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Abstract

Weaning and parent-offspring conflict were examined in bison (Bison bison). Maternal care and interactions with offspring during the first year were compared between mothers that did or did not conceive 4–5 mo postpartum; unlike barren mothers, pregnant mothers ceased nursing late in the first year. Pre-estrus nursing bouts were shorter for pregnant mothers; however, neither overall nursing time nor other forms of maternal care differed between groups. Offspring of pregnant mothers spent more time resting in Months 1–5; their grazing time also increased more slowly, suggesting higher rates of parental investment for pregnant mothers. For both groups, time spent suckling decreased due to decreasing bout frequency rather than duration. As soon as mothers differed in reproductive status, nursing time and frequency diverged dramatically, increasing for barren while decreasing for pregnant mothers. However, interactions with offspring differed little. The percentage of suckling bouts ended by calves decreased in Months 1–9 for both groups. Frequencies of rejected suckling attempts and maternal aggression increased in Months 2–4 but not subsequently. Contrary to the expectation that parent-offspring conflict is associated with the end of nursing, maternal aggression and nursing interruptions decreased in frequency with time. Because changes in interactions were gradual, there was no clear weaning period, i.e. when maternal care decreases most and conflict increases. Such changes were greatest in Months 1–3; however, suckling continued for 6–18 mo afterward. It appears that weaning occurs gradually, through maturational changes more than through parent-offspring conflict.