"I'm all ears!"

When H. Ross Perot declared 'I'm all ears' early in the 1992 presidential campaign, he really meant he was listening intently. But the articulate entrepreneur was blessed with robust ears and his statement earned him belly laughs that Jerry Seinfeld would envy.

Now that we're talking ears, how many times have you wondered why rabbits have such long ones? I mean, when you are cruising along and see a roadkilled jackrabbit, or think about the energizer rabbit . . .do you knit your brows and ponder:

"Why? Why is it that they have such large ears? Is it all about good hearing, or is something else going on?"

What? The question has never crossed your mind? You've got to be kidding!

Well then, how about this: the antelope jackrabbit (L. alleni) of our American southwest has ears that are 25% of its body surface. On the other hand, the ears of the black-tailed jackrabbit that you see at my water trough make up 19% of its body surface. Still impressive structures.

Back in the late 1800s a naturalist named J.A. Allen, who spent a lot of time measuring museum specimens (including the ears of rabbits) had a revelation. He noticed that birds and mammals from cold climates have smaller extremities than their relatives from warmer climates. He had hit on a general principle of the natural world, and he wrote it up and published it. Thus we got Allen's Rule.

Since then biologists have spent countless years measuring more museum specimens, and have found Allen's Rule to be less comprehensive than Allen thought. It does not apply to all species, nor to all extremities (legs, tails, ears, noses, beaks). In cottontails and brush rabbits, for example, no extremities 'obey' the rule. But it does apply to the ears (and tails) of jackrabbits.

It makes sense, because the ears of rabbits are like automotive radiators. You need bigger radiators in hot climates. The difference is that jackrabbits have more sophisticated thermostats than cars, and thanks to the studies of Professor Richard Hill and his associates, here's how they work in a nutshell.

Jackrabbits crank down blood flow to their ears when they are resting at temperatures below 24 C (=75F). Under these conditions their ear temperatures are slightly above air temperature (from 0.3 to 2.5 degrees C). When it's freezing on the plains, jacks have chilly ears. That way they conserve body heat.

At warm temperatures (above 31C = 88F) the ears' arteries dilate, blood flow increases, and the ears shed body heat by convection and radiation.

But when jackrabbits are exercising, their body temperature increases just as it does in people. Of course, running jackrabbits pant and thus cool their snouts by evaporation, but their ears can become considerably warmer than the air, and they also vibrate, which to some extent can help shed body heat by convection. After 15 to 25 minutes of rest the ears have cooled off, and the rabbits are back to their resting body temperature of 38C (=100F).

What about when it gets really hot? When temperatures reach 45.5C (=114F) jacks rely on panting and evaporation to shed body heat. Most of the time the ears stay cooler than the air because body temperature is less than air temperature, and their blood flow is restricted. However, every several minutes the arteries engorge for 10 to 20 seconds, at which time they absorb heat from the air. This adds significantly to the animal's overall heat gain.

If you are trying to get rid of heat this seems counterproductive. So, why let those big ears absorb more heat? Why not minimize their blood circulation so this doesn't happen? The most logical answer is that hot ears have their own metabolic demands for oxygen, blood sugar, and waste removal.

If you like the car radiator analogy, think of it this way. You've got to flush your radiator if you want to keep it working.




References

Griffing, J.P. 1974. Body measurements of black-tailed jackrabbits of southeastern New Mexico with implications of Allen's Rule. Journal of Mammalogy, 55(3):674-678. [the temperature data in this paper were in error, as noted in Hill et al (1980)].

Hill, R.W. and J.H. Veghte. 1976. Jackrabbit ears: surface temperatures and vascular responses. Science, 194(4263):436-438.

Hill, R.W., D.P. Christian, and J.H. Veghte. 1980. Pinna temperature in exercising jackrabbits, Lepus californicus. Journal of Mammalogy, 61(1):30-38.

Hinds, D.S. 1977. Acclimatization of thermoregulation in desert-inhabiting jackrabbits (Lepus alleni and Lepus californicus). Ecology 58(2):246-264.

Stevenson, R.D. 1986. Allen's Rule in North American rabbits (Sylvilagus) and hares (Lepus) is an exception, not a rule. Journal of Mammalogy, 67(2):312-316.