Radiators that also listen
Why Do Rabbits Have Such Big Ears?
A desert rabbit's ears are enormous, and an Arctic hare's are tiny. The difference is not about hearing.
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Step 1 of 5
Big ears catch quiet sounds.
A wide ear gathers sound from a large area and funnels it into a small hole, which makes a faint noise louder. Rabbits swivel each ear separately to sweep for danger.
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Step 2 of 5
They also throw away heat.
The ears are thin skin packed with blood vessels and almost no fur. Blood sent through them cools on the way past, so a rabbit dumps heat there instead of panting or sweating.
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Step 3 of 5
The rabbit turns the radiator up and down.
In heat the vessels widen and a lot of blood flows through, so the ears run hot and shed heat fast. In cold they narrow to a trickle and the heat stays in the body.
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Step 4 of 5
Climate decides the size.
Across the world, related animals in hot places have bigger ears, legs and noses than their cold-country cousins. A desert jackrabbit's ears run to about a quarter of its body length. More surface sheds more heat; less surface keeps it.
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Step 5 of 5
Eyes on the sides, and a blind spot in front.
A rabbit sees nearly all the way round without moving its head, which is what prey needs. The price is a small blind patch directly in front of its nose — it finds food there by smell and whisker.
A rabbit's teeth never stop growing. Gnawing wears them down at about the same rate, which is why a rabbit with nothing hard to chew gets into trouble.
The short version
Rabbit ears both funnel faint sounds and act as radiators, which is why desert rabbits have huge ones and Arctic hares tiny ones.
Try it yourself
Cup your hands behind your ears in a quiet room. Everything gets louder — that is what a bigger funnel does.