Time-space-displaced responses in the orangutan vocal system

Abstract

One of the defining features of language is displaced reference-the capacity to transmit information about something that is not present or about a past or future event. It is very rare in nature and has not been shown in any nonhuman primate, confounding, as such, any understanding of its precursors and evolution in the human lineage. Here, we describe a vocal phenomenon in a wild great ape with unparalleled affinities with displaced reference. When exposed to predator models, Sumatran orangutan mothers temporarily suppressed alarm calls up to 20 min until the model was out of sight. Subjects delayed their vocal responses in function of perceived danger for themselves, but four major predictions for stress-based mechanisms were not met. Conversely, vocal delay was also a function of perceived danger for another-an infant-suggesting high-order cognition. Our findings suggest that displaced reference in language is likely to have originally piggybacked on akin behaviors in an ancestral hominid.

Fig. 1. Vocal delay by Sumatran orangutan females.

Survival plot showing the delay of subjects’ vocal onset after presentation of predator models at time zero (A) and correlational plots between alarm probability and (B) subject height up the understory and (C) infant age. Shaded areas represent 95% confidence intervals. Yellow band denotes exposure time to predator model.

Fig. 2. Graphic representation of subjects’ ascent up the canopy during the 60 min after the first sight of the predator model.

Yellow band denotes exposure time to the predator model.

Fig. 3. Bar plot showing the vocal response delay and duration after presentation of predator models at time zero.

Yellow band denotes exposure time to the predator model.

Fig. 4. Density plots of the incidence of stress behaviors (i.e., urinating and defecating) during the 15 min after the first sight of the predator model.

(A) Count (density*n) of stress behaviors. (B) Scaled density estimate (maximum of 1) of stress behaviors, enabling a better visual comparison between the patterns of physiological stress between the two types of response by the subjects.