Research finds monkey calls are probabilistic

An analysis of a complex alarm call sequence in monkeys may provide clues to the evolution of meaning in human language.

Categorical meaning is the theoretical pillar of the study of animal communication, because it is how we as humans perceive and communicate. A word such as “eagle” denotes an eagle, no matter how often it is repeated in a sentence. 

Now, research reveals that the black-fronted titi monkey (Callicebus nigrifrons), a small, social primate from South America, communicates in another way, using what is known as probabilistic meaning. 

The monkeys encode information about a threat posed by a predator through the proportion of calls within an alarm call sequence. In a rough human equivalent, meaning is transmitted through how many times the word “eagle” is used, and where it falls in a sequence of sounds.  

A 2013 study determined that two different call types are the bedrock of the titi monkey alarm system: “A-calls” are used for aerial predators, and “B-calls” are used for terrestrial dangers. 

Researchers led by neuroscientist Cristiane Cäsar from St Andrews University in the UK established that each sequence of calls conveyed information about the location of a predator. {%recommended 5873%}

When an aerial danger, such as a bird of prey called a caracara (Caracara plancus), came to the ground, the monkeys used B-calls interspersed with A-call sequences. Conversely, when a usually terrestrial predator such as a southern tiger cat (Leopardus guttulus) was in the canopy, the monkeys used a B-call sequence that always began with a single A-call. 

In this most recent study, Mélissa Berthet from the University of Neuchatel, Switzerland, and colleagues used a larger sample size and a Bayesian approach to assess the information encoded in alarm call sequences and discovered that the contents were more complex than previously thought.

“By focusing on the proportion of B-call pairs, the monkeys were able to communicate gradual information about a categorical event (a predator on the ground) in a richly informative way,” the researchers write in the journal Science Advances.  

The scientists carried out the study in the titi monkey’s forest habitat, in a private nature reserve in Brazil.  Six family groups were exposed to call playback, and various predator models of taxidermy specimens placed in the trees and on the ground. 

 “We compared behavioural responses, such as whether the monkeys looked in the canopy, or on the ground, to the broadcasting of different call sequences to determine which information and sequence metrics titi monkeys attended to,” Berthet explains.

Their analysis revealed that information about predator type and location are encoded by the proportion of B-call pairs relative to all other call pairs in an alarm sequence. 

The use of probabilistic meaning in an alarm call system in monkeys raises intriguing possibilities and scope for further study.

“If common in other taxa, then a relevant next question to address is whether probabilistic meaning is the ancestral state and whether human categorical meaning evolved from it,” the researchers write. 

Berthet and colleagues also recommend that animal communication theory studies be extended beyond the classic linguistic framework, to “encompass communicative capacities that are not commonly found in humans to better understand what makes language unique”.