Final answer:
Extensive food processing in the mouth is a trait of endotherms because they require a higher intake of energy to maintain their internal body temperature, leading to the evolution of complex mouth structures like specialized teeth and secondary palates.
Step-by-step explanation:
A large amount of food processing in the mouth characterizes endotherms.
Endotherms, such as mammals and birds, have developed complex mouth structures to process food efficiently due to their high metabolic requirements. Unlike ectotherms like reptiles that absorb external heat, endotherms generate heat through metabolic processes and need more energy, which they obtain from thoroughly processed food. This need for efficient digestion drove adaptations such as specialized teeth, cheeks to hold food, and a secondary palate that allows them to chew without hindering respiration. The evolution of these characteristics can be traced from early synapsids to advanced ones such as cynodonts and ultimately to mammals. In contrast, reptiles typically have less specialized oral cavities and rely more on the external environment to regulate body temperature.