WEANING AND PARENT-OFFSPRING CONFLICT - VARIATION RELATIVE TO INTERBIRTH INTERVAL IN BISON

Weaning and parent-offspring conflict were examined in bison (Bison bi son). Maternal care and interactions with offspring during the first y ear were compared between mothers that did or did not conceive 4-5 mo postpartum; unlike barren mothers, pregnant mothers ceased nursing lat e in the first year. Pre-estrus nursing bouts were shorter for pregnan t mothers; however, neither overall nursing time nor other forms of ma ternal care differed between groups. Offspring of pregnant mothers spe nt more time resting in Months 1-5; their grazing time also increased more slowly, suggesting higher rates of parental investment for pregna nt mothers. For both groups, time spent suckling decreased due to decr easing bout frequency rather than duration. As soon as mothers differe d in reproductive status, nursing time and frequency diverged dramatic ally, increasing for barren while decreasing for pregnant mothers. How ever, interactions with offspring differed little. The percentage of s uckling bouts ended by calves decreased in Months 1-9 for both groups. Frequencies of rejected suckling attempts and maternal aggression inc reased 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 confl ict increases. Such changes were greatest in Months 1-3; however, suck ling continued for 6-18 mo afterward. It appears that weaning occurs g radually, through maturational changes more than through parent-offspr ing conflict.