Why do Arctic foxes have smaller ears than fennec foxes?

The difference in ear size between Arctic foxes and fennec foxes is a classic example of Allen's rule, a biological principle stating that warm-blooded animals in colder climates tend to have shorter, more compact protruding body parts than those in warmer regions.1

Arctic foxes, which inhabit the frigid Arctic tundra, have evolved short, rounded ears to minimize surface area and reduce heat loss, helping them maintain their body temperature in extreme cold. Conversely, the fennec fox lives in the hot, arid environments of North Africa and the Arabian Peninsula. Their exceptionally large ears serve as a vital thermoregulation mechanism, allowing them to dissipate excess body heat into the environment to prevent overheating.234

By adjusting the size of these appendages, both species effectively manage their internal heat balance according to the demands of their respective environments.14