The function of mounting behaviour was studied in farmed red deer (Cervus elaphus) calves. On the basis of previous work, we tested two alternative hypotheses about the function of this behaviour. The first hypothesis deals with the proximate function of the behaviour. Three predictions were tested: (1a) mounting behaviour attracts the attention of the mother and/or consolidates the mother–calf bond; (1b) mounting is intended to obtain more milk; (1c) mounting is intended to prevent other calves from sucking from the mother. The second hypothesis deals with an ultimate function in practising for future life with two predictions tested: (2a) mounting behaviour is part of calves' play behaviour promoting development of the calf's locomotor and social skills; (2b) calves gain sexual experience through mounting behaviour. For the study, 50 hind-calf pairs were observed. The maternity of individual hinds was confirmed by a genetical analysis. The hinds were classified as ‘maternal’ and ‘non-maternal’ and the calves as ‘filial’ and ‘non-filial’. We recorded 40 cases of mounting behaviour involving 25 hinds and 21 calves. Our results suggest that the mounting behaviour of red deer calves serves several different functions depending on the circumstances. In association with suckling, calves of both sexes mounted maternal hinds mainly to attract their attention and to achieve another suckling. In situations not associated with suckling, mounting by male calves might be considered part of their sexual training. On the other hand, mounting by female calves probably reflects the attempt to maintain contact with their mother in tense situations.