Abstract - Effects of Forced Weaning on Mother-Daughter Associations and Reproduction in Bison (Bison bison)
Brookshier, Jennifer S. 2000. Effects of Forced Weaning on Mother-Daughter Associations and Reproduction in Bison (Bison bison). M.S. Thesis. Utah State University, Logan, Utah. 64 p.
While the social organization of ungulates has received much attention, the nature and extent of the mother-daughter association is still not fully known for some species. In bison, mothers and daughters have a long-term association that may extend beyond the age of weaning. This association is believed to facilitate the daughters’ social integration into the herd. I compared the mother-daughter association for 29 yearling females that were forcibly weaned from their mothers to that of 19 yearling females that were naturally weaned. Forcibly-weaned yearlings did not reassociate with their mothers following winter separation, while naturally-weaned yearlings associated with their mothers significantly more often than with randomly-assigned control cows. Naturally weaned yearlings benefited from this mother daughter association by incurring less aggression from other herd members, and by spending more time in the center of the group rather than the periphery, where individuals are more at risk of predation. Forcibly-weaned yearlings were more likely to be nearest neighbors with other yearlings, and some forcibly-weaned yearlings developed cohesive partnerships with each other. Naturally-weaned yearlings spend more time near other cows compared to forcibly-weaned yearlings.
There were no significant differences between forcibly-weaned and naturally-weaned yearlings in terms of growth rates and age at first reproduction. Though not quite significant, naturally-weaned daughters did not appear to adjust gestation length. The year following forced weaning of their previous calves, calving dates were no different for mothers of forcibly-weaned daughters than for mothers of naturally-weaned daughters. Furthermore, subsequent weight gain, and calving and pregnancy rates were no greater for mothers of forcibly-weaned daughters compared to mothers of naturally weaned daughters. If forced weaning is having an effect on the reproduction of the herd, it was not long enough to be detected with this sample size. Give the social disadvantages of forced weaning for yearling daughters, forced weaning may not be an effective management tool for increasing productivity in this herd.