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Abstract
Human emotional expressions, such as laughter, are argued to have their origins in
ancestral nonhuman primate displays. To test this hypothesis, the current work examined
the acoustics of tickle-induced vocalizations from infant and juvenile orangutans,
gorillas, chimpanzees, and bonobos, as well as tickle-induced laughter produced by
human infants. Resulting acoustic data were then coded as character states and submitted
to quantitative phylogenetic analysis. Acoustic outcomes revealed both important similarities
and differences among the five species. Furthermore, phylogenetic trees reconstructed
from the acoustic data matched the well-established trees based on comparative genetics.
Taken together, the results provide strong evidence that tickling-induced laughter
is homologous in great apes and humans and support the more general postulation of
phylogenetic continuity from nonhuman displays to human emotional expressions. Findings
also show that distinctively human laughter characteristics such as predominantly
regular, stable voicing and consistently egressive airflow are nonetheless traceable
to characteristics of shared ancestors with great apes.