In this study 529 fallow deer fawns were captured and weighed at Doñana National Park, Spain. An annual index of male-biased maternal investment (MBMI) was calculated, and this index was found to vary in relation to environmental factors. Thus, the level of MBMI was sensitive to fluctuations of autumn rainfall and the male-biased maternal investment carried out by mothers in the previous year (MBMIP). Our results reveal that those factors related to the female's body condition at the beginning of gestation have a high contribution to the variation of differential pre-natal maternal investment. It has been hypothesized by other authors that male-biased maternal investment is limited by the level of total maternal expenditure and these authors classified the fallow deer as a species near to its maximum reproductive effort, in which no sex-biased maternal investment exists. Our study reveals that male-biased maternal investment in fallow deer does exist and supports the hypothesis that sensitivity of maternal investment to environmental conditions could be expected to occur in species with high reproductive effort and sex-biased maternal investment.