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Wild ruminants living in the moderate climate conditions follow periodical breeding. Calves are born during the period most favourable for their development, i.e. at spring time. The periodic character of the breeding process is the result of the physiological mechanisms, including the length of the pregnancy, but also internal population mechanisms, e.g. separation of both sexes except for the breeding period. Bulls do not head the herds, which is understandable since their presence in the herds is only temporary. Bison males reach sexual maturity at the age of 2 or 3. Young bulls from free roaming populations, at the age of 4 – 6, are not allowed to reproduce because of the presence of older bulls. The period of intensive sexual activity of the bulls is relatively short, since it starts at the age of 6 and ends at the age of 12. Old bulls typically have to give way to younger competitors, and their participation in the breeding process is limited. Breeding period for bison typically last for August and September. Adult bulls come to the herds composed at this time of females and young animals. Their presence in individual groups is not permanent. Bulls change groups looking for cows in the breeding period. Such changes are favourable, since it results in the greater genetic variation of the population. Males do not bellow but emit silent, resonant sound called chruczenie. There can be only one adult male in a herd, though it is not a rule. Frequently, it is possible to notice grand silhouettes of two or even more bulls in a herd. In large herds, several females can be in the breeding period at the same time. A bull assists a cow during the breeding period, creating a permanent couple for the period of 2-3 days. Bulls occupied with their female partners do not have time and are not willing to fight, though from time to time they fend off younger bulls if they come too close. Bull fights during the breeding period in free roaming herds are not observed very often, since weaker bulls typically stand down without a fight. Only two bulls with more or less the same strength can fight. Priori to the fight, bulls demonstrate their strength by breaking down or tearing out young spruce trees, tearing up soil with their front hoofs, urinating and wallowing. If these ritual activities do not fend off the rival, there is a violent clash and pushing with lowered heads. At this time, the great force and flexibility of these large animals can be admired to the full. Such a fight is typically accompanied by the crack of broken young trees and panting. Striking the rival down or dealing a strike to the back of his body can result in a serious injury. At the locations of such fights, undergrowth is typically completely destroyed and the characteristic, musky smell of bison stays in the air for quite some time. Rivals completely exhausted with the fight then rest for a longer time. Cows reach their sexual maturity typically around the age of 3, and the first calf is born around the age of 4. In a free roaming herd, approximately 20% of cows give birth to their first calf at the age of 3, though quite often only at the age of 5 or 6. Females can give birth until the end of their lives, though it is typically assumed that the upper limit for the breeding period of the cows in free roaming populations is the age of 20. In general, bison females give birth every second year. A bison pregnancy lasts 264 days (254 – 277 days). A female gives birth to a single calf – twin births are extremely rare. Even though in the period between 1957 and 1998 there were 1298 births, no twin birth was recorded. A cow leaves the herd for the period of the birth and comes back to the herd with the calf after a few days. A calf is milk fed for a year, though if the cow does not give birth to a new calf in the following year, the last year's calf can still feed on the cow's milk. Feeding two calves, a newly born one and the last year's one, was also recorded. Calves are born in a free roaming herd between May and July, though there are also cases of delayed births (August – October). A close relation between a female and a calf end when it becomes two years old. A relatively small foetus means that even experienced breeders have problems indicating whether a female is pregnant or not. Several hours priori to the birth, a pregnant female becomes anxious. Birth complications are very infrequent and are more common for females living in captivity. After the birth, a female licks the newly born calf clean, which is capable of lifting and standing on its feet after approximately 30 minutes and after the following 30 minutes it makes the first attempt to feed on its mother's milk. At this time, it is better to leave such a female alone, since in attempt to protect its calf it can attack any intruder. Luckily, free roaming bison females move to some secluded locations to give birth.
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