Nonadjacent dependencies and sequential structure of chimpanzee action during a natural tool-use task

Area of Science:

• Cognitive Ethology
• Comparative Psychology
• Evolutionary Anthropology

Background:

• Human complex behaviors often involve nonadjacent dependencies, potentially arising from hierarchical sequence organization.
• Studying nonhuman ape tool use offers insights into the evolution of these structural properties.
• Previous evidence for hierarchical organization in ape tool use lacked objective methodologies, and nonadjacent dependencies remained uninvestigated.

Purpose of the Study:

• To investigate the extent of nonadjacent dependencies in wild chimpanzee tool-use sequences.
• To determine the hierarchical organization of action sequences during naturalistic tool use.
• To explore interindividual variability in action sequence organization.

Main Methods:

• Utilized estimations of mutual information (MI) to measure dependency strength between sequence elements.
• Analyzed action sequences from wild chimpanzees performing a naturalistic nut-cracking task.
• Compared observed dependencies against non-hierarchical Markov models, accounting for repeated actions.

Main Results:

• Half of adult chimpanzees produced nonadjacent dependencies at significantly greater distances than predicted by Markov models.
• A power-law relationship in MI decay with sequential distance indicated hierarchical organization in most chimpanzees.
• The findings support a model where short action subroutines (2-8 actions) are hierarchically arranged.

Conclusions:

• Wild chimpanzee tool use demonstrates hierarchical organization and nonadjacent dependencies, mirroring aspects of human behavior.
• Interindividual variability exists in the mechanisms of action sequence organization.
• Results inform hypotheses on the coevolution of language syntax and tool-action across hominin history.