We begin by graphing the greatest length measurements of 1189

*Bison*calcanea over the past 40,000 years (younger on the right, older on the left). If you're interested why we choose the calcaneum, please go to 'How it Works'. Next, we add a smoothing trend-line that is a moving average; it averages 20 of the nearest neighbors for each measurement.

Next, we graphed existing oxygen isotope data from the Greenland Ice Sheet Project 2 (GISP2, Grootes and Stuvier, 1997). Using air bubbles that were permanently sealed in the ice, the GISP2 reconstructs the temperature at the time that the ice formed. Year after year, more bubbles are sealed with increased ice accumulation to create a timeline of temperature changes. The Greenland ice holds 800,000 years worth of data, but we are only interested in the past 40,000 years. In this graph, the older age is on the left and the younger age is on the right. Notice that the warmer temperatures are concentrated in the most recent years.

If we rotate the vertical axis as shown below, it begins to look similar to our first bone measurement graph.

When both data sets are incorporated in one graph, there is a strong correlation between temperature and body size, which is represented by the calcanea here. This relationship is well-correlated along the black trend line, including at the peaks and valleys. We acknowledge that there are considerable gaps in the bison data, but the overall trend follows the temperature. To our knowledge, this is the most complete record of body size reconstruction using only one bone, so as not to compound error.

Now that a tentative correlation is established, we then remove time from the data and only focus on temperature and body size. From the graph below, we see a linear relationship supporting that as temperatures increase, body sizes shrink. With a linear relationship, we can also predict the average body mass for bison, which is indicated on the right side of the graph below.

Now that a tentative correlation is established, we then remove time from the data and only focus on temperature and body size. From the graph below, we see a linear relationship supporting that as temperatures increase, body sizes shrink. With a linear relationship, we can also predict the average body mass for bison, which is indicated on the right side of the graph below.

A table of references can be found here.