Is the large size of the pinna of the ear of the European hare (Lepus europaeus) due to its role in thermoregulation or in anterior capital shock absorption?

Large pinnae are characteristic of the Leporids, and the pinna is known to have a thermoregulatory role. Another role has been hypothesized for the pinna of Lepus spp., as a part of a suspensory system for the greater portion of the head, absorbing shock that might otherwise interfere with vision during high-speed locomotion. We compared the lengths of the pinnae of adult European hares Lepus europaeus from the source population in the cooler climate of England with those of the introduced population in the warmer climate of Australia, and we compared the lengths of the pinnae of hares that had grown in cooler weather with those that had grown in warmer weather. There were no significant differences between each of the comparisons, indicating that the size of the pinna is not determined by thermoregulatory requirements at rest. We compared the growth in length of the pinnae and the legs with growth in body mass, and growth in the mass of the pinnae with the masses of the head and the eyeballs, and found support for the suspension hypothesis. We suggest that the rapid growth of the pinna is because visual acuity is a function of absolute eye size, not relative eye size, yet juvenile hares are subject to the same predator pressure as adult hares, and equally need to maximize visual acuity while running at high speeds in dim light. We believe that the large size of the pinna is determined by its role in anterior capital suspension, not in thermoregulation.