BobTHJ writes:
Also, look at the inverse: if selective pressure for prestin is so high then why have not all mammals evolved the enhanced prestin of dolphins and bats? I have a hard time picturing a situation where hearing higher frequency sound wouldn't be an increase in fitness.
I don't. Let's help your picturing.
Think of the times when you're trying to listen to something specific, but there's all kinds of background noise. The more background noise messages entering your brain, the harder it is to concentrate on what's important at the time.
For every mammal, because of the specific way it operates in its environment, there will be an optimum position on the hearing frequency levels that's best for them, and also an optimum breadth of their range. An increase in the breadth of the range will only be selected for if the advantages of hearing the extra sounds outweighs the "background noise" disadvantages.
Extending the breadth into higher frequencies could be an advantage for night flying hunters and swimming hunters in certain circumstances, but would be interference to those individuals of our own ancestors who mutated the characteristic, so it would have faced negative selection, rather than positive.
If I'm not right about this, all mammals would have much broader hearing ranges than we actually do. Instead, natural selection has focused on the priorities of different creatures in different circumstances, and hearing is specialised, rather than just bringing in maximum noise to the brain, and giving us unnecessary headaches.