Maternal investment and sex ratio adjustment in American bison calves

Summary Cow-calf behavior was observed in American bison (Bison bison) to determine if mothers invested differentially in sons and daughters. Cows nursed sons significantly longer than they did daughters in their first three months of life. The increased nursing time for sons was not compensated for by increased grazing time by their mothers. Grazing and activity patterns did not differ significantly between sons and daughters. Cows that had sons bred later in the breeding season than nulliparous cows, barren cows, or cows with daughters. Nine yearling sons compared to only two yearling daughters continued to suckle from their mothers for up to 15 months of age. Cows that had sons the previous year were more apt to be barren in the current year than cows that had daughters in the previous year.