It is very difficoult to study complex sounds as those produced by mammals. This because the social context in which sounds have importance is much more variable than the one we find in animal with simpler behaviour. Individual learning and experience, condition the developement of communication abilities and language expression. The message carries a meaning not only referred to the syntax and vocabulary used, but referred to emotional, physical, social and environmental conditions in which the animal acts too. This is true following the growth of behavioural complexity.
Today is possible to study sounds produced in very specific contexts, like mating or sonar surveing.
These sounds, expecially the second, are more uniform because constrained by physical unvariable laws. And so they are more easily comparable. Chiroptera , bats Bats are the only flying mammals. There are small bats, microchiroptera , which are mostly insectivorous, and large bats, megachiroptera , which eat fruit. In Europe only insectivorous bats exist.