Behaviour results from the interaction of an individual's genotype with prevailing environmental conditions, resulting in local adaptation to specific habitats. We investigated the development of exploratory behaviour in two closely-related species of African striped mice from the semi-arid Succulent Karoo (Rhabdomys pumilio) and moist grassland (R. dilectus chakae) localities. Irrespective of sex, R. pumilio displayed greater exploratory behaviour (open field) and greater use of the open arms of a modified plus maze, and thus were less anxious and bolder than R. d. chakae. When pups were cross-fostered between species, fostered individuals of both species showed an intermediate behavioural pattern between their foster and biological siblings: fostered R. pumilio explored more than their foster siblings but less than their biological siblings, whereas fostered R. d. chakae explored more than their biological siblings, but less than their foster siblings. Our study is one of the first to address how the underlying genotype and early postnatal experience interact to influence the expression of exploratory behaviour and personality. In particular, we showed that, in striped mice, the early postnatal environment shapes the anxiety responses and concomitant exploratory behaviour, but the genotype apparently modulates the phenotype and constrains the limit of behavioural flexibility.