In October (2015), I presented some of my research on Bison body adaption over the past 40,000 years at the Society of Vertebrate Paleontology conference in Dallas, Texas. Many folks came by my poster to discuss questions and offer opinions and feedback that were very helpful and useful to my studies. Thank you to those who came by!
I thought it would also be of great use for other people who could not make the conference to see what I am working on. Also, please feel free to comment below any alternative interpretations.
Post-cranial morphometric evaluation of the genus Bison: 40,000 years of body size change in response to abrupt temperature shifts, with implications for the Bison industry
Jeff M. Martin and Steven C. Wallace
University of Maine and East Tennessee State University
Influence on the body size of Bison has been forced prehistorically by climate, and historically by anthropogenic selection for breeding. Specifically, bison body size appears to be diminishing due to increasing temperatures and/or abrupt climate change. High resolution relative temperature changes occur within centuries and decades in many cases, as understood by the GISP2 ice core project research. Unfortunately, previous Bison studies have not evaluated direct responses from temperature changes during the late Pleistocene, when average global temperatures were 4°C cooler than today.
Furthermore, IPCC AR5 report forecasts temperature increases between 1°C - 4°C by year 2100. Consequently, six calcaneal linear metrics were used as a proxy to body mass from 53 fossil and archaeologic sites across the United States; with approximately 1132 specimens over a 40,000 year period. Adaptation response of bison body size is: larger during cooler periods and smaller during warmer periods. Correspondingly, Bison inversely adapted to these temperature changes approaching century resolution.
Preliminary data suggest for each 1°C of temperature change, average body mass changes by approximately 6%; placing total body mass loss from ∆4°C at 25% of modern body mass (~907 kg = 2000 lbs), or an estimated maximum body mass drop to 720 kg (1500 lbs).
Conventionally only distinguished by allometric size differentiation, the specific classification of Bison bison (Linneaus, 1758) and B. antiquus (Leidy, 1852) was also tested to understand the legacy phenotypic evolutionary adaptive capacity of Bison. Principal component and discriminant function analyses of both unstandardized and standardized metrics suggest that Bison bison and Bison antiquus are a conspecific evolutionary chronospecies. When abiotic climate is factored, trends appear.
As a result, we also propose that the two taxa be considered conspecific (Bison bison, Linnaeus, 1758).
What is a species?
It turns out that the definition that you use to define a species is critical to the assumptions that are built into your definition. I prefer to use a blend of the ecological, evolutionary, and phenotypic definitions over the others. More is available from: De Queiroz, K. (2007). Species concepts and species delimitation. Systematic Biology, 56(6), 879–86. http://doi.org/10.1080/10635150701701083.
The data I used was produced from 7 linear measurements (Figure 1) on 1132 calcanea from over 53 sites (Figure 2) across the U.S., Canada, and Mexico.
Figure 1. Calcaneum: A) Dorsal view: (DstBr) distal breadth of the tuber, (Min) minimal breadth of the tuber, (GBr) greatest breadth of the articular end from the tibial articulation to the medial end of the sustentaculum. B) Medial view: (GL) greatest length, (DstL) distal length of the tuber, (DstDp) distal depth of the tuber, (GDp) greatest depth of the proximal end from the tibial articulation to the plantar tuberosity. Scale = 10cm.
As Chris Bell (paleontologist and anatomist, Univ. Texas at Austin) would ask, “What are your apomorphies (defining characters that separate species from each other)?”
Well, to be honest, Bison bison and Bison antiquus don’t really have solid apomorphies distinguishing each, other than absolute size and horn core morphology.
In fact, they have more synapomorphies (characters that unify a species).
Climate as driver for post-cranial phenotypic appearance
Many authors have addressed morphological variance in Bison, however, none have integrated temperature as an indirect proxy to allometry. Using GISP2 ice core temperature proxy data (δO18), correlations are apparent (Figure 3).
Figure 3. The green dots are individual length measurements of the calcaneum (hock) and the black line is the average these lengths. The red/blue squiggle line is the temperature proxy from the Greenland Ice Core Project. According to this research, bison were larger when it was colder, such as during the Ice Age,(more than older than 11,500 years ago). Notice the sharp decrease at approximately 800 years ago, which was the time of the Medieval Climate Anomaly (a.k.a. Medieval Warm Period) period followed by a sharp increase in size during the Little Ice Age (300-500 years ago). Bison were remarkably small then compared to today’s animals. There is a strong correlation between the average length of the calcaneum and the temperature. HCO: Holocene Climatic Optimum, YD: Younger Dryas, B/A: Bølling/Allerød, OD: Older Dryas
Correlation and Causation...
It's difficult to assign a causation, but the correlation seen in Figure 4 would suggest a strong affect on body size of Bison from climatic change. To further test the correlation, creating a graph that plots relative temperature at a certain fossil site against the absolute size of the calcaneum, the trend-line elucidates the pattern of as temperature increases, bison shrink.
As skeletal components of an organism diminish, the relative mass of the animal decreases. This has major implications for bison industry meat production. Broadly, this extends to the cattle industry.
IPCC AR4 outlines two scenarios for average global temperatures increasing. One at 1˚C by year 2100, and 4˚C by 2100. The Ice Age, on average, was 4˚C colder than it is today.
Assuming a linear relationship, Bison will continue to diminish its body size and therefore diminish production of meat per animal.
I would like to thank Rachel Martin for helping create many of the figures used here. She has also offered many comments and edits for the improvement of this research. Jim Mead , Sandy Swift, Wally Wallace, Chris Widga, Eric Scott, Greg McDonald, and many of my peers were instrumental in producing the information, interpreting the data, and assisting in visualization.
Leidy, J. 1852a. Bison antiquus. Proceedings Academy of Natural Science 6: 117-118.
Leidy, J. 1852b. Memoir on the extinct species of American ox.
Van Zyll de Jong, C. G. 1993. Origin and geographic variation of recent North American bison. Alberta 3:21–35.
Marsolier-Kergoat, M.-C., P. Palacio, V. Berthonaud, F. Maksud, T. Stafford, R. Bégouën, and J.-M. Elalouf. 2015. Hunting the Extinct Steppe Bison (Bison priscus) Mitochondrial Genome in the Trois-Frères Paleolithic Painted Cave.
Poster as a whole