What's the purpose of the double flap on a dog's ear?

[Northern_Piper]

Although I’ve not done a scientific survey, most dogs have a little double flap, or double layer of skin, on one side of their ear, the side closer to the ground.

What’s the purpose of that little flap? just a quirk of evolution?

[rjk]

I think I know the bit you’re talking about. I don’t know much about dog ears, but it might be involved in locating the direction of sounds.

Ear shape modifies the sound you hear according to its direction, as it reflects off the convolutions. Take a look at http://www.sfu.ca/sca/Manuals/ZAAPf/b/binaural_local.html for a brief description, or http://www.binaural.com/binfaq.html for more than you wanted to know about synthesizing sound-position info.

Or maybe it is just a quirk.

[bibliophage]

In this thread http://boards.straightdope.com/sdmb/showthread.php?threadid=30964 I said

Are you talking about what I think you’re talking about? A pouch-like structure on the edge of the ear? If that’s not what you’re talking about, you can ignore what follows. I couldn’t find an anatomical atlas of the cat (“map of a cat” as R. P. Feynman would say) that considers this important enough to mention, but dogs appear to have more or less the same thing. The book Canine Anatomy: A systematic Study by Donald R. Adams (1986) says (pp 12-14),

quote:

A cutaneous marginal sac of unknown significance occurs in the antitragic margin.

If the anatomist doesn’t know what it’s for, I’m not going to speculate. I have scanned the accompanying diagram and you can see it here. I have underlined “cutaneous marginal sac” in red, since it’s hard to read

[Northern_Piper]

bibliophage, that’s what I meant - thanks. As you say, if the anatomy profs don’t know…

rjk, I’m not sure that it would affect the dog’s ability to hear, because it’s found on dogs with pointy-up ears (like in the illustration bibliophage posted), and also in floppy hang-down ears (like beagles and bassets). Although it’s located in the same anatomical region of the ear in both cases, I would think that the different physical location would mean that it would not have the same effect on the dog’s ability to hear.

[Squink]

If the anatomy profs don’t know, you can’t look it up, so you have to figure it out for yourself.

Cats and dogs apparently diverged from their common ancestor (Miacis) 30 or 40 million years ago. That seems like a very long time for such a complex structure to hang around if it were just a quirk of evolution.
If you grab a handy cat, or dog ?, and pull the back of their ear directly away from the head, you’ll see that the structure stretches like a spring, (or a folded piece of paper when pulled). The stress exerted by pulling on the ear also looks to be more spread out with the flap thing, than it would be if the ear were more simply constructed. So maybe the flap helps prevent torn ears during fights etc. ?

For a cat ancestral tree see: http://dspace.dial.pipex.com/agarman/bco/prehistoric.htm
A Google search on “Miacis”, “prehistoric cat” or “prehistoric dog” turns up scads of interesting stuff.