Our old friend, the bat, is the king of extreme hearing in the mammalian world. It uses echolocation, emitting ultrasonic sounds and measuring the length of time before the sounds echo back, in order to locate prey.
But it turns out there's an animal that uses an even more extreme variety of sounds--and it's theorized that it's a direct response to the bats.
Ultrasound simply refers to a sound that is outside a human's sonic range--which isn't that hard, really, as humans have modest auditory abilities. Researchers at the University of Strathclyde in Glasgow, Scotland have discovered that the greater wax moth (Galleria mellonella), a dull-colored, generally boring and common moth, has the most extreme hearing sense of any known animal. It's capable of hearing sounds frequencies of up to 300kHz, blowing away our piddly human abilities (at our best, humans can only hear up to about 20kHz).
The researchers suspect that the moth's extraordinary sense of hearing is largely used to outwit its main predator: the common bat. Greater wax moths are very common prey in their native Eurasia, as well as in North America, where they're an introduced species, for various types of bats. Echolocation is a particularly effective habit for bats partly because it's so unusual; a bat's prey will have no idea it's being targeted. So the wax moths have evolved to be able to hear the bat's greatest weapon for evasive purposes.
It's also possible that the moths use their even higher-frequency abilities to communicate with other moths outside the hearing range of bats.
The study appears in the journal Biology.