Pinna Temperature in Exercising Jackrabbits, Lepus californicus
Pinna surface temperatures of Lepus californicus were examined radiographically before and after forced exercise (e.g., running at about 6 km/h for 6 min) in an outdoor enclosure. Air temperatures were from −1.6 to +19.4°C, and radiant sky temperatures were from —27 to +22°C. In resting animals, average pinna surface temperature was only 0.3 to 2.5°C above air temperature, and pinna arteries were not evident radiographically. Immediately after exercise, average pinna temperature was often near 30°C and pinna arteries were evident, indicating vasodilation. Cooling of the pinnae to resting temperatures after exercise generally took 15 to 25 min. Calculations indicate that the pinnae can dissipate most of the excess metabolic heat generated during the exercise examined, and it is suggested that the role of the pinnae as dissipators of exercise-induced heat loads should be considered as a factor in the evolution of pinna size.