The Evolution of Bison bison: A View from the Southern Plains
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The Evolution of Bison bison:
A View from the Southern Plains
Patrick J. Lewis, Eileen Johnson, Briggs Buchanan, and Steven E. Churchill
ABSTRACT
Morphological changes in late Quaternary bison generally have been defined by a rapid decline in body size
and a reorientation of the horns. This morphological shift is used to mark the emergence of modern Bison bison,
a transition conventionally thought to have originated on the Northern Plains after 5,000 B.P. Hypotheses
regarding the impact of human hunting on this morphological transition vary from negligible to the driving force
behind a speciation event. New data from the Southern Plains, however, define better the transition and the role
that hunting may have played in driving this event. This research on Southern Plains bison indicates that fully
modern bison appear on the Southern Plains much earlier (between 8,000 to 6,500 B.P.) than on the Northern
Plains and that this morphological transition also involved a decrease in relative bone strength (robusticity). The
morphological transition in bison does not correlate with changes in tool technology, hunting intensity, or
hunting techniques. Therefore, the impacts of human hunting do not adequately account for the observed
morphological changes in bison morphology. The change in bison morphology on the Southern Plains is
correlated better with the spread of the C4 short grass ecosystem between 8,000 to 7,500 B.P. The properties of
C4 grasses explain both the size reduction and the gracilization of late Quaternary bison.
INTRODUCTION human hunting (e.g., Lott 2002; McDonald 1981) to
explain changes in the bison lineage, and are based
The North American bison was a critical spe- primarily on data from the Northern Plains. While
cies for people inhabiting the Plains during the late Southern Plains bison have been studied from an
Quaternary, serving as an important source of food archeological point of view (e.g., Bement 1999;
and raw materials for many cultures. More broadly, Johnson 1987), hypotheses concerning the emer-
however, bison impacted the lives of all Plains- gence of modern bison have yet to be explored fully
dwelling peoples as the keystone species of the in this region. New data, however, from a continu-
Plains ecosystem (Frison 1991; McHugh 1972). Any ous and well-dated sequence of bison metapodials
change in bison would have had a significant im- from the Lubbock Lake Landmark, supplemented
pact on the peoples of the Plains, and the evolution- with specimens from two other Southern Plains lo-
ary forces acting to change bison would have im- calities, allow an improved understanding of the
pacted all other species that were part of the Plains pattern of morphological change in the late Quater-
ecosystem, including humans. For these reasons, nary bison lineage. These new data have led to the
defining and understanding the changes in late Qua- formulation of a new hypothesis regarding the se-
ternary bison and the underlying causes for those lective force guiding these changes in bison.
changes are essential to the interpretation of prehis- Changes in the bison lineage during the termi-
toric Plains cultures. nal Pleistocene and early Holocene generally are
Hypotheses proposed to explain the morpho- described as a decrease in overall body size (ca.
logical changes in bison following the Wisconsin 20% reduction in linear measurements) and a re-
glaciation constitute some of the most compelling orientation of the horn cores (Dalquest and Schultz
and contentious questions surrounding the evolu- 1992; McDonald 1981; Wilson 1974a, 1975).
tion of Quaternary macrofaunas (McDonald 1981; These changes are suggested to have occurred
Reeves 1973). Most hypotheses implicate either cli- gradually beginning in the late Pleistocene and
matic changes (e.g., Wilson 1975) or the effects of continuing through the Holocene. Modern bison
Bulletin of the Texas Archeological Society 78 (2007)198 Texas Archeological Society
are reportedly present on the Northern Plains after Kaisan 1947; Wilson 1975; Wyckoff and Dalquest
ca. 5,000 B.P. (uncalibrated radiocarbon age) 1997), this study focuses on the metapodials. Metapo-
(McDonald 1981; Wilson 1975). Changes in horn dials are functionally important weight-bearing ele-
core morphology are vague, however, due to their ments for bison (Bedord 1974) and, more broadly,
highly variable nature (McDonald 1981; Wilson for the family Bovidae (Scott 1985). Metapodials
1975) and plasticity (Guthrie 1966), such that at- are conservative morphologically, showing little
tempts to use horn cores in taxonomic and phylo- variation in shape across the bovids (Scott 1985).
genetic analyses find them to be of limited utility. Any significant changes in metapodial morphology,
As such, horn cores, accepted as being fully mod- therefore, likely represent a functional, behavioral,
ern in morphology on the Southern Plains by or genotypic shift in bison relevant to hypotheses
10,000 B.P. (Dalquest and Schultz 1992), are not about the forces that were acting to shape the bison
addressed further. skeleton. Metapodials also are common elements in
Both human hunting and climatic change have late Quaternary faunal assemblages (Kreutzer 1992),
been posited as the principle force behind the de- allowing for robust statistical analyses.
crease in bison body size. Human hunting hypoth- Metapodial size, shape, and robusticity (rela-
eses generally suggest that hunting selected either tive cortical bone thickness) data were collected
for smaller body sizes or against larger body sizes from osteometric and radiographic examination of
(Lott 2002; McDonald 1981). Climatic hypotheses a temporal series of 463 late Quaternary bison
associate the warming temperatures of the Holocene metapodials from three sites on the Southern Plains
with decreases in body size, following Bergmann’s (Figure 1). Of those metapodials, 254 specimens
rule (Bergmann 1847) and the modern size cline (a were from Lubbock Lake (Johnson 1987) in west-
decrease in body size from north to south) seen in ern Texas. Dates were assigned to the Lubbock
today’s bison (Wilson 1975). Most of the evidence Lake specimens using multiple radiocarbon ages
given in support of these hypotheses is derived associated with archeological features and geologic
from Northern Plains populations from multiple lo- strata (Holliday et al. 1983, 1985). In addition, 75
calities, and is based principally on cranial charac- metapodials from Cooper (Bement 1999) and 134
teristics known to be highly variable. To define from Certain (Buehler 1997), both sites located in
better the rate and pattern of change in bison popu- western Oklahoma, were examined. These metapo-
lations, a morphologically conservative element dials were dated according to radiocarbon assays
should be used and the sample broadened to in- and known ages of diagnostic stone tools (Bement
clude populations beyond the Northern Plains. Cor- 1999; Buehler 1997).
relations between changing bison
morphology and parameters as-
sociated with human hunting and
climatic change hypotheses then
can be examined. This study,
therefore, focuses on Southern
Plains bison metapodials to de-
termine if the timing and pattern
of change is similar to that de-
fined on the Northern Plains and
which of the competing hypoth-
eses is best supported.
METHODS
While much of the historic
research regarding bison phylog-
eny has relied heavily on cranial
characteristics (Guthrie 1990; Figure 1. Southern Plains and archeological sites where the fossil bison
McDonald 1981; Skinner and metapodials in this analysis were recovered.Lewis et al.—The Evolution of Bison bison 199
Proximal and distal anteroposterior and only significant difference between the bison popu-
mediolateral width measurements were collected at lations is between the population dating to 8,000
the extremes on complete metapodials at the proxi- B.P. and the population dating to 6,500 B.P.
mal and distal ends using sliding calipers. Unfused A slight shift in metapodial shape appears to
metapodials were not included in the study. Overall accompany this size decrease (Figure 3). Metapo-
element length was taken at the extreme using an dials of similar length vary significantly by width
osteometric measuring board. Shaft diameters in (at the midshaft) when specimens from the group
the anteroposterior view and the mediolateral view dating to 8,000 years and older are compared to
also were collected from X-rays using sliding cali- specimens dating 6,500 years old and more recent
pers. Robusticity (relative cortical bone thickness) (see Figure 3). A decrease in bone robusticity also
was calculated using a published formula incorpo- is found, using a standard robusticity index ad-
rating medullary cavity area, cortical bone thick- justed for body size referred to as J/stand (Trinkaus
ness, and element size adjusted for estimated body et al. 1994). Again, this decrease in bone robusticity,
mass (Trinkaus et al. 1994). Data were tested using or gracilization, of the metapodials occurs between
ANOVA and Pairwise post-hoc Bonferroni/Dunn 8,000 B.P. and 6,500 B.P. in concert with the change
tests. A confidence level of 0.05 was set for all in body size and metapodial shape (Figure 4).
statistical tests.
DISCUSSION
RESULTS
In general, the pattern of morphological change
Data from the Southern Plains indicate a dif- on the Southern Plains is quite different than that
ferent pattern of size change than that seen on the reported on the Northern Plains. The Southern Plains
Northern Plains. The Southern Plains pattern is one pattern is best summarized as relative stasis before
of relative stasis in body size before 8,000 B.P. and 8,000 B.P., a rapid change in size, shape, and
following 6,500 B.P. (Figure 2). A rapid decrease robusticity between 8,000 B.P. and 6,500 B.P., fol-
in size occurs between 8,000 and 6,500 B.P. This lowed by stasis after 6,500 B.P. Bison reach their
pattern is found in both sexes and for both metatar- modern form on the Southern Plains, then, some-
sals and metacarpals. No statistically significant time in the 1,500 year period between 8,000 B.P.
difference exists between the populations 8,000 and 6,500 B.P., well before the appearance of mod-
years old and older, nor does a significant differ- ern morphology in Northern Plains bison at ca.
ence occur between those populations 6,500 years 5,000 B.P. (McDonald 1981; Wilson 1975). The
old and more recent. Pairwise tests find that the extremely rapid shift in Southern Plains metapodial
morphology also contrasts starkly
with the gradual pattern of change
reported for Northern Plains bison
(Wilson 1974a). With the pattern
of morphological change on the
Southern Plains established, vari-
ables associated with both the hu-
man hunting hypothesis and the
climatic change hypothesis can be
examined.
The period of rapid change in
bison did not correlate with an in-
crease in population pressure
(Haynes 2002; Meltzer 2000). Bi-
son were hunted intensively on the
Southern Plains both before and
after this change without any sig-
Figure 2. Change in metacarpal length for male and female bison for the nificant change in bison morphol-
Lubbock Lake populations. ogy. Also, no selection against200 Texas Archeological Society
dramatic change occur in tool
technology (Frison et al. 1976;
Johnson 1997). While it may be
argued that the change from
Paleoindian to Early Archaic tools
constitutes a major change in
technology, when undeniable
paradigmatic shifts in hunting
strategy and tool type do occur
(e.g., the introduction of the bow
and arrow, guns, and horses) no
change in bison morphology is
found. Likewise, increased preda-
tion from humans should be
associated with increased herd
movements. Such increases in
movement would increase the
Figure 3. Change in metacarpal shape as based on log length (lnL) to log bending forces and, therefore, be
mid-shaft width (lnW) measurements for ancient bison (Ba=8,000 B.P. and associated with more robust bones.
older) and modern bison (Bb=6,500 B.P. and more recent). Just the opposite is found, as bone
robusticity decreases sharply with
the decrease in body size. No
large animals occurred during the period of change, obvious mechanism is apparent that would connect
as kills continue to have females, juveniles, and changes in bison morphology with human hunting
males, and exhibit similar standard deviation levels based on the variables examined here.
for metric variables. Selection against large indi- In general, the changes in bison morphology
viduals also would not explain why a change in appear to be explained better by the changing cli-
shape and robusticity of the metapodials occurs. mate. Body size does decrease with warming tem-
McDonald (1981) suggests that the change in peratures, as would be expected (Wilson 1975). A
size was not driven by selection of large animals, correlation exists between metapodial shape and
but that smaller bison would have had an advantage vegetation density in modern bovids, whereby
over larger bison. Smaller bison
supposedly would be better able to
escape and to breed more quickly.
At issue with this hypothesis, how-
ever, is the fact that humans were
not the primary predator of bison
(Fuller 1959; Lott 2002; McHugh
1972). While humans were hunt-
ing bison during the period of rapid
morphological change in the lin-
eage, wolves remain their primary
predator. It does not follow from a
natural selection point of view for
bison to become smaller in order
to escape their secondary predator
if such a change increased the like-
lihood of their being killed by their
primary predator.
No major shift in hunting
strategy precedes the change in Figure 4. Boxplot of log J/standard (J/stand) for male metatarsals, estimating
bison morphology, nor does a bone robusticity. Median, 75th percentile, and 90th percentile are depicted.Lewis et al.—The Evolution of Bison bison 201
species associated with denser vegetation typi- (Fredlund 2002; Fredlund et al. 2002, 2003) offer
cally have relatively wider metapodials than those a possible explanation for both the decrease in
in areas where vegetation is sparse (Scott 1985). body size and in robusticity. A dramatic and rapid
As the amount of brushy vegetation on the South- shift from C3 to C4 grass occurs on the Southern
ern Plains decreased, bison metapodials become Plains beginning at ca. 10,000 B.P. and completed
thinner. Lastly, given the earlier date of modern by 8,000 B.P., just when changes in bison mor-
morphology on the Southern Plains than on the phology begin to appear in the fossil record. C4
Northern Plains, the pattern of morphological short grasses have an advantage over C3 long
change spreads south to north along with the grasses under warm Holocene conditions, as C3
temperature gradient. grasses are less tolerant of warm, dry conditions.
When changes in temperature and rainfall are The decrease in bison body size begins at approxi-
examined with the changes in bison morphology, mately the same time that tall grass (C3) ecosys-
however, several shortcomings of the climatic hy- tems begin to give way to short grass (C4) ecosys-
pothesis become evident. Based on both floral and tems on the Southern High Plains in the early Ho-
faunal changes (see Johnson 1987), the late Pleis- locene. Additionally, both C4 grasses and smaller
tocene and early Holocene are characterized by a bison spread northward during the Holocene. Once
warming and drying trend on the Southern Plains. C4 grasses become dominant on the Southern High
This trend peaks at ca. 7,000 to 5,000 B.P., during Plains ca. 8,000 to 7,500 B.P., C3 grasses never
the deposition of stratum 3 at Lubbock Lake (Fig- rebound despite the cooling climate of the late Ho-
ure 5), and is followed by a return to cooler, moister locene (Fredlund 2002; Fredlund et al. 2002, 2003).
conditions. Bison morphological change, therefore, The timing and pattern of the short grass eco-
occurs as the climate reaches its warmest and driest system’s spread follows that of all morphological
conditions. But, if bison morphology were tracking changes in bison metapodials on the Southern
closely with temperature and rainfall, then the de- Plains. The productivity and nutrition of C4 grasses
crease in body size would have begun much earlier may explain why the transition in grass types
when the climate first began to change. While the would cause such a suite of changes in bison mor-
warming and drying trends appear to explain the phology. C4 grasses are more productive per acre
progression toward relatively smaller bison in the than are C3 grasses (Howe 2000). With greater
late Quaternary, it does not explain the period of productivity, bison would not be required to range
rapid change between 8,000 and 6,500 B.P. as far to find similar amounts of food. Such a
Likewise, if the morphological changes during change in grass types may have led, therefore, to a
this period were strictly pheno-
typic responses to changing tem-
peratures, then they should have
reversed when the conditions
cooled after ca. 4,500 B.P. (stra-
tum 4 at Lubbock Lake). Also, the
decrease in metapodial robusticity
during the late Quaternary is in-
consistent with a drying, degrad-
ing habitat where bison would
have been forced to range further
to find adequate food. This in-
creased movement would have in-
creased bending forces on the skel-
etal elements of the limbs and led
to more robust metapodials, not
less robust metapodials. The cli- Figure 5. Changes in temperature (T, grey points) and precipitation (P, black
matic model, as it stands, fails to points) for the 5 strata at Lubbock Lake. Stratum 1 dates from ca. 11,500-
address these inconsistencies. 11,000 B.P., stratum 2 dates from ca. 11,000-8,000 B.P., stratum 3 dates
Recent phytolith analyses from ca. 8,000-5,000 B.P., stratum 4 dates from ca. 5,000-1,000 B.P., and
from Southern Plains localities stratum 5 dates from ca. 1,000 B.P. to today (Johnson 1987).202 Texas Archeological Society
decrease in herd movements and a subsequent re- environment rather than hunting pressure from humans.
duction in range during the late Quaternary. This
reduction in movement patterns would have led to
decreased bending forces on the skeletal elements ACKNOWLEDGEMENTS
of the limbs and ultimately to less robust meta-
podials. The pattern of robusticity change would The authors thank Dr. Leland Bement and
be a rapid shift during the period of transition Kent Buehler (Oklahoma Archeological Survey)
from C3 to C4 grasses, followed and preceded by for access to Cooper and Certain site materials,
relative morphological stasis. their help in collecting data, and their insights into
Despite higher productivity, C4 grasses pos- bison evolution; and Dr. Glen Fredlund (Univer-
sess a larger cell wall and less protein than their sity of Wisconsin-Milwaukee) for sharing data.
C3 counterparts, and are thus less nutritious (Howe Research was funded by the Museum of Texas
2000). Such a change in nutritional quality may Tech University. This manuscript represents part
have selected for bison with smaller body sizes, as of the ongoing Lubbock Lake Landmark regional
smaller mammals generally require absolutely research into late Quaternary adaptations to eco-
fewer calories and have reduced protein demands logical, climatic, landscape, and cultural changes
(Calder 1984). Larger mammals have higher total on the Southern Plains.
rates of metabolism than do smaller species, and
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