 Modern non-avian reptiles exhibit some form of cold-bloodedness (i.e.some mix of poikilothermy, ectothermy, and bradymetabolism) so that they have limited physiological means of keeping the body temperature constant and often rely on external sources of heat.Due to a less stable core temperature than birds and mammals, reptilian biochemistry requires enzymes capable of maintaining efficiency over a greater range of temperatures than in the case for warm-blooded animals.