As the seasons change throughout the year, the size and herd dynamics of the American bison vary. These herd variations are known as seasonal aggregation or seasonal segregation---depending on the time of year. Seasonal aggregation occurs when large, male bison join mixed-age and -sex groups to form larger groups. The biggest aggregations occur during the rut, or the breeding season, when individuals are trying to find mates. In non-rut season, seasonal segregation occurs, when large bulls leave the group to form smaller, separate groups. Seasonal group size is considerably variable as forage conditions, seasonal behavior, population size, landscape composition, and environmental conditions influence group dynamics. In each of these groups, patterns of hierarchical social dominance are also well documented.
Group size increases during the summer breeding season and then rapidly decreases during the fall after the rut. Throughout the year during these non-rut seasons, bison herds vary in structure. Adult females, calves, and immature males will form mixed-age and -sex groups and large bull males will form separate groups. Research infers that males segregate from females because they require more abundant forage due to their large body sizes. They will temporally separate to use the same grasslands as females but at different times.