Trade-offs between growth and reproduction in female bison

The extent to which (a) larger females produce more or larger offspring, (b) reproduction imposes costs, and (c) the benefits of early reproduction outweigh the costs was examined for female bison (Bison bison) with data from an 8-year period. Description of ontogenetic patterns indicated that reproductive performance reached a plateau near the end of the growth period; changes with age in fecundity were little related to those of body weight during the typical reproductive lifespan (age 3-18 years). Examination of inter-and intra-individual variation showed that the relationship between growth and reproduction depends on the age of subjects. Larger juveniles matured earlier and were more fecund (though smaller) as adults. In contrast, post-pubertal growth was negatively related to reporductive performance, although intra-individual weight variation was positively associated with pregnancy rate. Fecundity was not generally lower in years after reproduction than after barren years, in spite of weight loss, suggesting that reproduction did not impose significant fitness costs. On the other hand, offspring quality was lower when mothers reproduced in successive rather than alternate years. Early maturation resulted in immediate costs, including reduced growth and infertility in the year after first parturition. However, there were long-term benefits: fecundity in the first 7-9 years was highest for early-maturing and lowest for late-maturing females, suggesting that reproductive success is greater for early-maturing females despite reduced growth. In the present study, trade-offs between growth and reproduction did not appear to reflect substantial costs as measured by long-term reproductive success.