The heart is a critical organ in vertebrate animals, driving the circulatory system that delivers oxygen to cells. Animals should be designed economically, and having excess structure represents a waste of resources that should be selected against. The cost of supplying oxygen to the body should be proportional to the rate that oxygen is consumed. Despite its importance, heart-work (the overall cost of the circulation) has rarely been measured. The purpose of this thesis was to investigate the relative cost of the circulation in animals of varying body mass, by measuring heart-work in several different species of wild African antelope.