The expensive tissue hypothesis suggests that tissues with a high metabolic demand like the brain will become reduced if they confer little selective advantage and so do not help to increase food intake. The paucity of large predators means that insular species can afford to become slower and less alert without suffering from massively increased predation risk. As such, reduction in relative brain size is often seen in insular species as this reduces basal metabolic rate without increases in predation risk. For example, the endocranial volume of the extinct Malagasy dwarf hippos is 30% less than that of an equally sized continental ancestor. Similarly, the early human, Homo floresiensis, had a brain of similar size to that of the significantly earlier Australopithecus specimens from mainland Africa and 3.4 times smaller than that of Homo sapiens which evolved later (see Evolution of human brain size).
