Rabbits were designed with big ears to be able to thrive in their natural habitat. Often preyed upon by larger animals, rabbits need to monitor their environment and detect incoming predators. The rabbit's outer ears, or pinna, can change directions and pick up faint sounds from considerable distances.
The rabbit's long ears detect movements from predators from different directions, as each is able to rotate up to 270 degrees. The ears are so sensitive to sound that rabbits can detect two sounds at once, allowing them to escape before predators can lunge.
Another function of their ears is thermoregulation. The outer ears of rabbits are thin and lined with an extensive network of blood vessels that provide ample surface area for heat transfer. When the weather is hot, the vessels swell to give off heat and allow the blood to circulate again and cool off their bodies through evaporative cooling. In winter, the vessels shrink to prevent heat loss and preserve their body heat. The dilated vessels are hardly noticeable during cold weather.
Aside from rabbits, hares also use their ears to detect distant sounds and to regulate their temperatures. Jackrabbits, which are actually hares, cool off in their desert habitats through their large ears, allowing them to survive in the extreme environment.