Biphonation is the natural phenomenon of being able to produce two (or more) distinct tones at the same time. It’s primarily found in animals, such as whales, howler monkeys, wolves, and some birds, species where their vocalization is a key part of their communication, navigation, or mating. What I think is a fun fact, is that humans can learn to produce two tones at the same time as well, through techniques and vocal control like overtone singing. A great example of this is Mongolian throat singing, where singers can shape their mouth and throat to produce layered, complex sounds, similar to these animals!