This paper examines the development of children’s understanding of animals’ minds and mental capabilities over two experiments. Experiment 1 modified standard false-belief and discrepant-desire tasks in two age-groups (3-4 and 7-8 years) to test developmental and species differences in children’ basic cognitive reasoning about animal and human minds. Experiment 2 incorporated a modified mental-capacity rating task in three age-groups (3-4-, 7-8 years, and adults) to investigate developmental changes in children’s judgements of appropriate mental-capacity terms in animals of different species. Mixed-model two-way ANOVA was used in both experiments to calculate age-group, species and age-group species interaction effects, and post-hoc analysis was used where appropriate. Analysis revealed developmental trends in both experiments. In Experiment 1, modified task performance improved between 3-4 years, and peaked at 7-8 years, but showed no species differences. In Experiment 2, there was a developmental trend towards more extensive and consistent species differentiations. Findings are discussed with reference to the origins of animal mentality concepts in relation to naïve biology and psychology and the implications for children’s everyday interactions with animals.