The relationship between brain and body weight in vertebrate groups, plotted in grams on logarithmic scales on which each tick marks a tenfold change. The members of each particular vertebrate group, such as the mammals, fall within a well-defined polygon. The teleosts are a large group of bony fish, distinct from the cartilaginous sharks and rays.
The brain of a typical fish, a trout, compared with the brain of the elephantnosed fish, a mormyrid. The overall brain size, and especially the blue-shaded component, the cerebellum, is much larger in the mormyrid.
A comparison between maps in the somatosensory cortex of the raccoon and the coatimundi. The representations of the different parts of the body in the brains are shown below. The opposite side of the body is represented in each hemisphere of the brain. The representations of the forepaw, greatly enlarged in the raccoon and much smaller in the coatimundi, are outlined in red. As shown in the enlargement of the forepaw map, Wally Welker and his colleagues also found that the representations of the individual digits of the highly sensitive raccoon forepaw are separated by small fissures; the skin between the digits is less sensitive and is represented in the bottoms of these fissures. Their observation suggests that one mechanism for the formation of cortical fissures results from the differential expansion of the representation of the more sensitive parts of the receptive surface, in this case the skin of the forepaw.
A logarithmic plot of fe tal brain weight against body weight for a sample of mammalian species. Note that for every fetal size primate brains are larger than those of non primates.
Evolving brains / John Morgan Allman. 1999