This question has no background information here. I just want to know if there are any evolutionary advantages for a species to have 4 ears?(The kind a mammal has).

  • $\begingroup$ Yes there is. This answer has no background information :) $\endgroup$ – Samuel Aug 24 '15 at 23:37
  • $\begingroup$ You mean organ used for hearing, a spider's legs contain many hairs which detect vibration in the air to sound out prey, akin to the design of our ear. $\endgroup$ – user6760 Aug 24 '15 at 23:40
  • $\begingroup$ By ear I mean mammal ears $\endgroup$ – TrEs-2b Aug 24 '15 at 23:41

Evolution is thrifty. One of the most effective processes I have seen for determining if evolution might make something happen is to look at what is simpler to do (i.e. can be done with 2 ears), and see if the environment can encourage development of something more complicated.

In the case of ears, I'll look at 2 ears, and what the limits of this approach are. I could look at 3, which is also simpler than 4 eared creature, but symmetry tends to prefer even numbers, and it turns out that an ear right in the center line isn't too useful.

So what are the limits of 2 ears:

  • 3d sound is difficult. We can easily discern the direction of a sound left to right, because of the travel time of the soundwave to our ears. For up and down, we actually rely on frequency distributions. If we know what a sound should sound like, such as our own name called by a friend, we can hear the effects of all those little funny shapes in our ear, and our shoulders, and determine by frequencies which direction the sound came from. With 3 or even 4 ears, more options show up. In fact, with 4 ears, you have enough information to not only get a direction but a distance, just from the time delays (we use that effect in GPS!)
  • Redundancy. 2 is the bare minimum number of ears for our particular environment. If one gets damaged, we become weak on that side.
  • Alternate frequencies. While nature vastly prefers having one auditory organ over two for simplicity reasons, if we had a reason to listen to two different sets of frequencies, it might make sense to have two fundamentally different kinds of ears.
  • Alternate synergy. Perhaps we get a freak mutation that gives us 4 ears, but we find that having 4 earlobes is is useful (maybe having more space for earrings is a sexually selected trait). Evolution is not embarrassed by anything. If it's useful, it's kept!

I'm sure there's others, but that might be enough to get your creative juices flowing.

  • $\begingroup$ Dang, I was just writing about the 3D localization. It's one of those interesting things about hearing that people often forget to think about. +1 (in a few minutes when my vote count is reset :) ). $\endgroup$ – Samuel Aug 24 '15 at 23:41
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    $\begingroup$ I ran an awesome experiment a while back. I grabbed a pair of spoons so that I could click them together, and got a friend to sit down and close their eyes. I then had them reach out in the direction they thought the click was coming from, and clicked the spoon a few times. Every time, they got the left-to-right axis perfect, but their up and down was wrong. Then I let them open their eyes, and observe a few clicks. Then I had them close their eyes and do the experiment over again. This time, they reached for the spoon dead-on, every time. $\endgroup$ – Cort Ammon - Reinstate Monica Aug 25 '15 at 0:44
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    $\begingroup$ Literally they were able to use the visual information from seeing the spoons click to determine the real direction they were in, and then back out whatever transform their ears did, to hear the undistorted true sound of the clicking. Then, when they heard it again, they could use the frequency distributions to get the location right. I think it is fair to say that they actually heard with their eyes, and it is the coolest example of sensory fusion I have seen (or heard). $\endgroup$ – Cort Ammon - Reinstate Monica Aug 25 '15 at 0:46
  • $\begingroup$ @Samuel (should have linked the above comments to you) $\endgroup$ – Cort Ammon - Reinstate Monica Aug 25 '15 at 0:52
  • $\begingroup$ Very cool. It's an interesting problem for video game and virtual reality designers too. It also raises interesting metrics for use in the design of cochlear implants and hearing aids. Not only is captured tone important (especially in tonal languages) but the phase information also need to be preserved (and likely recorded in ear). $\endgroup$ – Samuel Aug 25 '15 at 1:09

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