Green, Wendy C.H. 1990. Reproductive effort and associated costs in bison (Bison bison): Do older mothers try harder? Behavioral Ecology 1. pp. 148-160.

Abstract

To examine the prediction that reproductive effort should increase with age, I compared female bison (Bison bison) of varying ages in terms of maternal behavior, reproductive costs, and the quality and quantity of offspring. In support of predictions, older mothers showed more tolerance toward and spent more time nursing their calves in months 1-5 than expected based on age differences in size and presumed milk production. The two oldest mothers, who died before calving again, appeared to make the greatest effort. When mothers were barren between calves, which occurred more often in older females, their increased investment enhanced the weight, dominance, maturation rate, and/or fecundity of previous and subsequent offspring. In contrast to predictions, however, offspring quality did not generally increase with maternal age; in fact, primiparae tended to produce larger daughters. Nursing behavior varied little in month 1, when offspring condition was apparently most affected. Young mothers contacted their calves more often and tended to graze more during the first month. Reproductive costs appeared greater for young mothers; their fecundity in years after calving was relatively low. Proportional weight loss following reproduction decreased with age, although changing growth rates complicate interpretation. The results suggest a general decrease in reproductive effort, or at least in associated costs, during the reproductive life span. This pattern may be due in part to the increased experience and dominance status of older mothers, through which they may avoid some of the reproductive costs incurred by primiparae.