Intra-annual bison body mass trajectories in a tallgrass prairie

Climatic warming and shifts in precipitation patterns could reduce the growth and reproduction of North American bison (Bison bison). To quantify rates of body mass changes for different classes of bison grazing on native tallgrass prairie, a remote autonomous weighing system was positioned to passively record weights throughout the growing season of radio frequency identification (RFID)-tagged yearling, 2-year-old, and adult female bison. Over a 152-day period, yearling bison gained weight longer into the year than adult females, but adult females had higher intrinsic growth rate. The midsummer asymptote of weight for adult females could not be ascribed entirely to the demands of lactation. Adult females that did not have a calf were heavier than those that did, but individuals of both groups leveled off in body mass at similar times in midsummer. We also assessed dietary quality of adult females approximately every 10 days from fecal samples. Adult female weight gain decreased with decreasing dietary crude protein ([CP]) concentrations suggesting that adult female bison cease gaining weight when [CP] = 54.5 mg g(-1) and gained weight less efficiently at moderate dietary quality than expected. Broader adoption of this technique will begin to generate the data necessary to understand the effects of variation in weather and climate on seasonal patterns of large herbivore body mass.