Mammals and birds produce their own body heat and control their body temperatures. This process is known as endothermy, or warm-bloodedness, and it may be one of the reasons why mammals tend to dominate almost every global ecosystem. Warm-blooded animals are more active during both days and nights than their cold-blooded counterparts and they reproduce faster.
...Mystery solved: when mammals’ ancestors became warm-blooded
Julien Benoit, University of the Witwatersrand; Kenneth D. Angielczyk, University of Chicago; Ricardo Miguel Nóbrega Araújo, Universidade de Lisboa , and Romain David, Natural History Museum