Which of the following is not a characteristic feature of reptiles?

Reptiles are ectothermic (cold-blooded) organisms that rely on external heat sources to regulate their body temperature. They cannot generate internal metabolic heat to maintain a constant temperature. Characteristics that DO define reptiles include: scaly skin (preventing water loss), oviparous reproduction (egg-laying), and pulmonary respiration (breathing through lungs). Being 'warm-blooded' is a characteristic of birds and mammals, not reptiles.