Playback Stimuli

Calls used for the playback stimuli were taken from a databank of vocal recordings from all individuals at the three zoos amassed by SK in 2013. Calls were selected on the basis of an acoustic similarity to vocalisations recorded during an actual transfer event–where two individuals were transferred together to La Vallée des Singes in 2012–and can be described as peep-yelps²⁸ (See Supplemental Table S1 for acoustic details). Individual call sequences contained 4–6 calls and had decreasing intercall intervals along the progression of the sequence, as in the call sequence heard during the real transfer, with a mean total sequence time of 5.70 s (range of 3.79 s–7.882 s) for Apenheul and Planckendael. Because the enclosure at La Vallée des Singes is more than two-times larger than the enclosures at the other two zoos, the call sequences played there were followed by two additional calls 10 seconds later, to ensure all bonobos heard the playback sequences. All sequences were broadcast between 65–80 dB, SPL measured at 1 meter from the loudspeaker.