Vocal recruitment for joint travel in wild chimpanzees

Abstract

Joint travel is a common social activity of many group-living animals, which requires some degree of coordination, sometimes through communication signals. Here, we studied the use of an acoustically distinct vocalisation in chimpanzees, the 'travel hoo', a signal given specifically in the travel context. We were interested in how this call type was produced to coordinate travel, whether it was aimed at specific individuals and how recipients responded. We found that 'travel hoos' were regularly given prior to impending departures and that silent travel initiations were less successful in recruiting than vocal initiations. Other behaviours associated with departure were unrelated to recruitment, suggesting that 'travel hoos' facilitated joint travel. Crucially, 'travel hoos' were more often produced in the presence of allies than other individuals, with high rates of recruitment success. We discuss these findings as evidence for how motivation to perform a specific social activity can lead to the production of a vocal signal that qualifies as 'intentional' according to most definitions, suggesting that a key psychological component of human language may have already been present in the common ancestor of chimpanzees and humans.

Figure 1. ‘Hoo’ spectrograms obtained from an adult male (HW) and female (NB) of the Sonso community.

Above: ‘hoos’ given during travel events (‘travel hoos’); below: ‘hoos’ given during resting events (‘resting hoos’). Compared to ‘resting hoos’, ‘travel hoos’ are significantly shorter (0.125s vs. 0.336s, t-test, N=20, t=4.455, p=0.001), have a lower maximum fundamental frequency F₀ (178.83Hz vs. 220.47Hz, t-test, N=20, t=3.139, p=0.006), are less modulated (difference D between F_0MAX and F_0MIN: 37.17Hz vs. 89.23Hz, t-test, N=20, t=3.796, p=0.001), and consist of more elements (mean 2.7 vs. 1.0, t-test, N=20, t=-3.042, p=0.014). Analyses were based on N=20 calls (N=5 travel hoos, N=5 resting hoos recorded from HW and NB, respectively).

Figure 2. Mean plot showing the sequential order of behaviours observed during travel events that included at least one ‘travel hoo’.

‘Initial gazing’ and ‘hooing’ (p=0.975), and ‘waiting’ and ‘checking’ (p=0.971), were not significantly different from each other, but differed from ‘initial moving’ (p=0.002, p=0.015, p<0.001 and p<0.001, respectively, Tukey HSD pairwise comparisons).

Figure 3. Profile plot showing the successes of focal individuals in recruiting other individuals as a function of the presence of ‘hooing’ and ‘waiting’.

The production of ‘hoos’ had a significantly positive effect on recruitment (GLMM, t=4.857, p<0.001), while the presence of ‘waiting’ had a significantly negative effect (GLMM, t=-2.457, p=0.015).