Tool use in wild orang-utans modifies sound production: a functionally deceptive innovation?

Abstract

Culture has long been assumed to be uniquely human but recent studies, in particular on great apes, have suggested that cultures also occur in non-human primates. The most apparent cultural behaviours in great apes involve tools in the subsistence context where they are clearly functional to obtain valued food. On the other hand, tool-use to modify acoustic communication has been reported only once and its function has not been investigated. Thus, the question whether this is an adaptive behaviour remains open, even though evidence indicates that it is socially transmitted (i.e. cultural). Here we report on wild orang-utans using tools to modulate the maximum frequency of one of their sounds, the kiss squeak, emitted in distress. In this variant, orang-utans strip leaves off a twig and hold them to their mouth while producing a kiss squeak. Using leaves as a tool lowers the frequency of the call compared to a kiss squeak without leaves or with only a hand to the mouth. If the lowering of the maximum frequency functions in orang-utans as it does in other animals, two predictions follow: (i) kiss squeak frequency is related to body size and (ii) the use of leaves will occur in situations of most acute danger. Supporting these predictions, kiss squeaks without tools decreased with body size and kiss squeaks with leaves were only emitted by highly distressed individuals. Moreover, we found indications that the calls were under volitional control. This finding is significant for at least two reasons. First, although few animal species are known to deceptively lower the maximum frequency of their calls to exaggerate their perceived size to the listener (e.g. vocal tract elongation in male deer) it has never been reported that animals may use tools to achieve this, or that they are primates. Second, it shows that the orang-utan culture extends into the communicative domain, thus challenging the traditional assumption that primate calling behaviour is overall purely emotional.

Acoustic models of orangutan hand-assisted alarm calls

“…The median duration for hand kiss-squeaks was 0. was not significant according to Wilcoxon's rank sum test (P=0.40, W=103.5). This is in line with earlier findings (Hardus et al, 2009a).…”

“…The difference between hand and unaided kiss-squeaks is congruent with the hypothesis that the use of the hand serves to exaggerate size as suggested in earlier work (Hardus et al, 2009a). Even though establishing directly whether the hand is indeed used for size exaggeration may be very hard, if not impossible (Hardus et al, 2009a), our results do support this hypothesis.…”

“…The median for unaided kiss-squeaks was 0.15 mW m −2 (0.14 and 0.18 mW m −2 ) (it should be noted that these values can only be compared within the recording, as we have no accurate reference signal to gauge the sound pressure level). This difference was not significant according to Wilcoxon's rank sum test (P=0.052, W=74.5) and this is again in line with earlier findings (Hardus et al, 2009a). In any case, the effect size is very small and is therefore not expected to influence the spectral analysis.…”

“…This is in accordance with the observation Hauser, 1995, 2002;Ohala, 1984) that lower frequencies are in general associated with larger body size, as it is indeed the case in orangutan kiss-squeaks (Hardus et al, 2009a;Lameira et al, 2013b). It also is congruent with the observation that modified kiss-squeaks were emitted in circumstances perceived to be more dangerous (Hardus et al, 2009a).…”

“…Recently, a study (Hardus et al, 2009a) showed that Bornean orangutans, Pongo pygmaeus wurmbii (Tiedemann 1808), alter the acoustic characteristics of an alarm call, the kiss-squeak, by positioning a hand in front of their lips during call production, and in some cases holding detached leaves in their hand simultaneously. Orangutans produce hand and leaf kiss-squeaks in the same context of their unaided version of the call -moments of distress and towards disturbances on the forest ground, such as humans, sun bears and snakes, as well as to realistic and unrealistic predator models (Lameira et al, 2013a).…”