Tarsiers communicate on ultrasound ranges of incredibly high frequencies. In fact, they use it all the time and make vocalisations at 70,000 hertz. This helps them communicate over the background noise of the jungle and above the endless whining of human tourists and self snapping morons. It also allows them to hone in on the sound of insects as well. But they really use it to communicate because it is way up in a range where humans can’t hear.