It is a trend that the larger the animal gets, the smaller the brain-to-body mass ratio is. Large whales have very small brains compared to their weight, and small rodents like mice have a relatively large brain, giving a brain-to-body mass ratio similar to humans. One explanation could be that as an animal's brain gets larger, the size of the neural cells remains the same, and more nerve cells will cause the brain to increase in size to a lesser degree than the rest of the body. This phenomenon can be described by an equation of the form E = CSr, where "E" and "S" are brain and body weights, "r" a constant that depends on animal family (but close to 2/3 in many vertebrates), and "C" is the cephalization factor. It has been argued that the animal's ecological niche, rather than its evolutionary family, is the main determinant of its encephalization factor "C".