 In mammals, UCP1 functions within brown adipose tissue to protect newborns against hypothermia.In modern birds, skeletal muscle serves a similar function and is presumed to have done so in their ancestors.In this view, bipedality and other avian skeletal alterations were side effects of muscle hyperplasia, with further evolutionary modifications of the forelimbs, including adaptations for flight or swimming, and vestigiality, being secondary consequences of two-leggedness.