Social behavior and activity patterns vary considerably as well. Some mammals live in groups of tens, hundreds, thousands or more individuals. Other mammals are generally solitary except when mating or raising young. Mammals may be nocturnal, diurnal, or crepuscular.Mammals have hair or fur; are warm-blooded; most are born alive; the young are fed milk produced by the mother's mammary glands; and they have a more complex brain than other animals.Extant mammals are divided into three subclasses based on reproductive techniques (monotremes, marsupials, and placentals) consisting of 27 orders.According to the University of Michigan Museum of Zoology, there are only three characteristics unique to mammals. The three characteristics are mammary glands, hair and three middle ear bones. Other characteristics often thought to be unique to mammals are found in other species including birds, insects and reptiles.