To find the causes of faster growth in males, we studied the growth and behaviour of six male and five female fallow deer, Dama dama, fawns, hand-reared on the same amount and quality of milk, and compared them with naturally reared fawns. The bottle-reared fawns grew more slowly. In particular, the difference in weight gain between bottle- and mother-reared fawns was significantly larger for males during the time when they consumed only milk. Male fawns also sucked harder and were more motivated to obtain milk. These results indicate that male fallow deer fawns receive more milk from their mothers than female fawns under natural conditions and, hence, maternal investment seems male biased in this species. Males grew slightly, but significantly, faster until 10 weeks old, which revealed that sex-differential growth during the period of maternal investment is possible in the absence of differential maternal investment. The results could not reveal whether this small difference in growth is brought about by physiological or behavioural differences between the sexes.Copyright 1998 The Association for the Study of Animal Behaviour