake – Nyarling River region of north-western
Wood Buffalo National Park (WBNP). In 1958 a remote herd of bison was observed in the
Needle Lake region, and due to their isolation by geographic distance and inhospitable terrain,
combined with their wood bison phenotypic expression, the group was thought to be a remnant
population of pure wood bison (Banfield and Novakowski 1960). Shortly after the discovery of
this herd an outbreak of anthrax along the eastern portion of WBNP impelled conservationists to
8
attempt to salvage this herd before it too was decimated by the disease. This led to the live
capture and subsequent translocation of 23 wood bison from WBNP to EINP in August of 1965
(Blyth and Hudson 1987).
Despite rigorous testing for the presence of Bovine brucellosis and tuberculosis prior to and
following their arrival in EINP, in 1968 the first confirmed case of TB was found. This
discovery eventually led to the slaughter of all founder bison and the bottle-raising of their calves
in an attempt to eradicated the disease. This was successful and by 1970 the herd was certified as
disease free (Blyth, Gates and Reynolds 1991) and has remained so since that time (Olson
2007c).
To have contracted the disease in WBNP, the wood bison must have had physical contact
with the diseased hybrids, and thus probably shared genetic material along with the disease. This
realization led to a phenotypic survey of the EINP population (Geist and Karsten 1977) who then
described seven traits which served to define the physical and pelage differences between plains
and wood bison. While they concluded that “to a trained observer, the wood bison bull and cow
differ significantly from those of prairie bison” it is generally accepted that the EINP wood
bison must have some plains bison genetic introgression.
11. Mackenzie Bison Sanctuary (MBS), Northwest Territories
The MBS population was established in 1963 with the translocation of 18 bison from the
Needle Lake region of WBNP, as part of the project described above. At the time of the capture
of these bison it was assumed that they had not come into contact with the introduced plains
bison or their descendant hybrids. The fact that the population has remained free of the
introduced diseases bovine tuberculosis and brucellosis supports this. No further additions have
been made to the herd (Gates et al 2001, Gates et al 1991).
12. Wood Buffalo National Park, Central, (WBNP-C), Northwest Territories
13. Wood Buffalo National Park, Lake Claire, (WBNP-LC), Northwest Territories
The WBNP population has a complex and controversial genetic and management history
(Babbage 1969; Banfield 1960; Fuller 2002; Gates et al 1992; Karsten 1976; Novakowski 1957;
Novakowski 1963a; Novakowski 1963b; Novakowski and Stevens 1965; Polziehn et al 1996;
9
Reynolds and Parkinson 1983; van Camp 1989; Wilson and Zittlau 2004; Wilson, Olson and
Strobeck 2002 ).
WBNP was established in 1913 to protect the last wild population of about 250 extant wood
bison (Soper 1941). By the early 1920’s the herd had expanded in range and in numbers to about
1500 pure wood bison (Bradley and Wilmshurst 2005). Coincident to the recovery of wood bison
in WBNP the plains bison population in Buffalo National Park was experiencing a rapid
increase. Severe over-grazing, poor rangeland management, and poor fiscal management let to
the decision to remove 6,673 plains bison and translocate them into the eastern portion of WBNP
(Brower 2002, Fuller 2002). This occurred during 1922-24 and by the 1940’s it was generally
accepted that wood bison, as a distinct subspecies, had been replaced by a hybrid population
(Soper 1941).
The Buffalo National Park plains bison were introduced to the eastern portion of WBNP
along the Slave River. They dispersed west into the central regions of the park, and south into
Lake Claire, Peace – Athabasca Delta region. Photographs were collected throughout the park,
but were concentrated in the Central and Lake Claire regions, due to the larger and more
accessible populations that inhabit these areas.
DATA COLLECTION
Ernest Thompson Seton (1896) in his “Studies in the Art Anatomy of Animals” was the first
author to describe and illustrate the use of proportional body measurements, “for the careful
analysis of the visible forms and proportions of the living animal.” He did this as a resource for
fellow artists, but over a century later it provided the inspiration for the work that follows here.
The use of morphometric analyses to delineate inter- and intra-specific variation has been
employed for a wide range of species (Baryshnikov, Bininda-Emonds, and Abramov 2003 with
Mustelids; Kennedy and Lindsay 1984 with racoons; Imre, McLaughlin, and Noakes 2002 for
fish; Patterson, Ratnaswamy and Winn 1993 for whales; van Zyll de Jong et al 1995 for bison;
van Zyll de Jong 1992 with weasels; Wimberger 1992 for fish, Meyer 1987 for fish; Thorpe
1975 with snakes; Turner and Worthy 2003 for dolphins; and Zhang and Malmqvist 1977 for
insects). These studies relied upon physical access to the specimen upon which their
measurements were made.
10
Due to the obvious difficulties associated with obtaining morphometric measurements from
live bison, this research was conducted using photographs of bison. Skogland (1983) employed
photographs in his analysis of Norwegian reindeer (Rangerifer tarandus ) to obtain heights of his
subject animals. These data in turn yielded data on the length of long bones which he used to
document growth rates. Ratnaswamy and Winn (1993) used aerial photogrammetric estimates of
allometry to segregate whale calves from adults and thus determine calving rates and calf
survival. These studies were designed to produce an estimate of the size (length, height, etc) of
the species in question. Kays, Kasiki and Sebestyen (2006) documented differences in mane
development of the African lion through the use of photographs.
The morphometric measurements obtained and presented here are not designed to produce an
estimate of body size for comparison between individuals or populations. It is used to produce
size-independent ratios of body morphology for comparing individuals within and between
populations.
Data was obtained from two photographic databases. The first was the original photo database
compiled during 1991-92 from across North America as part of the research conducted by van
Zyll de Jong et al (1995). The second photographic database was comprised of photographs
taken opportunistically of plains and wood bison in the populations described above, subsequent
to 1995. Photographs were taken during the months of July and August, the period during which
adults are most phenotypically distinct, to eliminate variation caused by season. Bison undergo
two annual moults (Meagher 1986), each spring and fall, and consequently there can be
considerable between-season variation in pelage expression / development. The animal had to be
standing or walking in a normal manner; i.e., the head was carried in a relaxed manner, not
grazing, and not running. All photographs are of the bison in full body profile, with all of its
surface area available for inspection, i.e. from the ground up.
Age and Sex Classes Sampled
When a bison herd was encountered the photographers obtained photographs of every bison,
regardless of age or sex, that presented itself in the required posture to the camera lens. This
resulted in a photographic collection with every age and sex represented and includes over 2300
individual bison from both subspecies. While attempts were made to obtain photographs of every
11
age class and both sexes in every population, this was not always possible. Some populations did
not contain individuals in the “aged” group for example (due to management practices) and in
some free-roaming populations only one age class was photographically represented (ADJ, and
NBR for example). Only EINP-P, EINP-W, YNP, WCNP, and MBS had all three bull cohorts
represented. These photographs were then sorted by their herd of origin, by sex, and then by age
class. The aging and sexing techniques employed follow those described in Olson (2005). He
described the physical characteristics for five age classes of females (calf, yearling, two-year old,
adult and aged), and seven classes for males (calf, yearling, two-year old, three-year old, adults,
breeding and aged). The age classes of adult males (aged 4 to about 7), breeding (aged 8 to about
15) and aged (aged 16 years and older), and the adult females (aged 3 to 15 years) and aged (16
plus) were selected for study.
Males had to be at least four years of age to be included in the study, since this is the age
when they first attain their adult pelage and begin to develop their mature body profile. Prior to
this age, juvenile males mimic female characteristics, perhaps as an anti-predator strategy.
Use of the Photograph
Each photograph first had a series of parallel lines drawn on it, with the first line drawn along
the ground at the contact point of the animals hooves (see Figure 2, Appendices 5.1.1. and 5.1.2.)
This line served as a reference line upon which all other lines were based. Once these were in
place a series of reference points were placed at specific, repeatable, recognizable and immobile
points on the body.
These reference points provided the required angles, measurements and ratio proportions
which were used to detect inter- and intra-population variation in hump morphology (see Figures
1 to 4).
The size and shape of the neural spines of the vertebrae, combined with tissue and pelage,
determine the dorsal outline of bison, and create their distinctive humps. The angles formed by
the intersection of the previous lines, overlaid on the photographs of bison, were used to describe
differences between plains and wood bison, and between their sexes.
Body or torso morphology was examined through the use of size-independent comparative
ratios. This research sought to determine whether there were differences in the relative
12
proportions of bison, and were chosen because they were not affected by changes in hair coat.
For example, is the hump depth deeper in wood bison than it is in plains bison of the same age
and sex? These measurements were converted to ratios to be independent of photograph scale.
The hump is composed of several definable components, which in turn can be measured
through the use of the previous points and lines. As the measurements were entered into the
Excel spreadsheets, formulas for the calculation of the area of triangles determined the relative
area of each of four triangles. These four areas were the Anterior Shoulder Hump (ASH, or
Triangle 1), the Dorsal Shoulder Hump (DSH, or Triangle 2), the Ventral Shoulder Hump (VSH,
or Triangle 3). These three hump components, combined together, comprise the Shoulder
Hump. The fourth hump is the Back Hump (BH, or Triangle 4). All four were added together to
obtain the total area of the hump, and from this a percent composition for each component was
determined.
13
Results
A total of 358 plains and wood bison males in three age classes were photographed from 13
populations across North America. Data from all plains bison populations were combined for
each age class for each sex, and for each subspecies, to represent the meta-population mean. For
example all plains bison B4 males, from all source populations, were combined to obtain the
mean BHA for this age class and it was then compared to the B4 wood bison meta-population
mean.
A total of 23 measurements (Four hump angles, five hump area calculations, and 14 body
proportion measurements) were performed on each photograph.
Are age classes different within a subspecies?
1. The Back Hump
The Back Hump angle measured 14 degrees for plains bison B4, B5 and B6 within the plains
bison meta-population. (Table 1). There was considerable variation within a cohort, for each
population. For example, while the mean BH angle for B4 plains bison from EINP was 14
degrees, it ranged from 8 to 17 degrees for B4, 9 to 20 for B5 and from 10 to 18 for B6 bulls.
14
Discussion
While it is possible to place bison of both sexes into identifiable age classes, there is a degree
of variation within each of these classes. As the bison matures its body contours and profiles
change as it gains weight (body mass), strength (muscle firmness), and size (height, length, etc).
Since the age class “adult” contains bulls that range in age from four to seven years, there is
naturally some variation between individuals in each of these characters. The same holds true for
breeding aged bulls (8 to 15 yrs) and for the aged bulls (>15 yrs). The characteristics that placed
a bison into its age class were readily visible in photographs. For example, to be included in the
adult class (4 to 7 yrs), the bulls horn tips had to be pointed straight up, with a slight inward tilt
to the tips. This usually happens in their fourth summer. To be included in the breeding bull
group, the horn tip had to be exfoliated. In Elk Island this occurs consistently at around seven or
eight years of age. Bulls became aged when characters such as belly sage, broomed horns, and
massive size were assessed. The degree of variation is most extreme when a four year old is
compared to an aged bull. However, by using the means of the characteristics for each age class,
and then comparing those means, a reasonable comparison of characteristics can be made.
15
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