More bison data are rolling in off the field scale. At Konza, we set up a scale for bison to walk over and weigh themselves. So far, they seem to be obliging.
As to the data, first, are examples of a 3-year-old cow (red) and a 2-year-old cow (blue). These are the individuals that we've had the most data for. You can see that over ~60 d they've put on almost 100 kg. As far as we know, neither of these animals have had a calf this year. Although the sample size is small here, it looks like weight gain has started to level off here.
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| Bison mass (kg) vs. day of year for 2 cows at Konza. |
With 90 calves tagged last year, we have a lot more data for this year's yearlings. If I standardize for individual variation in weight among animals (for those animals that have enough data to look at trajectories), this is the general trajectory. More or less linear increases in weight over the past 60 d. About 80kg of weight gain.
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| Residual weight vs. day of year for yearlings at Konza. |
For everything we know about seasonal patterns of dietary quality, weight gain should be leveling off right about now.
This technique is going to be pretty important in order to understand seasonal weight dynamics and eventually how climate change will affect grazers in grasslands. For example, we see that bison in the north reach greater size than those in the south. Is this because they have a longer period of weight gain, or gain more weight at a given time? As it gets warmer, what mechanism will climate affect our grazers?
How much weight different animals will gain in the next month is going to be super interesting.
[Note: the average yearling mass as of July 12 is approximately 245 kg. If you look at the 2013 Oikos paper, average yearling mass in the October roundups ranged among years from 234.1–281.6 kg. That means, if this was a typical year the animals would gain just another 20 kg, but they might put on another 35 kg.]
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