An immigrant female chimpanzee copies an inefficient behaviour without clear function to mitigate risk of rejection in the new tribe

Zoo-housed female chimpanzee adopts local female-specific tradition upon immigrating into a new group. Zoë Goldsborough et al. Behaviour, Feb 25 2021. https://doi.org/10.1163/1568539X-bja10075

Abstract: Wild female chimpanzees typically migrate to a neighbouring community at the onset of sexual maturity, a process that can be dangerous and unpredictable. To mitigate the risk of rejection in the new community, immigrants may employ several behavioural strategies. During the integration of two chimpanzee females at Royal Burgers’ Zoo (Arnhem, The Netherlands) one of the immigrant females rapidly copied a local tradition — the crossed-arm walk — which has been present in the group for over 20 years. She copied the behaviour after meeting only one resident female, and showed the behaviour frequently throughout a 6-month observation period following the introduction. The other immigrant female never adopted the crossed-arm walk, highlighting the variation in behaviour by immigrants upon integration, as well as the potential associated consequences: in a separate observation period 2 years later, the female who copied the local tradition appeared more socially integrated than the other immigrant female.

Keywords: chimpanzees; social integration; social learning; behavioural copying; tradition

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4. Discussion

In this report, we describe how an immigrant female chimpanzee (Moni) copies an idiosyncratic group-specific behaviour soon after being introduced to a new group. Two days after first contact with a resident female (a crossed-arm walker), before meeting any other resident individuals, Moni engages in crossed-arm walking: an inefficient means of locomotion without clear function that is enacted by the majority of females in the new group. The immediacy of the acquisition of this behaviour by the immigrant, as well as the introduction context during which this behavioural copying occurred, both invite us to speculate that Moni may have enacted an integration strategy that facilitated social favourability (as suggested by Moni’s steep increase in social integration over time). Another immigrant female (Erika) who similarly encountered the crossed-arm walkers first and for similar durations during integration — yet never adopted the crossed-arm walk — did not exhibit marked improvements in social integration over time. These findings provide anecdotal evidence that behavioural copying might be an expression of social motivation which in conjunction may facilitate social acceptance in chimpanzees.

Anecdotal evidence should be treated with care (Sarringhaus et al., 2005; Sándor & Miklósi, 2020). Yet, it is noteworthy that the anecdote reported here does not stand on its own. There are at least two other reports showing that chimpanzees may copy behaviourally ineffective/functionless behaviour (Hobaiter & Byrne, 2010; van Leeuwen et al., 2014) and some indications that wild female chimpanzees may gradually (>1 year) adopt local conventions upon immigration (Luncz et al., 2012; Luncz & Boesch, 2014). Hence, it is conceivable that Moni strategically adopted the local convention. Moni’s behavioural copying upon immigration is further reminiscent of studies reporting conformity in migrating animals (Van de Waal et al., 2013; Luncz & Boesch, 2014; Aplin et al., 2015). Especially when individuals are uncertain of their predicament, for instance during migrations, the inclination to align oneself with the (new) majority can take precedence (van Bergen et al., 2004; Morgan et al., 2012; Smolla et al., 2016; Jones et al., 2018). It is noteworthy that chimpanzee introductions in captive settings are notoriously difficult, often resulting in protracted aggression and injuries (Brent et al., 1997). This may further incentivize immigrating individuals to gain social acceptance from the group. In this light, it is relevant to emphasize that Erika, the immigrant who did not copy the crossed-arm walk, was introduced to the new group together with her mother, Marlene. As such, Erika might have been less uncertain/insecure in her new environment compared to Moni, who was introduced alone and had been housed solitary for years prior to her arrival. By extending her behavioural repertoire with the local customary behaviour of crossed-arm walking, Moni might have alleviated her uncertainty by gaining social benefits as expressed by her expedited social integration. The possibility that Moni engaged in behavioural copying in order to increase social favourability is consistent with the fact that Moni selectively directed her grooming efforts towards the two most prolific crossed-arm walkers (Morami and Moniek). Interestingly, though, while crossed-arm walk is shown by the majority of the Burgers’ group, Moni first engaged in the behaviour after having met only one resident female. This suggests the workings of another yet related mechanism that could be sparked by uncertainty, namely copying the behaviour of the first individual one encounters (Galef & Whiskin, 2008; van Leeuwen et al., 2016).

Similarly, given that close affiliative relationships are thought to facilitate social learning (Bonnie & de Waal, 2006; Hobaiter et al., 2014; van Leeuwen et al., 2014; Lamon et al., 2017), it is also possible that Moni copied the crossed-arm walk from Morami given the nature of the social bonds between the females. According to the caretakers, Moni (but not Erika) and Morami responded very positively to one another when introduced, which is supported by the fact that they went from visual contact to sharing an enclosure within one day. However, as Moni engaged in the crossed-arm walk already two days after first meeting Morami, in our view, it is more plausible to infer that Moni proactively tried to establish connections by means of behavioural copying than the inverse alternative where Moni established close social bonds of which the behavioural copying was but an expression. This interpretation is corroborated by the observation that although Moni directed her grooming efforts selectively to Morami and Moniek, this directional social bonding behaviour was not selectively reciprocated. Interestingly, when an entire group of chimpanzees migrated to a resident group, the adoption of local behaviour by immigrant chimpanzees only occurred after affiliative social relationships had been formed (Watson et al., 2015). In conjunction, these findings suggest that chimpanzees may cope with immigration by converging toward locally adaptive information, but that the extent of experienced stress owing to uncertainty (higher when migrating alone — like Moni — than with familiar group members — like Erika and in the study by Watson and colleagues) moderates its immediacy.

To our knowledge, the crossed-arm walk is the first female-specific tradition in chimpanzees ever reported. While sex-specific preferences in play object choices have been found in juvenile chimpanzees (Kahlenberg & Wrangham, 2010), and sex differences in rates of social learning (Lonsdorf et al., 2004), there are no reports on sex-specific conventions in chimpanzees or primates in general. The crossed-arm walk tradition was presumably instigated by Morami, who was pregnant at the time. Given that the form of the crossed-arm walk (which closely resembles how a chimpanzee would carry an (deceased) infant), and the female-specific expression of the behaviour, it is tempting to speculate that maternal instincts could have played a role in the inception and spread of the behaviour. In this light, it is of interest to note that Erika is the only female in the group who has never had an infant.

The reported observations and inferences warrant differential scrutiny (Sándor & Miklósi, 2020). Moni’s adoption of the crossed-arm walk provides an objective account of chimpanzees’ capacity to copy behavioural sequences (Hobaiter & Byrne, 2010; van Leeuwen et al., 2014), yet proponents of the “zone of latent solutions” approach (Tennie et al., 2020) would challenge this notion by arguing that the crossed-arm walk was merely individually reinvented by Moni. In our view, however, the speed at which Moni adopted the crossed-arm walk, in conjunction with the idiosyncrasy of the crossed-arm walk posture/gait, defies — probabilistically — the reinvention hypothesis. The link between Moni’s crossed-arm walk adoption and her facilitated social integration compared to the immigrant who did not copy the crossed-arm walk (Erika) is impossible to prove based on the reported observations. There are myriad reasons why Moni might have become more socially integrated than Erika — for instance, a difference in personality, a difference in life histories (i.e., Moni was hand-raised while Erika was mother-reared), and the differences in introduction contexts. Events during the observation period could also have affected differences in integration, for instance, Moni experienced a stillbirth during Period 1 and received increased affiliation from certain group members afterwards (Goldsborough et al., 2020). Therefore, our anecdotal report could best be interpreted as (i) evidence that chimpanzees can (and are motivated to) alter their behaviour upon integration into a new group (likely owing to social influences), and (ii) a tentative pointer to the possibility that in non-human animals behavioural similarity may induce relative favourability, just like in humans (e.g., McPherson et al., 2001). Tracking behavioural responses of animals integrating into new groups may thus represent an interesting approach for examining animals’ phenotypic plasticity and exploring the evolutionary origins of behavioural copying and its ramifications for social acceptance.