Final answer:
Endotherms have a higher BMR than ectotherms of the same size because they regulate body temperature internally, requiring more energy. Smaller endotherms additionally have higher BMRs due to greater surface area to mass ratio, leading to faster heat loss and increased energy needs to maintain body temperature.
Step-by-step explanation:
Endotherms have a higher basal metabolic rate (BMR) compared to ectotherms of the same size because endotherms use metabolically-generated heat to regulate and maintain a constant body temperature, regardless of external temperatures. This is in contrast to ectotherms, which rely on external thermal energy to help regulate their body temperatures. Due to this internal heat regulation, endotherms have to consume more energy and food to keep their body temperature stable, which results in a higher BMR.
Energy requirements are also related to body size. Smaller endothermic animals have a greater surface area relative to their mass compared to larger ones, which makes them lose heat more rapidly and thus require a higher metabolic rate to maintain constant internal temperatures. Consequently, a smaller endothermic animal will have a higher BMR per body weight than a larger endothermic animal.