The jackrabbit's ears are a cooling system, and a good one. At around 30 degrees Celsius, studies have shown a jackrabbit can shed more than 100 percent of the heat its body produces straight out through those big, thin, blood-filled ears — enough to cover its entire metabolic output and then some. It does this mostly by convection, moving air carrying heat off the warm skin, not by radiating it to the sky the way the tidy old textbook had it.
The catch is that it only works while the air is cooler than the animal. Once the afternoon climbs hotter than the hare itself, the flow reverses: the ears begin collecting heat from the air instead of dumping it, and the only tools left are panting and shade. So the ears are not a magic radiator. They are a fair-weather one, brilliant in the morning and useless at the worst of the day, which is exactly when the animal is holed up in shade anyway.
The other half of surviving here is the gut, and it runs on a sliding scale. A jackrabbit pulls energy from good spring annuals at around 65 percent efficiency; by summer that falls to about 45 percent, and on the woody, dried-out browse of winter it drops to roughly 18. Winter, not summer, is the hard season — the worst food arriving exactly when the animal burns the most just to stay warm.
It helps to know it is not a rabbit at all but a hare, and the difference shows at birth. A cottontail is born blind, naked, and helpless in a fur-lined nest. A jackrabbit is born furred, eyes open, and able to move within minutes, because a hare of the open ground keeps no burrow to hide its young and cannot afford a helpless week. It is fast, too; the figure of 30 to 35 miles an hour gets repeated, though nobody seems to have actually clocked one, and in any case speed is the backup plan. The ears are the main one.
Catch the ears backlit against a low sun and you can see the blood in them. That is the radiator, running.