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Despite a low probability of achieving copulations, yearling male (1.5-year-old) fallow deer Dama dama display behavioural changes during rut, interacting with both females and older males. Juvenile interactions in polygynous ungulates may affect further reproductive success through both motor training for future interactions and/or competition for current resources that affect growth. To test this hypothesis, we intensively studied the behaviour of 12 yearling males in two captive populations during a rutting season. Interactions with females were interpreted as motor training aimed at learning to recognize oestrous females, because yearling males rarely interacted with their mother and because interactions essentially consisted of olfactory investigations focused on oestrous females. Because agonistic interactions with older males never involved physical contact and were mainly initiated by the most dominant buck in each population, we concluded that these interactions resulted from the presence of the yearlings close to oestrous females defended by rutting males. Heavier yearlings tended to be dominant over lighter ones and tended to interact more frequently with other animals, suggesting that early growth may influence the ontogeny of social behaviours.