Social Brain Hypothesis: Vocal and Gesture Networks of Wild Chimpanzees

Abstract

A key driver of brain evolution in primates and humans is the cognitive demands arising from managing social relationships. In primates, grooming plays a key role in maintaining these relationships, but the time that can be devoted to grooming is inherently limited. Communication may act as an additional, more time-efficient bonding mechanism to grooming, but how patterns of communication are related to patterns of sociality is still poorly understood. We used social network analysis to examine the associations between close proximity (duration of time spent within 10 m per hour spent in the same party), grooming, vocal communication, and gestural communication (duration of time and frequency of behavior per hour spent within 10 m) in wild chimpanzees. This study examined hypotheses formulated a priori and the results were not corrected for multiple testing. Chimpanzees had differentiated social relationships, with focal chimpanzees maintaining some level of proximity to almost all group members, but directing gestures at and grooming with a smaller number of preferred social partners. Pairs of chimpanzees that had high levels of close proximity had higher rates of grooming. Importantly, higher rates of gestural communication were also positively associated with levels of proximity, and specifically gestures associated with affiliation (greeting, gesture to mutually groom) were related to proximity. Synchronized low-intensity pant-hoots were also positively related to proximity in pairs of chimpanzees. Further, there were differences in the size of individual chimpanzees' proximity networks—the number of social relationships they maintained with others. Focal chimpanzees with larger proximity networks had a higher rate of both synchronized low- intensity pant-hoots and synchronized high-intensity pant-hoots. These results suggest that in addition to grooming, both gestures and synchronized vocalizations may play key roles in allowing chimpanzees to manage a large and differentiated set of social relationships. Gestures may be important in reducing the aggression arising from being in close proximity to others, allowing for proximity to be maintained for longer and facilitating grooming. Vocalizations may allow chimpanzees to communicate with a larger number of recipients than gestures and the synchronized nature of the pant-hoot calls may facilitate social bonding of more numerous social relationships. As group sizes increased through human evolution, both gestures and synchronized vocalizations may have played important roles in bonding social relationships in a more time-efficient manner than grooming.

Intentional gestures predict complex sociality in wild chimpanzee

“…In contrast shorter duration of time spent in proximity would be associated with higher 150 rate of response absence and response by communication. Roberts and Roberts 2016b). We therefore predict that 156 the centrality of individual chimpanzees will be associated with the rate of singe, rapid and control for these biological factors in all models.…”

Gestural repertoire size is associated with social proximity measures in wild chimpanzees

“…266 Next, to ensure that the sampling procedure did not bias our results, we examined 267 whether there was a similarity in association patterns between scans taken at 2 (scan 1), 4 268 (scan 2) and 18 minutes (scan 9) interval of the focal sample including both sexes. These 269 analyses showed that there was no significant difference in the number of times focal and 270 non-focal subjects were in close proximity within the samples but there was a difference 271 between the samples [Roberts and Roberts, 2016b]. The behavioural measures were then 272 derived and the method can be summarized by the following equations: 2002; Roberts and Roberts, 2016a;Roberts and Roberts, 2017;Roberts and Roberts, 2018a; community of chimpanzees have been previously determined and we used this information to 299 classify the dyads as maternally related or not [Reynolds, 2005].…”

“…Chimpanzees live in fission-fusion communities within which they associate temporarily in 108 subgroups ('parties') that vary in size, composition and duration [Goodall, 1986]. Individuals 109 in the community are often spatially and temporarily separated but nonetheless recognise 110 each other and maintain long-term relationships [Foerster et al, 2015; Langergraber et al, 111 2009; Mitani, 2009; Mitani et al, 2002;Roberts and Roberts, 2016b]. Maintaining the 112 differentiated set of social relationships that arises from this fission-fusion structure is 113 hypothesised to be a key driver of communicative complexity [Freeberg et al, 2012; Roberts 114 and Roberts, 2016b].…”

“…to initiate or maintain travelling), whilst taking into account the ability of the recipient to 265 perceive the signal [Roberts and Roberts, 2016b]. 266 Next, to ensure that the sampling procedure did not bias our results, we examined 267 whether there was a similarity in association patterns between scans taken at 2 (scan 1), 4 268 (scan 2) and 18 minutes (scan 9) interval of the focal sample including both sexes.…”

“…The reduction in uncertainty associated with 569 greater communicative complexity (a larger repertoire size) in tactile gestures may therefore 570 be related to the nature of social relationship. A larger repertoire of tactile gestures was 571 associated with affiliative contexts (reassurance, gestures to play and greeting) and these 572 contexts are important in reestablishing social relationships with chimpanzees encountered 573 less frequently than the strong proximity bonds [Roberts and Roberts, 2016b]. Thus, large 574 repertoire of tactile gestures appears to be important for these partners were the individuals 575 maintain affiliative relationship but stay in proximity less often.…”