Among the given items,statement 3 is not true for mammals. It is because heart in mammals is four-chambered.General characteristics of mammals are: 1. Mammals are endothermic vertebrates; 2. Mammals have hair and fur on the body; 3. Mammals have mammary glands; 4. Mammals have four-chambered hearts; 5. Mammals possess one single jaw bone; 6. Mammals have three small bones in the middle of the ear.