European bison live in small social groups. Cow groups consist of female animals and their offspring. The group is led by an old female. She decides on where to go and what to do: rest, ruminate, graze or drink.
When a calf is about to be born, its mother retreats to a spot she has chosen in advance. When the calf is born, the mother licks it untill it is dry and helps it find her nipples. At birth the calf weighs only 30 kilograms. After a few days the calf is strong enough to walk along with the herd and then mother and calf will join the group again. The young animals grow up in the cow group, where the daughters are to stay permanently. So a cow group consists in fact of a ruling grandmother, her daughters and granddaughters and their young. At the age of 3 or 4 a cow gives birth to her first calf. If there are more calves in a group, they will rest in a nursery watched over by an older animal.
Young bulls will leave during adolescence and join a bull group. At the age of 4 they are grown up. Bulls and bull groups have home ranges which may partly overlap. The hierarchy among them is established by fights and other behaviour to impress other males. Particularly during mating season the bulls look for the cow groups. Dominant bulls then do not tolerate any competitors in their neighbourhood. The strongest bulls have territories at sites used by several cow groups. Less dominant males live beyond these areas. Young bulls or very old animals avoid dominant bulls and live in home ranges hardly visited by cow groups. A female decides which bull will be her mate.