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).
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.