# A primate grammar enabling incremental processing

## TL;DR

This study shows that olive colobus monkeys use a simple call sequence structure to communicate about threats, allowing listeners to predict the nature of the threat incrementally.

## Contribution

The study identifies a minimal grammar in primate call sequences that enables predictive processing of referents.

## Key findings

- Sequence-initial calls reliably indicate the urgency of a threat.
- Subsequent calls in the sequence refine the specific referent, such as eagles or leopards.
- The structure of call sequences allows for incremental information gain and entropy reduction.

## Abstract

Characterizing the structure and function of animal communication systems provides insights into the cognitive and evolutionary processes shaping signal complexity. One key question is whether and how call sequences allow potential listeners to make predictions about the call-eliciting referents. Here, we investigated whether primate call sequences contained properties that enabled such predictive processing. We analyzed several years of experimentally elicited alarm responses from a West African forest primate, wild olive colobus monkeys. Using Kullback-Leibler divergence and prediction gain approaches, we identified a simple primate grammar that allowed predictions of referents from only minimal input. In particular, sequence-initial positions reliably discriminated urgent from non-urgent threats while the following positions increased the referential specificity regarding two main predators (eagles and leopards) and non-predatory disturbances (falling tree parts). Sequences often contained further calls, which may allow callers to either confirm the referent or to alter the conveyed information. We concluded that animal communication can contain features adapted for predictive, incremental processing, suggesting evolutionary roots older than language.

•Olive colobus monkeys have a minimal vocal repertoire with only two calls•Calls are produced in context-specific sequences that adhere to a simple grammar•Initial calls state urgency, later calls refine referents and allow eventual corrections•Information gain and entropy reduction is incremental, allowing predictive processing

Olive colobus monkeys have a minimal vocal repertoire with only two calls

Calls are produced in context-specific sequences that adhere to a simple grammar

Initial calls state urgency, later calls refine referents and allow eventual corrections

Information gain and entropy reduction is incremental, allowing predictive processing

Biological Sciences, Zoology, Evolutionary biology