The Incredible Shrinking Bison

Toward the end of the 19th century the bison faced extinction by extermination.  Today, even after more than a hundred years of restoration efforts, the plains bison is faced with another threat of extinction—the accelerated warming of the Great Plains.

The Plains Bison (Photograph by Kailyn Komro, West Bend, WI.  (

Even before the immense public attention on climate change, there has been great scientific interest in climate processes and extinction events in the Earth’s natural history.    Evaluation of fossil evidence has shown an inverse correlation between warming trends and body size and mass of large mammals.  As temperatures rise, body size shrinks over large geological time scales. Along with this negative correlation a consequent, positive correlation has been established between shrinking body size and extinctions [1].  

The warming trend, which began at the end of the last Ice Age, has been accelerating in recent decades. [2].  Since the beginning of the 21st century the northern Great Plains’ average summer temperature increased by 0.8˚C while for the southern Great Plains the mean summer temperature rose by 0.4˚C with winters rising by 0.25˚C for both the southern and northern Great Plains [3]. Consequently, the IPCC (Intergovernmental Panel on Climate Change) Working Group 1 predicts a 4˚C increase in global temperatures by 2100 over the 20th century—a period of 100 years.   This rate of temperature change is much greater than for the Bolling-Allerod period [4]—a warming period 14,700 to 12,500 years ago with a mean temperature 6˚C cooler than that for the 20th century.

The evolutionary history of bison has shown an absolute increase of 4˚C is not unprecedented.  However, the time frame in which the bison has had to adapt needs to be considered. From the end of the Last Glacial Maximum (approx. 14,700 years ago) to the 20th century the earth warmed 6˚C. During the Last Glacial Maximum, bison mass was, on average, approximately 910 kg. (2006 lbs.). The greatest decline in body size of 26% occurred between 12,500 and 9250 years ago. Given a generation time between 3 and 10 years, the change in body size occurred in 325 to 1080 generations, producing an average rate of change of 0.2 to 0.7 kg per generation.   If the current warming trend continues as predicted for the 21st century, bison body mass will likely decline from 665kg (current average body mass) to 357kg.  It is unclear whether bison can adapt their body size to a 4˚C temperature increase within 10 generations [5].

Changes in body size and mass of animals have long been used to indicate large-scale environmental processes over geological time scales, and have become predictors of extinction risk in mammals [6].  In regard to bison B. antiquus and B. occidentalis, these species did go extinct, but through phenotypic [7] and morphologic [8] adaptation to changing climatic conditions, they evolved into what is known today as the North American bison (Bison bison) which has existed throughout the Holocene epoch-the current geological epoch. The importance of body size in dictating extinction proneness is likely due to the fundamental association between size and other key life history traits such as fecundity, longevity, mating system, trophic level (step in a nutritive series, or food chain), dispersal ability and energetic requirements [9].

Bison Size Comparison (from

Fossil bison shrank with global warming probably because large-bodied grazers are disadvantaged both by heat dissipation and by the phenological [10] shifts in plant quality and abundance in warming conditions [11].  Impacts of climate change, then, are two-fold: 1) direct effects of temperature on the animal, demanding energy to compensate for heat, and 2) indirect effects of temperature on the animal’s food supply [12].

Maximum body size of endotherms–an animal that is dependent on or capable of the internal generation of heat; a warm-blooded animal—depends on optimal maintenance for the efficient production of tissues.  This is especially true in seasonal environments when food availability and environmental demands constrain the annual windows for growth.   Optimal maintenance is dependent on thermal loads (amount of heat energy).  High thermal loads increase cost of body maintenance to balance internal and external loads through thermoregulation, which reduces energy for growth.

Thermoregulation is the mechanism by which heat balance is achieved.  It affects the use of energy, water and nutrients such as electrolytes and organic nitrogen which affect resting and foraging behaviors. Thermoregulatory processes usually increase energy use by increasing heart rate and blood flow.  In hot weather thermoregulation increases the flow of body water because water is used for evaporative cooling (e.g., panting, perspiration).  In cold weather thermoregulation generates body heat through such efforts as shivering, increased metabolic heat production, and muscular activity in an effort to conserve core body heat through control of blood flow to the periphery [13].

The negative climate-body size correlation, then, reinforce feedbacks that may increase extinction rates [14].  Both excessive heat (> 40˚C) and excessive cold (< -30˚C) directly increase demands for energy, water and nutrients because thermoregulation outputs increase, whereas indirect effects of rising temperature decrease forage quantity and quality—ultimately affecting the supply of energy, water and nutrients [15].  Smaller body size, then, is more efficient in regulating increased thermal loads due to rising temperatures.

Conceptual model of the direct and indirect effects of elevated ambient temperature on body size of Bison bison from Martin, et. al., 2018.

In regard to food supply, climatic warming tends to exacerbate nutritional stress and reduce weight gain in large mammalian herbivores by reducing plant nutritional quality.  Warming trends have the potential to not only reduce the nutritional quality of plant species, but also by decreasing the relative abundance of nutritionally critical plant species.  For the North American plains bison this is likely to result in an increase protein stress, reducing bison growth and reproduction [16].

Compounding the issue of decreased nutritional quality of grasses, the warming trends have resulted in an increase of droughts in the Great Plains.  The lack of water availability reduces the availability of critical plants necessary for bison growth.  Consequently, droughts cause declines in the number and body size of bison [17].

One of the driving factors in the rising temperatures may be the increasing CO2 concentrations which reduce plant protein concentrations in grasslands [18].  Increasing atmospheric CO2 concentrations have been causing Nitrogen to become progressively more limiting to ecosystem productivity.  Nitrogen is a crucial element for many structures and metabolic processes in plants. Plants are required to manufacture the complex molecules by use of minerals from the soil that contain nitrogen such as nitrate ions. Plants too, like animals, need some important macro and micro nutrient elements including nitrogen, oxygen, hydrogen and carbon to keep them healthy. The wellness of plant parts (leaves, roots, trunks, etc.) depends on the availability of essential nutrients like nitrogen to enhance the plant’s biological processes including growth, absorption, transportation, and excretion [19].

Science has offered information and theories concerning the effect of warming trends on the size and survival of bison.  The question for us is: how do we respond?   The Great Plains are predicted to warm, resulting in longer, hotter summers accompanied by more severe droughts.   The anticipated warming and drying along the Great Plains will shift the distribution and protein efficacy of vegetation types by mid-and-late century, altering the supply of digestible energy and digestible nitrogen to bison, native wildlife and domestic livestock [20].  Bison are very good at adapting to shifts in environmental processes given the rates of change in the past.  But with the acceleration of warming rates, their adaptive ability comes into question.

With decreasing body mass life history traits that are dependent on body mass will also shift. Age of maturity, reproduction rates and growth rates will be reduced.  Preliminary data already indicate a decrease in the life span of female bison, reducing reproductive rates [21].

In response, there are ways to mitigate the observed effects of the climate shifts on bison according to Dr. Jeff Martin— an integrative conservation ecologist.  Prescribed burns to the land to boost available energy and protein in grasses are one example.   More generally work is needed to determine how best to create landscape heterogeneity for bison to select the best available forage [22].

To achieve such a goal, management questions arise.  For instance, bison diet remains poorly understood which limits the ability to determine the plant species most critical, and consequently prohibits a full understanding of the required management of dietary needs.  Plains bison are considered strict grazers.  This implies they primarily consume grasses and grass-like flowering plants—such as sedges—as opposed to browsing on forbs, shrubs or trees (woody species).  Being strict grazers would suggest that climatic warming may reduce bison performance by altering the productivity and nutritional quality of different grass species. However, earlier analyses may have overemphasized the contribution of grasses and underemphasized the amount of herbaceous and woody species in their diet.  Recent studies have suggested that bison utilize eudicot species to some degree.  If eudicot species constitute a critical component of bison diet, then managers will need to take into account the relative abundance of these and their nutritional quality when considering mitigation strategies [23].

Bison have been wonderfully adaptive to environmental and climatic changes over the course of their history. Until recent times, though, they have had great expanses of time to acclimate to new conditions.  The recent accelerated warming trends have placed another hurdle in their evolutionary path—a shortened time frame in which the species has to respond.  It is unclear whether the species will be able to offset the induced biological stress with a shift in body mass in the allotted time. It is highly unlikely the climate shift underway can be halted or reversed.  Mitigation efforts, then, need to focus on land management to provide the requisite forage.  This will, however, require additional studies and the implementation of known effective practices. 

Simply restoring bison numbers is not enough. To ensure the survival of this keystone species, land and vegetation management practices which will mitigate the current climate effects need to be developed.  

End Notes:

[1] Isaac, Joanne L. 22-May-2008. Effects of climate change on life history: Implications for extinction risk in mammals.  Endangered Species Research. Vol. 7:115-123, 2009.

[2] See IPCC-AR5, 2013; USGCRP, 2018.

[3] Martin, Jeff M. Perry S. Barboza. 06-Dec- 2019. Decadal heat and drought drive body size of North American bison (Bison bison) along the Great Plains. Wiley. Retrieved 19 Oct 2020.

[4] The transition of from the last Glacial Maximum (12,500 years ago) to the Holocene

[5] Martin, Jeff M., Jim I. Mead., & Perry S. Barboza. 10-Apr-2018. Bison body size and climate change. Wiley.

[6] Isaac.

[7] Pertaining to the appearance of an organism resulting from the interaction of the genotype and the environment—Webster’s

[8] Pertaining to the form and structure of an organism considered as a whole—Webster’s

[9] Isaac

[10] Pertaining to the influence of climate on the recurrence of annual phenomena of animal and plant life—Webster’s Unabridged Dictionary of the English Language. 2001. Random House.

[11] Craine, J. M., Towne, E. G., Joern, A., & Hamilton, R. G. (2009). Consequences of climate variability for the performance of bison in tallgrass prairie. Global Change Biology15(3), 772– 779.  See also

Martin, Jeff M., & Perry S. Barboza. 06-Dec-2019. Decadal heat and drought drive body size of North American bison (Bison bison) along the Great Plains. Wiley.

[12] Martin, Jeff M., Jim I. Mead., & Perry S. Barboza. 10-Apr-2018. Bison body size and climate change. Wiley.

[13] Martin, Jeff M. Perry S. Barboza. 08-Jul-2020. Thermal biology and growth of bison (Bison bison) along the Great Plains: examining four theories of endotherm body size. ESA Journals.

[14] Isaac.

[15] Martin 2019.

[16] Craine, Joseph M. E. Gene Towne, Mary Miller & Noah Fierar. 16-Nov-2015. Climatic warming and the future of bison as grazers. Nature.

[17] Craine, J. M., Nippert, J. B., Elmore, A. J., Skibbe, A. M., Hutchinson, S. L., & Brunsell, N. A. (2012). Timing of climate variability and grassland productivity. Proceedings of the National Academy of Sciences109(9), 3401– 3405. Retrieved 19 Oct 2020. Also Martin, Jeff M., Perry S. Barboza. 06-Dec-2019.

[18] McKauchlan, K.K., Ferguson, C.J., I. E. Ocheltree, T. W. & Craine, J.M. 2010. Thirteen decades of foliar isotopes indicate declining nitrogen availability in central North American grasslands. New Phytol, 187, 1135-1145.

[19] Tajir, Amir. 02-Nov-2016. What’s the function of Nitrogen (N) in plants? Greenway Biotech.  Retrieved 19-Oct-2020.

[20] Martin, 2020.

[21] Martin,, 2019.

[22] Kobilinsky, D. 16-Dec-2019. Droughts and high temperatures are shrinking bison. The Wildlife Society.  See also Jeff Martin’s website

[23] Craine, et al., 2015.