Probabilistic calls of the titi monkeys

A fascinating paper in Science Advances today looks at the way that a small platyrrhine monkey species conveys information about predators in its vocal communication system: “Titi monkeys combine alarm calls to create probabilistic meaning”.

The titi monkeys have two kinds of alarm calls, which they can combine together in complex sequences. The research by Mélissa Berthet and coworkers shows that the sequences carry information about not only the type of predator but also the location of the predator. Unlike human sentences, which are comprised of words that have distinct meanings, these titi monkey call sequences are probabilistic, that is, it is not the precise order or number of calls, but their quantity in combination that predict to a feature of the environment.

A couple of paragraphs from the discussion of the paper are enlightening:

Human and nonhuman animals (hereafter referred to as animals) live in environments where most stimuli appear in a continuous form, but perception is often categorical (9). For example, although rainbows consist of continuously changing wavelengths, they are perceived by humans as color bands. Similar effects are found in communication systems, including human speech. Acoustically, the human vocal tract can gradually alter the second formant of the syllable from the sound “b” (as in “beer”) to “d” (as in “deer”) and then to “g” (as in “gear”), although they are perceived in sharply categorical ways by listeners (10). Another example comes from the American Sign Language, where the hand configuration gradually differs between the words “please” (the thumb and all the fingers are selected) and “sorry” (only the thumb is selected) but is perceived categorically by deaf signers (11).

Linguists have focused quite extensively on the categorical encoding of human language. In looking for precursors or analogues of human communication in other animal communication systems, linguists and animal behaviorists have often paid attention to such categorical systems – for example, the alarm calls of vervet monkeys, which seem to form clear categories relating to predator types.

Yet maybe there is more to be seen in the simple call systems of non-human primates than correspondences between calls and features of the environment:

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.

I think this is cool. It suggests that natural language learning systems, which include strongly probabilistic features, might make some headway with animal communication systems.