Animal behaviour

Today I am pondering a question about animal behaviour.  Why do animals make a noise when injured or in pain.... what is the evolutionary advantage of this?

The only plausible answer I can come up with so far is that it may stem from the need for baby animals to alert their mother when they need assistance.  But shouldn't animals grow out of this when their mothers are no longer around to protect them?  

A farmer's son once told me that an easy way to catch a fox is to make a squealing noise like an injured rabbit.  Foxes will come running if they are within earshot of the easy prey of an animal in distress.  So... why do the rabbits squeal?   (Is it just that they have not needed to evolve the survival mechanism of suffering quiet distress, because the strength of their breeding speed off-sets these losses?)

A slightly related question is this:  are humans the only species that will run to the aid of a fellow creature in distress, assuming that the creature is not one of their own immediate offspring?  I hear tales of dolphins rescuing drowning men...  is this for real?  

I once saw an interesting (and tear-jerking) David Attenborough documentary about lemurs.   A baby lemur died while calling for its mother - she did not reach it in time and it fell from the tree (believe me it was *really* tear jerking).  Immediately afterwards the other lemurs in the social group clustered around the mother and started to stroke and groom her, as if comforting her on her loss.  I found this quite a profound example of animal behaviour that hints at a lot more going on in the minds of animals than we currently understand or assume.