Some mammals cannot always maintain a constant temperature within. Most bats are unable to maintain a constant temperature and cool down considerably when they sleep or rest. Rat Heterocephalus glaber , from Africa. This unique mammal has lost the the ability to regulate its body temperature endothermically. This does not effect their lives because they live underground in an environment with a very stable temperature regime.
Mammals are unique in possessing hair and in many mammals hair varies throughout the year.
Why Warm Blooded Being warm blooded gives mammals a distinct advantage in may habitats, allowing them to be active when reptiles are hardly able to move. It also allows mammals to live in habitats where reptiles cannot live at all, such as the arctic, mountain tops, etc. The reason behind this is that muscle activity is basically dependent on chemical reactions, as are all biological functions. Chemical reactions run slowly when it is cold, but quickly when it is hot. The more energy you have the faster the chemical reaction occurs.