Mammals need more energ.y to maintain their body temperature that's why they have a four-chambered heart. In the mammal heart, the oxygenated and deoxygenated blood is not allowed to mix, and this is achieved by the separation of the heart into the right side and left a side. Such a separation of the heart provides an efficient method of supplying oxygen to the body. Fish have two-chambered heart and oxygenation of blood takes place in the gills and passes directly to the rest of the body.