Although typical anatomical features of the digestive tract of carnivores are well known, such as the presence or absence of a caecum in various carnivore taxa, and although a large number of length measurements have been published, the body mass measurement of the corresponding specimens has mostly not been reported. Here, we add original mass and intestine length measurements for 36 carnivore species to literature data. Using Phylogenetic Generalized Least Squares, we demonstrate that marine Carnivora (pinnipeds and the sea otter Enhydra lutris) have significantly longer total and small intestines relative to body mass than terrestrial Carnivora, and both pinnipeds and mustelids in general have particularly long total intestines amongst terrestrial Carnivora. The natural diet explains little about variation in relative intestinal length measures. However, amongst species that do have a caecum, a higher proportion of plant material in the diet might be associated with a longer caecum. In particular, a diet with higher proportions of plant material provided by humans could have led to a particularly long caecum in the domestic dog.