Which principle best describes why larger animals have a relatively lower metabolic rate per unit mass?

The principle that larger animals have a relatively lower metabolic rate per unit mass is best described by the surface area to volume ratio. As animals increase in size, their volume (which relates to mass) increases at a faster rate than their surface area. This relationship means that larger animals have less surface area available per unit of volume to lose heat. Consequently, they tend to be more efficient in retaining heat and may have lower metabolic rates compared to smaller animals, which have a higher surface area relative to their volume and thus lose heat more rapidly, leading to higher energy demands for thermoregulation and other metabolic processes.

Understanding this principle is fundamental in animal physiology, as it helps explain how body size influences metabolic rates, energy requirements, and even adaptations to different environments. Larger animals, needing less energy per unit of mass, can sustain their size more efficiently than smaller ones, which must often consume energy at a higher rate to maintain their body temperature and overall physiological functions.