Most critters have some sort of vocal range. Rhinos do too. Our ability to produce sound is related to what another Rhino can hear, obviously. (Rhinos do not sing to hyenas, right?)

For our Readers, high C is about it, so We are showing that as a communication device. Rhinos can squeak higher, but only under extreme duress. The Wild can provide that sort of duress, believe Us.

By the way, high notes take practice. Lots of it.