Probabilistic Vocalizations

Titi monkeys have a communications system with each other that is not categorical. Even the concept as explained in this linked blog post from John Hawks is hard to get your head around. From the paper discussed in the blog post:

Although the notion of categorical meaning is intuitively compelling, it is not necessarily the default mode of animal perception. Categorical perception has been a major theoretical pillar in animal communication research, particularly because of its intuitive link to linguistic theory. For example, Macedonia and Evans [(16), p. 179] presupposed that external events are processed in categorical terms (“…all eliciting stimuli must belong to a common category”). Although this approach has been fruitful and productive, it has also generated enigmas suggesting that the underlying theory may have to be revised. For example, in a seminal paper, Cheney and Seyfarth (17) were puzzled by the fact that animals appeared to have very few categorical semantic labels, mostly limited to predator classes and a few social events. One possibility is that graded meanings are the default way of animal communication [e.g., (18)], although this hypothesis has been much ignored and considered as less interesting than categorical perception (16). Our study suggests that explaining animal communication on categorical terms alone may be too restrictive and anthropocentric and may explain the struggle to extract meaning from some animal communication systems.