Expert: Dana Krempels, Ph.D. - 6/30/2007

Question
Dear Dr. Krempels,
Are there any exceptions to the general pattern that only mammals have external ears?  If not, could you briefly speculate?  Can you imagine conditions under which the development of a non-mammalian external ear would be plausible?  Is it imaginable that a reptilian would ever develop an ear like an elephant-- with all those groovy functions of cooling, signaling, identification and WOW! infrasonic communication?  Or is something about reptilian biology utterly incompatible with such a structure?  It is a curious thing that almost all cultures that come up with dragons give them external ears.  Hmmm...  
Sincerely,
Keith McCune, PhD (University of Michigan, 1984)

Answer
Dear Keith,

This is an interesting question, but the answer might be really simple.  

Animals don't evolve things because they need them or because they would be adaptive.  Traits are selected and kept in the population if they evolve "by accident" (incrementally or due to sudden genetic events, which is quite possible) and are adaptive (i.e., they increase an individual's chances of leaving offspring).

I suppose it wouldn't be detrimental for a lizard to have big ears.  But it's just never come up in the genome.  Only in the Most Recent Common Ancestor (MRCA) of the mammals did the external pinna show up and become a trait upon which natural selection did a real dance number to produce such wild shapes and sizes!  (I live with 16 jackrabbits, and so have a special appreciation for spectacular pinnae!)

I don't know of any animals besides mammals that have anything like an external pinna.  If a mutant bird was hatched that had them, we might be able to find out whether this trait would be adaptive or not.  But apparently, it either has never happened, or if it did, it was so maladaptive that it was out of the gene pool in short order.

In some groups, such as aquatic mammals, one can easily see why an external ear would be a disadvantage, since it would decrease the slipstream of the fusiform shape that whales and dolphins have.  Among marine mammals, only the sea lions retain external pinnae, and those are pretty small.

Your list of all the advantages of the external pinna points up how adaptive and advantageous the external ear has been for mammals.  I wonder how much of our success can be attributed to that wonderful strucure!  :)

And yeah, dragons always seem to have deer ears, don't they?  I guess that's one more piece of evidence to assure us that they are just a fantasy....

What do you think?  

Dana