Cat intelligence
The brain of the domesticated cat is about long, and weighs . If a typical cat is taken to be long with a weight of , then the brain would be at 0.91% of its total body mass, compared to 2.33% of total body mass in the average human. Within the encephalization quotient proposed by Jerison in 1973, values above 1 are classified big brained, while values lower than 1 are small brained. The domestic cat is attributed a value of between 1–1.71 relative to human value that is 7.44–7.8. The largest brains in the Felidae family are those of the tigers in Java and Bali, of which the largest relative brain size within the "panthera" is the "tigris". It is debated whether there exists a causal relationship between brain size and intelligence in vertebrates. Correlations have been shown between these factors in a number of experiments. However, correlation does not imply causation. Most experiments involving the relevance of brain size to intelligence hinge on the assumption that complex behavior requires a complex (and therefore intelligent) brain; however, this connection has not been consistently demonstrated.
Evolution of nervous systems
There has been a gradual increase in brain volume as the ancestors of modern humans progressed along the human timeline of evolution (see Homininae), starting from about 600 cm in "Homo habilis" up to 1736 cm in "Homo neanderthalensis". Thus, in general there is a correlation between brain volume and intelligence. However, modern "Homo sapiens" have a smaller brain volume (brain size 1250 cm) than neanderthals; women have a brain volume slightly smaller than men, and the Flores hominids ("Homo floresiensis"), nicknamed "hobbits", had a cranial capacity of about 380 cm, about a third of the "Homo erectus" average and considered small for a chimpanzee. It is proposed that they evolved from "H. erectus" as a case of insular dwarfism. In spite of their threefold smaller brain there is evidence that "H. floresiensis" used fire and made stone tools as sophisticated as those of their proposed ancestor, "H. erectus". Iain Davidson summarizes the opposite evolutionary constraints on human brain size as "As large as you need and as small as you can".