Third-order self-embedded vocal motifs in wild orangutans, and the selective evolution of recursion

Recursion, the neuro-computational operation of nesting a signal or pattern within itself, lies at the structural basis of language. Classically considered absent in the vocal repertoires of nonhuman animals, whether recursion evolved step-by-step or saltationally in humans is among the most fervent debates in cognitive science since Chomsky's seminal work on syntax in the 1950s. The recent discovery of self-embedded vocal motifs in wild (nonhuman) great apes-Bornean male orangutans' long calls-lends initial but important support to the notion that recursion, or at least temporal recursion, is not uniquely human among hominids and that its evolution was based on shared ancestry. Building on these findings, we test four necessary predictions for a gradual evolutionary scenario in wild Sumatran female orangutans' alarm calls, the longest known combinations of consonant-like and vowel-like calls among great apes (excepting humans). From the data, we propose third-order self-embedded isochrony: three hierarchical levels of nested isochronous combinatoric units, with each level exhibiting unique variation dynamics and information content relative to context. Our findings confirm that recursive operations underpin great ape call combinatorics, operations that likely evolved gradually in the human lineage as vocal sequences became longer and more intricate.