European bison have an extremely similar social system to their American relatives, and are believed by some to represent the same species (7). Outside of the mating season the males form bachelor herds, whilst females are found in maternal groups of around 20 individuals and their young (2). These female groups occupy vast home ranges of up to 100 km2 and are led by a dominant cow (5). The 'rutting' season takes place between August and October and during this time males join up with the female herds and compete for access to receptive females (5). Once he has found an appropriate cow the bull will attempt to separate her from the remaining herd and particularly from the advances of other males in the group (3). He will attend to her in this way for around a day before mating (3). Aggressive clashes between rival males may occur at this time. Cows give birth after a gestation period of about 264 days, typically in May and July, usually leaving their herd to do so; young calves are able to run after only a few hours of being born and are fully weaned at around one year old (5). Bison feed predominantly on grasses although they will also browse on shoots and leaves; in summer months, an adult male can consume 32 kilograms of food in a day (5). Bison in the Bialowieza Forest in Poland have traditionally been fed hay in the winter for centuries, and vast herds may gather around this diet supplement (5). Bison need to drink every day and in winter can be seen breaking ice with their heavy hooves (2). Despite their usual slow movements, bison are surprisingly agile and can clear three metre wide streams from a standing start (2).