Week 6 – Exploring cultural techniques in non-human animals: how are flexibility and rigidity expressed at the individual, group, and population level?

This early draft was authored by Sadie Tenpas, Manon Schweinfurth, and Josep Call.

Non-human animal (henceforth animal) culture has been demonstrated across a variety of species and, for some, includes technical traditions. Like human culture, animal culture faces the dilemma of balancing flexible and rigid behaviour. On one hand, a culture must allow for the creation, acquisition, or novel application of cultural behaviours. On the other hand, a culture must ensure a high degree of similarity between individual performances, leading to stable traditions that persist across generations despite disruptive influences. The dominant argument to understand the rigidity of cultural transmission within animals has focused on social learning, with mechanisms such as imitation leading to the highest fidelity of transmission. Thereby animal culture has often been limited to examples explained by social learning alone and has historically excluded behaviours that could be explained by genetic or ecological factors. In this chapter we argue that the effect of these factors, rather than exclude, may assist social learning in explaining cultural differences. To explore these concepts further, we will examine and define flexible and rigid behaviour at the individual level, then expand to the group and population level. We propose a framework that combines social learning theory and the Cultural Attraction Theory to non-human primate cultural techniques, who are our closest living relatives and thus can give insights into human culture. We argue that cultural traits are the product of three poles of attraction, i.e. individual predispositions, social influences and ecological contexts. This new framework can contribute to better understanding the origins, persistence, and eventual demise of cultural techniques.

Introduction

The existence of animal culture, understood as those group typical behaviour patterns that are shared by members and that rely on socially learned and transmitted information (Laland & Hoppitt, 2003), is commonly accepted today. Following over 50 years of debate, we have convincing evidence of cultural behaviour in birds (Aplin, 2019), cetaceans (Day et al. 2001; Allen et al. 2013), and primates (Whiten & van de Waal, 2017), among others (Laland & Evans, 2017; Whiten, 2019). Cultures consist of traditions that are distinct behaviour patterns, shared by at least two members of a group, which persist over time and that new practitioners acquire, in part, through socially aided learning (Fragaszy & Perry, 2003). These traditions can include techniques that produce material change (Lamon et al., 2017). For example, chimpanzees (Pan troglodytes) create and use stick tools during termite fishing (Goodall, 1964). However, for the purpose of this chapter we will extend our definition of techniques beyond cases that produce material changes to include more cultural traditions, such as different manual techniques to perform a certain action. In such, we understand techniques as complex actions whose instrumental function is to produce changes in the environment, both physical and social.

The ability to ratchet up the complexity of cultural traditions, leading to both increased efficiency and productivity, is created by cumulative culture (Tennie et al. 2009; Dean et al. 2014); a phenomenon primarily observed, and by some exclusively described, in humans (Dean et al.2014). Although the learning of many animal species is influenced by the observation of or interaction with another animal or its products (Heyes, 1994), there is minimal evidence of cumulative culture in animals (but see – Sasaki & Biro, 2017). The richest evidence of animal culture comes from one of our closest living relatives, chimpanzees (Marshall-Pescini & Whiten, 2008). Therefore, we will focus primarily on this species even though we will also consider data from other species to shed light onto our own cultural evolution.

Long-term research of chimpanzees has provided us with extensive documentation of cultural traditions, highlighting variation between communities and distinct cultural repertoires. Most famously, Whiten et al. (1999) collected candidate cultural behaviours from seven long-term field sites across Africa. Using a systematic approach, i.e. the method of exclusion or the ethnographic method, candidate behaviours shown only to occur within some communities were determined to be cultural when the variability between groups could not be otherwise explained by ecological of genetic differences, thus leaving social learning as the remaining source of the observed variation (Whiten et al. 1999).

While chimpanzees show a great variety of traditions, there is little evidence that these traditions are frequently updated through additional knowledge or behaviours (e.g. Gruber et al., 2009; but see Biro et al., 2003). This tendency toward stasis is especially striking considering that chimpanzees demonstrate an impressive ability to innovate solutions to novel problems (Bandini & Harrison, 2020). So, while chimpanzee cultures may not reflect the same process or complexity of human cumulative culture, chimpanzee culture still undergoes cultural evolution in which a balance must be struck between the novel creation, variation, or application of cultural behaviours and the high degrees of similarity between individual performances, leading to stable traditions that persist across generations despite potential disruptive influences. Thus, the questions arise, which mechanisms and factors drive chimpanzee cultures toward stasis, which drive them toward change, and how can we understand this in respect to cultural techniques?

To address these questions, we will begin by exploring the relationship between flexibility and rigidity in culture at the individual, group and population level. Using these concepts, we will investigate how flexibility and rigidity in chimpanzee culture are currently understood and how social learning explains them. After highlighting the limitations of the social learning perspective, we will address an alternative theory utilized in the cultural evolutionary literature that may shed insights into factors supporting cultural variation and stability. We propose a framework that combines these two theories to provide a more satisfactory explanation of the occurrence of flexibility and rigidity in chimpanzee techniques.

Defining flexibility and rigidity in primate techniques

To fully capture the concept of cultural evolution, we will illustrate different levels of flexibility and rigidity in primate cultural techniques, i.e. individual, group, and population. Let’s imagine a scenario in which a chimpanzee uses a wooden hammer to crack open nuts. Beginning at the individual level, we define a spectrum with liberalism, describing an individual’s tendency or disposition to change, on one end and conservatism, describing an individual’s tendency or disposition to remain the same, on the other end (Figure 1a). Note that we expect to see variation both within and between individuals along this spectrum. In some cases, driving a change in behaviour is necessity, which we understand to occur when an individual can improve the efficiency of their technique by updating it through modification, variation, or invention of behaviour. By distinguishing the necessity for a change in an individual’s behaviour, or lack thereof, we can further breakdown liberal and conservative behaviour in respect to adaptive decisions. Consider, for example, that an environmental change resulted in increased hardness of the nuts used for extractive foraging such that a wooden hammer was no longer hard enough to crack the nut. Therefore, the liberal individual selects a different hammer, such as a stone to crack the nut. Under this condition, the individual is behaving innovatively, responding to the necessity for change. Alternatively, if we consider an example in which the nutshell remains the same hardness, yet the liberal individual switches hammers anyway, the individual is behaving creatively because it changes its behaviour despite no need for change. When considering the individual behaving conservatively, the same scenarios apply. If the conservative individual maintains their behaviour, but there is need for change, the individual is behaving perseveringly. If the conservative individual maintains their behaviour, and there is no need for change, the individual is behaving consistently.

Broadening to the group level, we introduce social factors guiding behavioural change (or lack of) using the same context and framework as before. First, in defining a spectrum at the group level, we have flexibility, describing a group’s tendency to change a behaviour, on one end and rigidity, describing a group’s tendency to maintain a behaviour, on the other end (Figure 1b). Envisioning the same nut cracking scenario, when a flexible group needs to change their behaviour and does so through successful social learning and transmission, this change can be described as an advancement. When a flexible group does not need to change their behaviour, yet does anyway, this change can be described as a shift. Alternatively, when a rigid group needs to change their behaviour but does not, this lack of change can be described as fixedness. When a rigid group does not need to change their behaviour, and does not, this lack of change can be described as stasis.

Expanding further, we can examine the cultural repertoire of a group to understand the population dynamics as a product of evolution. Problems arise when a group leans too far toward either end of the flexible-rigid spectrum. For example, if a group leans too far toward the flexible end, such that they are constantly creating and abandoning cultural behaviours, they are not able to retain long-term cultural information and risk disadvantage when facing old problems again. Over evolutionary time, an overly flexible group can lead to cultural breakdown (Figure 1c). Conversely, if a group leans too far toward the rigid end, such that they rarely acquire new cultural behaviours or update existing ones, they are unlikely to innovate new solutions or adapt to changing environments. Over evolutionary time, an overly rigid group can lead to cultural stagnancy (Figure 1c). Therefore, communities must find a balance between flexible creation, variation, and application of cultural behaviours while also maintaining relatively rigid transmission and performance, allowing cultural traditions to form and persist across generations.

One way we can understand these dynamics is through frequency dependent evolution. Cultural traits, such as nut cracking, provide an advantage to performers because they can gain access to a resource, for instance, and thus are favoured by natural selection (Whitehead et al., 2019). At the same time, the behaviour is not fixed, but flexible and thus individuals and groups can adapt to their environment by changing their behaviour. Changed behaviour that is adaptive may become part of the repertoire of an individual conforming to others and thus spread to the group. However, if there would be only conformists in a group, no new behaviours would ever emerge (Kendal et al., 2009). This highlights the interplay of conformists and anti-conformists, which are both needed, and their frequency is dependent on the other strategy. Only if there is the right frequency of both, cultural traits can emerge and persist.

When considering primate cultural techniques, comparative researchers explain the relative stability through the fidelity of social learning mechanisms. A social learning mechanism of high fidelity would allow for rigid transmission of techniques, retaining a high degree of similarity between performances of the demonstrator and observer, such that the technique remains stable as it is transmitted between individuals over a long period of time, protected from disruptive influences such as miscopying or the loss of necessary information required to sustain a tradition (Charbonneau, 2018). Mechanisms of lower fidelity offer less rigidity in the transmission of a technique, such that subsequent performances may differ from that of the demonstrator, not able to retain a high degree of similarity and being more susceptible to said disruptive influences. In this use of fidelity, comparative researchers are specifically employing ‘propensity fidelity’, outlined by Charbonneau (2018) as focusing on the specific mechanisms involved in the transmission of a cultural trait, where certain transmission mechanisms are more or less faithful than another in perpetuating a tradition. We will illustrate these mechanisms and their associated fidelity in the next section by applying them to chimpanzee behavior.

Social learning and cultural fidelity in chimpanzees

Historically and to this day, social learning mechanisms have been the dominant source for understanding the spread and persistence of chimpanzee cultural behaviour. As stated in the previous section, it has been argued that high-fidelity social learning mechanisms have the ability to stabilize cultural traditions through faithful transmission from one individual to the next. Imitation, the copying of another’s actions (Tomasello 1996), and teaching are understood to result in a high degree of fidelity and plays an important role in the evolution of culture (Whiten et al., 2009). Other social learning mechanisms such as emulation, copying the environmental or end-results of another’s actions (Tomasello, 1996), and local and stimulus enhancement, when the actions of another draw the attention of the observer to particular locations or stimuli, respectively (Hoppitt & Laland, 2013), are thought to be of lower fidelity. In such, when low-fidelity mechanisms are used, the techniques performed by the demonstrator are not retained and performance can be dissimilar between episodes of transmission from one individual to the next (Hoppitt & Laland, 2013).

Following the onset of research into the culture of wild chimpanzees, the idea of apes’ ability to ape (i.e. imitate) and the existence of culture went hand in hand (Whiten et al. 2009). However, subsequent experimental research into the social learning abilities of chimpanzees questioned and critiqued this notion, reporting that unlike humans, chimpanzees are not natural imitators, but rather emulators (Whiten et al. 2009). In an experiment, chimpanzees were exposed to a demonstrator using a rake tool to acquire a food-reward (Tomasello et al. 1987). Individuals who observed the demonstrator were more likely to adopt the behaviour than those who did not observe the demonstrator. However, they did not acquire the technique used by the demonstrator to obtain food from awkward positions. These results suggest an intermediate mechanism between stimulus enhancement and imitation, termed emulation (Tomasello 1990). Later studies reaffirmed chimpanzees’ tendency to emulate end-results over imitate a demonstrator’s actions (Tomasello & Call, 1997; Whiten et al. 2004; Call et al. 2005; Whiten et al. 2009).

Many of the studies mentioned earlier relied on a single episode of transmission, between one demonstrator and an observer, but cultural transmission requires multiple iterations of these episodes to sustain traditions. To address this, Whiten et al. (2005) conducted a transmission chain experiment with chimpanzees to experimentally replicate spontaneous imitation through the transmission of a novel food processing technique. In doing so, an ‘artificial fruit’ was presented to three groups of chimpanzees that could be opened by either using a stick to lift a hook (‘lift’ method) or poking a stick into a trap (‘poke’ method). For each group, one individual was selected and exposed to the artificial fruit during which one was trained under the lift method, one was trained under the poke method, and another was not trained at all. The results revealed that no individual in the untrained group was able to access the food either via the lift or poke method. Importantly, individuals of both trained groups were able to learn the planted technique through observation of group members. However, the initial technique introduced into the trained groups was not exclusively retained by all group members, nor were all individuals able to open the artificial fruit. This variation attracted criticism.

Claidière and Sperber (2010) highlighted two features of their results, weakening their argument that imitation offers sufficient fidelity to explain the stability of the observed transmission. First, because not all individuals performed the demonstrated technique, stimulus enhancement by increased manipulation of the device could have led other individuals to engage with the device. Thereby, the naïve individual could have explored the device individually and discovered their own technique, leading to several techniques in the group. Even if strictly adhering to imitation to solve the problem, increased interaction may still have resulted in the observing individual spontaneously discovering the alternative technique. Second, had the groups received more naturalistic exposure to the artificial fruits, such as unrestricted access for a longer period of time, one might expect that the individuals eventually perform the most efficient method, or both if equally efficient, rather than that technique initially propagated through imitation. This is illustrated by the fact that members of the lift group more often converted to the poke method than the other way around. Thus, they suggested that the social learning mechanisms demonstrated in the experiment may act as propagation mechanisms, but that complementary stabilization mechanisms, such as ecological availability, reward-based factors (that combine an ecological and a psychological aspect), content-based psychological factors and source-based psychological factors (Sperber & Claidière, 2008), must exist to explain wild observations of cultural transmission that are not being modelled in these experiments.

This criticism highlights a more fundamental problem, demonstrated by the method of exclusion which defines cultural behaviour based on social learning alone (Whiten et al., 1999). This method isolates behaviours that are shown by groups, but are not shared species wide, which cannot be explained by genetic or ecological factors. Using this method, chimpanzees have been described to demonstrate at least 39 cultural traits (Whiten et al., 1999). These group behaviours might be difficult to explain by social learning mechanisms of lower fidelity than imitation (i.e. such as stimulus enhancement) alone. For example, variations observed in tool-use techniques like ant-dipping, in which one group uses a long wand with one hand and the ants are wiped off with the other (McGrew, 1974) and another group uses a short wand and ants are transferred directly to the mouth (Nishida, 1973), could utilize differential copying of either technique. Here, a mix of imitation, other social learning mechanisms, and individual learning are likely at play as implied by previous experimental research (Whiten et al., 2004).

In addition, while the method of exclusion enables cataloguing and identifying cultural traits, genetic and ecological factors are often difficult to entirely exclude (Laland & Hoppitt, 2003; Schuppli & van Schaik, 2019). For example, ant-dipping techniques in chimpanzees are dependent on the nature of the ants’ behaviour (Humle and Matsuzawa, 2002). Techniques differ based on the abundance of ants, their aggressiveness, and the severity of their bite, such that when each of these were high, they used the long wand and hand swiping technique. Under the method of exclusion, this ecological explanation would lead to the exclusion of ant-dipping as a cultural trait. However, we would argue that by excluding behavioural variation understood to be influenced by ecological factors, we may be overlooking the very mechanisms that support cultural variation and stability similar to those complementary mechanisms proposed by Claidière and Sperber (2010).

To illustrate how ecological factors are arguably necessary to the existence of culture, let us consider human cuisine. The use of olive oil is a cultural feature of Mediterranean and middle eastern cuisine, which has evolved due to the historic availability of olive trees. However, we would not exclude dishes involving olive oil from being cultural traditions of these regions because olives have historically not been available in other parts of the world, as one would do if employing the method of exclusion here. Rather, we understand that these recipes are created, taught, and otherwise passed down through generations and are adapted to and likely continue to be supported by the availability of olives. While this example is most certainly over simplified, one can understand that such ecological features not only make cultures distinct, but may additionally provide a consistent feature, which maintains these cultural traditions over generations. This might also apply to animal cultures. Take again the example of the ant-dipping variations, one could imagine that a distinct technique observed in one population is propagated through the group via social learning mechanisms but is stabilized by the nature of the ant species. Unlike the transmission chain experiments, individuals interacting with more aggressive ants may be less likely to switch to or discover an alternative, potentially more efficient, technique, dissuaded by the severe bites and thus maintaining the behaviour over time.

Both the transmission chain experiments and the method of exclusion highlight the emphasis placed on imitation and other high-fidelity social learning mechanisms in explaining cultural variation and stability by those adopting the social learning perspective.  In the next section, we would like to explore factors outside of social learning mechanisms that may have been excluded or overlooked, but that may assist in the propagation and stabilization of cultural techniques. Here, we will shift our focus to psychological and ecological factors by addressing cultural attraction theory in the realm of animal culture and applying it to examples chimpanzee cultural techniques.

Cultural attraction theory and animal culture

Cultural attraction theory (henceforth, CAT) can explain how cultural traits (such as norms, beliefs, skills, etc.) change in their distribution and form over time (Sperber, 1985; Sperber, 1996; Buskell, 2017; Scott-Phillips et al. 2018). Initially introduced to understand human culture, CAT was intended to reconcile two observations: i) that at the micro-level, transmission of information between humans is generally not a copying process and typically results in modifications; and ii) that at the macro-level, cultural information is relatively stable within entire populations and often remains so across generations (Claidière & Sperber, 2007). These observations suggest that the micro-process of transmission is not itself faithful enough to explain macro-stability. Thus, it contrasts the social learning perspective in which cultural traditions are mainly based on high-fidelity social transmission (Claidière & Sperber, 2007). Instead, CAT posits that the cognitive mechanisms producing social transmission at the micro-level create cultural chains of causally related events. Thereby the micro- and macro-level are interconnected. Mental representations (knowledge, beliefs, intentions etc.) allow for public productions (artefacts, behaviour, speech, etc.) which influence the mental representations of others and thus both levels act as positive feedback loops (Scott-Phillips et al. 2018). Transformations in these chains are biased because they are not totally random. Hence, over time cultural traits emerge within a population. In a system of cultural chains, cultural attraction is the probabilistic favouring of specific traits. Factors of attraction that bias cultural traits influence an individual’s mental representation and thus the production of traits, which can be applied to non-human cultures.

CAT suggests that factors of attraction can be divided into three categories, i.e. reconstructive learning, motivational factors, and ecological factors (Buskell, 2017). To understand factors related to reconstructive learning, we must turn to the transformative nature of transmission assumed by CAT. In such, an individual acquiring a new cultural trait never strictly copies the variant(s), but instead draws on the information transmitted in addition to personal background knowledge, inferential abilities, and interests to produce their own variant(s) (Claidière & Sperber, 2007). Therefore, reconstructive learning consists of processes involved in inferential learning which are influenced by individual beliefs, emotions, judgements, and cognition (Buskell, 2017). This is distinct from motivational factors, that make us want to use or transmit a particular variant (Morin, 2015; Buskell, 2017). Finally, ecological factors encompass environmental elements influencing cultural traits. Ecological factors can range from features of the biological or physical environment, such as food and material resources, to behaviours and artefacts, including public representations used for communication (Heintz & Claidière, 2015; Morin, 2015; Buskell, 2017; Scott-Phillips et al. 2018).

When considering the factors of attraction that may bias animal cultural variants, we imagine three distinct, but highly intertwined ‘poles’: one social, one individual, and one ecological. These categories are similar to the ones described for CAT, but also incorporate the social transmission mechanisms highlighted by the social learning perspective. It is our position that both theoretical approaches may complement one another and can be jointly pursued. The social pole encompasses factors of attraction related to the social environment, including accessibility to observe con-specifics or their products and community structure. The individual pole encompasses factors of attraction related to an individual’s psychology including emotional, motivational, and sensorimotor processes, be they determined or learned. Finally, the ecological pole encompasses factors of attraction related to ecological availability and opportunity and environmental state, ranging from climate considerations, to the available vegetation and landscape features. As stated previously, we imagine these categories separately, but recognize that they are highly interactive, influencing one another greatly. For example, when considering individual psychological mechanisms and the social environment, combinations of the two might uniquely influence cultural variants through social learning strategies such as dominance rank based social learning, in which chimpanzees selectively observe and copy group members of higher dominance (Kendal et al. 2015; Kendal et al. 2018). Further, given the species, we may see that the strength of each pole’s ability to bias cultural traits varies. As factors of attraction can influence across different levels, we would also expect to see variation of pole strength both within and between individuals, which in turn guides liberal and conservative behaviour. In the next section we will illustrate how a combined approach offers new understanding toward the flexibility and rigidity of chimpanzee cultural techniques.

Applying cultural attraction theory to chimpanzee techniques

Techniques can be complex actions that produce changes in the environment, both physical and social. This can be extended to cultural traditions that include actions beyond tool-use, such as specific social grooming techniques. We analyse techniques by focusing on three aspects: i) the function of a target behaviour, ii) the material means necessary to produce the target behaviour, and iii) the actions with which the target behaviour is achieved. For example, in the case of ant-dipping, extracting ants is the function; selecting the type of tool is the material means; and hand wiping the tool is the action. Between groups and subgroups, we expect variation in techniques to occur at one or more of these aspects.

We will focus on three techniques to illustrate our approach. Table 1 presents these techniques along with potential social learning mechanisms and cultural attractors supporting them. Through our integrated conceptualization of social learning and cultural attraction, we will illustrate how each technique is differentially influenced by the three poles of attraction and how this impacts the expression of flexible and rigid behavior, with sponging demonstrating strong individual influence, ant-dipping demonstrating strong ecological influence, and hand-clasping demonstrating strong social influence (Figure 2).  Crucially, even if a technique is mainly affected by one of the poles of attraction, the others are still relevant.

Behavior	Technique	Social learning	Cultural attraction
Sponging	Insert item in mouth Chew and suck on item Insert item in hole Repeat	Local + stimulus enhancement	Chew plant material repeatedly Insert fingers in hole Water aversion
Ant-dipping	Insert stick in ant nest or near trail May swirl stick Remove stick Harvest ants by (2 techniques): a. Direct stick to mouth b. Sweep stick with hand and pop ants from hand into the mouth Repeat	Emulation or Imitation	Avoid insect bites Insert tool in hole
Hand-clasping	Grab and lift arm high in the air Hold arm against own’s by (2 techniques): a. Interlocking hand palms b. Pressing wrists or forearms Groom armpit Switch sides	Ontogenetic ritualization or Imitation	Low branch nearby absent Prevent partner from lowering her arm Increase own arm’s comfort

In Figure 2, we attempt to visualize our three-pole framework with those techniques more strongly influenced by a given pole, appearing closer to them. The stronger the influence of a pole, the more conservative the individual tends to be and in turn, the more rigid a group will be such that when novel innovations appear, they are unlikely to diffuse within the group. Under conditions where the influence of a pole lessens and/or the influence of two or more poles cancel each other out, the individual can behave more liberally, leading to innovations that can be adopted and demonstrated flexibly at the group level. Let us consider, for example, sponging behaviour, which we consider to be strongly influenced by the individual pole. Sponging is a foraging technique in which a wad of leaves and/or other vegetation is folded and/or chewed and used to collect water then squeezed in mouth (Goodall, 1964). Considered to be a universal behaviour, sponging is not recognized as a cultural trait by the method of exclusion (Whiten et al. 1999). Further, when captive chimpanzees have been provisioned with the materials to sponge, they spontaneously invented the technique without prior experience or observation (Kitahara-Frisch & Norikoshi, 1982). Such findings suggest that given the correct environment, chimpanzees can perform sponging behaviour relying predominantly on their individual dispositions. However, a recent observation of the transmission of a novel sponging variation using moss as different material means has provided evidence for social learning and therefore suggests sponging might be a cultural trait (Hobaiter et al., 2014).

These observations also highlight the roles of the ecological and social poles. For example, this novel variation occurred under an unusual ecological context of discovering a novel clay-pit, which had been repeatedly flooded by the nearby river.  As a consequence, the clay-pit attracted larger groups which may have increased competition. The social influence of competition combined with the unusual ecological context may have allowed for the initial innovation of moss-sponging, allowing space for variation in material means that could diffuse via social learning through the group.

Ant-dipping is a foraging technique in which an individual selects a wand tool, inserts the wand into a nest or near a trail of ants, may move or hold the wand still to collect ants, then removes the ants by either bringing the wand directly to the mouth or by using the opposite hand to remove ants and put into the mouth (Humle, 2011). In contrast to sponging, ant-dipping is only present in some chimpanzee populations and the technique differs in terms of material means and actions (McGrew, 1992; Whiten et al., 1999; Yamakoshi, 2001; Humle & Matsuzawa, 2002 Humle, 2011). Depending on the aggression of the ants present, chimpanzees will select wands of different length and apply different ant gathering techniques to avoid severe bites. Under conditions with high probability for bites, the ecological influence becomes more important. The behaviour is predicted to be more rigid to avoid discomfort. In a setting where ants vary in aggressiveness or show low levels, behaviours are predicted to be more flexible, allowing for more variation. Indeed, chimpanzees from sites with more aggressive ants use longer tools and collect the ants by hand wiping them from the wand whereas those from sites with less aggressive ants were dipped with shorter tools associated with the direct to mouth method (Schöning et al. 2008). Strikingly, the ecology can probably not exclusively explain ant-dipping techniques as chimpanzees from two sites with ants that show the same level of aggression use differently sized wands (Möbius et al. 2008). This suggests that social learning may play an important role in maintaining these variations (Humle, 2011). In combination with social learning, individual experience of developing chimpanzees may further reinforce a group-specific technique as they encounter ant bites. Through combining strong ecological influences with social learning and individual experience, we can imagine that variation of techniques between groups is maintained.

In addition to these two material-based examples, hand-clasping is a social custom that is only present in some groups and varies between groups. Hand-clasping is a social grooming technique where two individuals clasp hands or press wrists or forearms (or other combinations of two) together overhead and groom the other with their free hand (McGrew & Tutin, 1978). Due to the dyadic nature of this grooming technique, it is unlikely that variations of this behaviour would arise out of, or be maintained by, individual dispositions alone. It might have originated by holding arms overhead in a comfortable manner for which branches are needed (McGrew et al. 2001), highlighting an ecological influence. Still, groups kept under the same ecological condition, show different techniques, suggesting that variations are socially determined (McGrew et al. 2001; Nakamura & Uehara, 2004; van Leeuwen et al. 2012). While it is not known exactly which mechanisms support this transmission, we could speculate that high-fidelity mechanisms such as ontogenetic ritualization, in which two individuals shape one another’s behaviour through repeated interaction (Tomasello & Call, 1997), or imitation in combination with individual and ecological influences maintain group-specific variations. In this context, one could imagine that the strong influences of social learning and the dyadic nature of the technique maintain group rigidity, in addition to the appropriate ecological context (i.e. no available branches) and limited individual influence.

In summary, pole strength can limit the variations and inventions an individual can produce and in turn limit how innovations spread through a group. However, when polar influences are less strong or cancel each other out, opportunity for change can arise and be supported both at the individual and group level. Depending on the species, the potential impact each pole exerts over the transmission and performance of techniques may vary. For example, an animal group that does not highly affiliate or cohabitate between group members may not see as strong an influence by the social pole and may not have as many socially transmitted and sustained techniques. Conversely, we can imagine that humans are very strongly influenced by the social pole allowing for the use of mechanisms like teaching and the accumulation of techniques beyond the individual. Further, individual factors may more strongly influence humans allowing for a wide array of stylistic preferences liberally demonstrated. In such, an appropriate balance between the polar influences can provide the opportunity for liberal invention and variation by individuals and flexible, yet stable adoption through social learning by groups, resulting in cultural evolution.

Conclusion

Our multipolar framework combines the insights of cultural attraction and social learning theories to produce a more complete understanding of animal cultural behaviour. We do not view flexibility and rigidity along a single dimension in which individuals or groups behave one way or the other. Rather, we understand the expression of flexible and rigid behaviour as the outcome of the dynamic process created by the unique influence of each pole in a given context.  This framework helps us to unify CAT and the social learning perspective, reconciling their limitations; namely, CAT’s de-emphasis on social learning mechanisms and the social learning perspective’s inability to meaningfully incorporate the role of ecology and genetics. We do not believe that these differences should represent mutually exclusive reasoning, but rather that when brought together, offer solutions for understanding one another. In such, we use this combined framework to illuminate that chimpanzee populations are not strictly flexible or rigid, but rather that their flexibility and rigidity arise differentially as an outcome of dynamic polar attraction. Through this lens we understand that the traditions and techniques demonstrated in chimpanzee cultures are a product of combined social, individual, and ecological influences which have allowed for the evolution of the distinct repertoires we observe today.

References

Allen, J., Weinrich, M., Hoppitt, W., & Rendell, L. (2013). Network-based diffusion analysis reveals cultural transmission of lobtail feeding in humpback whales. Science, 340(6131), 485-488.

Aplin, L. M. (2019). Culture and cultural evolution in birds: a review of the evidence. Animal Behaviour, 147, 179-187.

Bandini, E., & Harrison, R. A. (2020). Innovation in chimpanzees. Biological Reviews.

Biro, D., Inoue-Nakamura, N., Tonooka, R., Yamakoshi, G., Sousa, C., & Matsuzawa, T. (2003). Cultural innovation and transmission of tool use in wild chimpanzees: evidence from field experiments. Animal cognition, 6(4), 213-223.

Buttelmann, D., Carpenter, M., Call, J., & Tomasello, M. (2007). Enculturated chimpanzees imitate rationally. Developmental science, 10(4), F31-F38.

Buskell, A. (2017). What are cultural attractors?. Biology & Philosophy, 32(3), 377-394.

Call, J., Carpenter, M. & Tomasello, M. 2005 Copying results and copying actions in the process of social learning: chimpanzees (Pan troglodytes) and human children (Homo sapiens). Animal Cognition 8, 151–163.

Charbonneau, M. (2018). Understanding cultural fidelity. The British Journal for the Philosophy of Science.

Claidière, N., & Sperber, D. (2007). The role of attraction in cultural evolution. Journal of Cognition and Culture, 7(1-2), 89-111.

Claidière, N., & Sperber, D. (2010). Imitation explains the propagation, not the stability of animal culture. Proceedings of the Royal Society B: Biological Sciences, 277(1681), 651-659.

Custance, D. M., Bard, K. A., & Whiten, A. (1995). Can young chimpanzees (Pan troglodytes) imitate arbitrary actions? Hayes & Hayes (1952) revisited. Behaviour, 132(11-12), 837-859.

Day, R. L., Kendal, J. R., & Laland, K. N. (2001). Validating cultural transmission in cetaceans. Behavioral and Brain Sciences, 24(2), 330.

Dean, L. G., Vale, G. L., Laland, K. N., Flynn, E., & Kendal, R. L. (2014). Human cumulative culture: a comparative perspective. Biological Reviews, 89(2), 284-301.

Fragaszy, D. M., & Perry, S. (2003). Towards a biology of traditions. The biology of traditions: models and evidence, 1-32.

Goodall, J. (1964). Tool-using and aimed throwing in a community of free-living chimpanzees. Nature, 201(4926), 1264-1266.

Gruber, T., Muller, M. N., Strimling, P., Wrangham, R., & Zuberbühler, K. (2009). Wild chimpanzees rely on cultural knowledge to solve an experimental honey acquisition task. Current biology, 19(21), 1806-1810.

Heintz C., Claidière N. (2015) Current Darwinism in social science. In: Heams T., Huneman P., Lecointre G., Silberstein M. (eds) Handbook of evolutionary thinking in the sciences. Springer, Dordrecht

Heyes, C. M. (1994). Social learning in animals: categories and mechanisms. Biological Reviews, 69(2), 207-231.

Hobaiter, C., Poisot, T., Zuberbühler, K., Hoppitt, W., & Gruber, T. (2014). Social network analysis shows direct evidence for social transmission of tool use in wild chimpanzees. PLoS Biol, 12(9), e1001960.

Hoppitt, W., & Laland, K. N. (2013). Social learning: an introduction to mechanisms, methods, and models. Princeton University Press.

Horner, V., & Whiten, A. (2005). Causal knowledge and imitation/emulation switching in chimpanzees (Pan troglodytes) and children (Homo sapiens). Animal cognition, 8(3), 164-181.

Humle, T., & Matsuzawa, T. (2002). Ant‐dipping among the chimpanzees of Bossou, Guinea, and some comparisons with other sites. American Journal of Primatology: Official Journal of the American Society of Primatologists, 58(3), 133-148.

Humle, T. (2011). Ant-dipping: how ants have shed light on culture. In The chimpanzees of Bossou and Nimba (pp. 97-105). Springer, Tokyo.

Kendal, J., Giraldeau, L. A., & Laland, K. (2009). The evolution of social learning rules: payoff-biased and frequency-dependent biased transmission. Journal of theoretical biology, 260(2), 210-219.

Kendal, R., Hopper, L. M., Whiten, A., Brosnan, S. F., Lambeth, S. P., Schapiro, S. J., & Hoppitt, W. (2015). Chimpanzees copy dominant and knowledgeable individuals: implications for cultural diversity. Evolution and Human Behavior, 36(1), 65-72.

Kendal, R. L., Boogert, N. J., Rendell, L., Laland, K. N., Webster, M., & Jones, P. L. (2018). Social learning strategies: Bridge-building between fields. Trends in cognitive sciences, 22(7), 651-665.

Laland, K. N., & Hoppitt, W. (2003). Do animals have culture?. Evolutionary Anthropology: Issues, News, and Reviews: Issues, News, and Reviews, 12(3), 150-159.

Laland, K., & Evans, C. (2017). Animal social learning, culture, and tradition. In J. Call, G. M. Burghardt, I. M. Pepperberg, C. T. Snowdon, & T. Zentall (Eds.), APA handbooks in psychology. APA handbook of comparative psychology: Perception, learning, and cognition (pp. 441-460). Washington, DC, US: American Psychological Association.

Lamon, N., Neumann, C., Gruber, T., & Zuberbühler, K. (2017). Kin-based cultural transmission of tool use in wild chimpanzees. Science Advances, 3(4), e1602750.

Marshall-Pescini, S., & Whiten, A. (2008). Chimpanzees (Pan troglodytes) and the question of cumulative culture: an experimental approach. Animal cognition, 11(3), 449-456.

McGrew, W. C. (1974). Tool use by wild chimpanzees in feeding upon driver ants. Journal of Human Evolution, 3(6), 501-508.

McGrew, W. C., & Tutin, C. E. (1978). Evidence for a social custom in wild chimpanzees?. Man, 234-251.

McGrew, W. C., & McGrew, W. C. (1992). Chimpanzee material culture: implications for human evolution. Cambridge University Press.

McGrew, W. C., Marchant, L. F., Scott, S. E., & Tutin, C. E. (2001). Intergroup differences in a social custom of wild chimpanzees: the grooming hand-clasp of the Mahale Mountains. Current Anthropology, 42(1), 148-153.

Möbius, Y., Boesch, C., Koops, K., Matsuzawa, T., & Humle, T. (2008). Cultural differences in army ant predation by West African chimpanzees? A comparative study of microecological variables. Animal Behaviour, 76(1), 37-45.

Morin, O. (2015). How traditions live and die. Oxford University Press.

Nakamura, M., & Uehara, S. (2004). Proximate factors of different types of grooming hand-clasp in Mahale chimpanzees: implications for chimpanzee social customs. Current Anthropology, 45(1), 108-114.

Nishida, T. (1973). The ant-gathering behaviour by the use of tools among wild chimpanzees of the Mahali Mountains. Journal of Human Evolution, 2(5), 357-370.

Kitahara-Frisch, J., & Norikoshi, K. (1982). Spontaneous sponge-making in captive chimpanzees. Journal of Human Evolution, 11(1), 41-47.

Sasaki, T., & Biro, D. (2017). Cumulative culture can emerge from collective intelligence in animal groups. Nature communications, 8(1), 1-6.

Schöning, C., Humle, T., Möbius, Y., & McGrew, W. C. (2008). The nature of culture: technological variation in chimpanzee predation on army ants revisited. Journal of Human Evolution, 55(1), 48-59.

Schuppli, C., & van Schaik, C. P. (2019). Animal cultures: How we’ve only seen the tip of the iceberg. Evolutionary Human Sciences, 1.

Scott‐Phillips, T., Blancke, S., & Heintz, C. (2018). Four misunderstandings about cultural attraction. Evolutionary Anthropology: Issues, News, and Reviews, 27(4), 162-173.

Sperber, D. (1985). On anthropological knowledge. Cambridge University Press, Cambridge.

Sperber, D., & Claidière, N. (2008). Defining and explaining culture (comments on Richerson and Boyd, Not by genes alone). Biology & Philosophy, 23(2), 283-292.

Sperber, D. (1996). Explaining culture: A naturalistic approach. Cambridge, MA: Cambridge.

Tennie, C., Call, J., & Tomasello, M. (2009). Ratcheting up the ratchet: on the evolution of cumulative culture. Philosophical Transactions of the Royal Society B: Biological Sciences, 364(1528), 2405-2415.

Tomasello, M., Davis-Dasilva, M., Camak, L., & Bard, K. (1987). Observational learning of tool-use by young chimpanzees. Human evolution, 2(2), 175-183.

Tomasello, M., 1990. Cultural transmission in the tool use and communicatory signaling of chimpanzees. In “Language” and Intelligence in Monkeys and Apes. Comparative and Developmental Perspectives. S.T. Parker & R.K. Gibson (Eds.) Cambridge, Cambridge University Press.

Tomasello, M. (1996). Do apes ape. Social learning in animals: The roots of culture, 319-346.

Tomasello, M., & Call, J. (1997). Primate cognition. Oxford University Press.

Van Leeuwen, E. J., Cronin, K. A., Haun, D. B., Mundry, R., & Bodamer, M. D. (2012). Neighbouring chimpanzee communities show different preferences in social grooming behaviour. Proceedings of the Royal Society B: Biological Sciences, 279(1746), 4362-4367.

Whitehead, H., Laland, K. N., Rendell, L., Thorogood, R., & Whiten, A. (2019). The reach of gene–culture coevolution in animals. Nature communications, 10(1), 1-10.

Whiten, A., Goodall, J., McGrew, W. C., Nishida, T., Reynolds, V., Sugiyama, Y., … & Boesch, C. (1999). Cultures in chimpanzees. Nature, 399(6737), 682-685.

Whiten, A., Horner, V., Litchfield, C. A., & Marshall-Pescini, S. (2004). How do apes ape?. Animal Learning & Behavior, 32(1), 36-52.

Whiten, A., Horner, V., & De Waal, F. B. (2005). Conformity to cultural norms of tool use in chimpanzees. Nature, 437(7059), 737-740.

Whiten, A., McGuigan, N., Marshall-Pescini, S., & Hopper, L. M. (2009). Emulation, imitation, over-imitation and the scope of culture for child and chimpanzee. Philosophical Transactions of the Royal Society B: Biological Sciences, 364(1528), 2417-2428.

Whiten, A., & van de Waal, E. (2017). Social learning, culture and the ‘socio-cultural brain’ of human and non-human primates. Neuroscience & Biobehavioral Reviews, 82, 58-75.

Whiten, A. (2019). Cultural evolution in animals. Annual Review of Ecology, Evolution, and Systematics, 50, 27-48.

Yamakoshi G. 2001. Ecology of tool use in wild chimpanzees: toward reconstruction of early hominid evolution. In: Matsuzawa T, editor. Primate origins of human cognition and behavior. Tokyo, Japan: Springer Verlag. p 537–556