of the herd to protect the group. Bulls begin to serve a more active role in the herd when a danger to the group's safety appears, as well as during the mating season – when they compete with each other. Territory held by bulls is correlated by age, with young bulls aged between five and six tending to form larger home ranges than older males. The European bison does not defend territory, and herd ranges tend to greatly overlap. Core areas of territory are usually sited near meadows and water sources. Reproduction The rutting season occurs from August through to October. Bulls aged 4–6 years, though sexually mature, are prevented from mating by older bulls. Cows usually have a gestation period of 264 days, and typically give birth to one calf at a time. On average, male calves weigh at birth, and females . Body size in males increases proportionately to the age of 6 years. While females have a higher increase in body mass in their first year, their growth rate is comparatively slower than that of males by the age of 3–5. Bulls reach sexual maturity at the age of two, while cows do so in their third year. European bison have lived as long as 30 years in captivity, but in the wild their lifespans are shorter. The lifespan of a bison in the wild is usually between 18 and 24 years, though females live longer than males. Productive breeding years are between four and 20 years of age in females, and only between six and 12 years of age in males. Diet European bison feed predominantly on grasses, although they also browse on shoots and leaves; in summer, an adult male can consume 32 kg of food in a day. European bison in the Białowieża Forest in Poland have traditionally been fed hay in the winter for centuries, and large herds may gather around this diet supplement. European bison need to drink every day, and in winter can be seen breaking ice with their heavy hooves. Differences from American bison Although superficially similar, a number of physical and behavioural differences are seen between the European bison and the American bison. The bison has 14 pairs of ribs, while the American bison has 15. Adult European bison are (on average) taller than American bison, and have longer legs. European bison tend to browse more, and graze less