The jackrabbit’s large ears provide an expansive surface area of exposed skin loaded with blood vessels. When the surrounding air temperature is slightly below the rabbit’s body temperature, as when it retreats from hot desert sun into shade, the blood vessels in the outer part of its ears widen in a process called vasodilation. This results in greater circulation of warm blood from the body’s core to the jackrabbit’s ears, where heat is lost to the cooler surrounding air.
This cooling mechanism based on blood circulation helps to prevent overheating and maintain the jackrabbit’s body temperature within set boundaries. It’s also an important water conservation technique given the jackrabbit’s arid habitat, as it reduces the need for evaporative cooling mechanisms, such as panting or sweating, which involve the loss of water. At air temperatures around 30° Celsius, convection from the jackrabbit’s ears can shed all of the animal’s excess heat.
“Many desert animals have large ears, and the jack rabbit is no exception. It has been suggested that large ears, with their network of blood vessels, may serve to radiate heat to the sky while the animal is resting in the shade, so helping to lower its body temperature.” (Foy and Oxford Scientific Films 1982:165)
“Blood flow to the ear pinnae [outside portion of the ear] is curtailed at ambient temperatures of between 1.4° and 24.0° C, which minimizes heat loss across the pinnae and allows the surfaces of erect pinnae to approach ambient temperature. The pinnae are warmed by steady or pulsatile vasodilation in some animals when the ambient temperature is between 1° and 9° C below body temperature, a response favoring heat loss. When ambient temperature exceeds body temperature by 4° to 5° C, the pinnae are circulated with blood cooler than ambient temperature; this response favors heat influx.” (Hill and Vegth 1976:436)
“Convection from the ears…could account for the loss of over 100% of the animal’s metabolic heat at an air temperature of 30°C. If air temperature exceeds body temperature, the animal must either store heat or resort to the evaporation of water.” (Wathen et al 1971:1030)