Rank and payoff biases influence subject choices in a foraging task among sanctuary chimpanzees (Pan troglodytes)

doi:10.21203/rs.3.rs-8321136/v1

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Rank and payoff biases influence subject choices in a foraging task among sanctuary chimpanzees (Pan troglodytes)

https://doi.org/10.21203/rs.3.rs-8321136/v1

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Social learning is essential to the development of culture in both humans and non-human primates. Transmission biases, or social learning strategies, are a key but understudied aspect of social learning. They are the cognitive processes that determine when and from whom we should learn. Primatological research has explored a handful of single biases in controlled experiments, yet few studies have considered how simultaneous strategies interact when subjects are given a dichotomous choice between two potentially adaptive biases. This study used 35 sanctuary-dwelling chimpanzees at Ngamba Island Sanctuary in Uganda to determine whether chimpanzees will copy high-ranking or low-ranking female demonstrators when those demonstrators present either a low- or high-payoff version of a token-reward task. Individuals in Group 1 observed a low-ranking demonstrator collecting the high-payoff reward (pineapple), and a high-ranking demonstrator collecting the low-payoff reward (carrot). Results concluded that chimpanzees in Group 1 did not significantly copy either demonstrator, suggesting the biases confound one another. In Group 2 the demonstrator roles were reversed, and the frequency of high-payoff selection was higher. Therefore, whilst humans may be able to overlook demonstrator characteristics to ratchet culture in beneficial ways, chimpanzees might not be able to do the same. This contributes to explanations as to why chimpanzee culture is simple compared to that of humans.

Chimpanzee

transmission bias

payoff

rank

social learning

Some primate species possess the capacity for culture (McGrew, 1998; Laland & Hoppitt, 2003; Humle & Newton-Fisher, 2013), including our closest relatives, chimpanzees (Pan troglodytes; Goodall, 1964; Whiten et al., 1999; McGrew, 2010; Whiten, 2019). Several chimpanzee field studies have documented variations in cultural behaviors between groups that cannot be explained solely by ecological or genetic differences (e.g., Whiten et al., 2001; Kamilar & Marshack, 2012; Luncz et al., 2012; Eleuteri et al., 2025). These differences are developed and maintained through social learning, defined as the ability to learn from conspecifics (Heyes, 2012). New behaviors can arise through innovation (Boesch, 1995; Biro et al., 2003; Osvath & Karvonen, 2012) or the movement of wild chimpanzees between culturally different groups (Gunasekaram et al., 2024). Not all new behaviors or innovations propagate across groups once they have been seeded (Hrubesch et al., 2009). Instead, behavior is driven by social learning in a directed manner (Laland, 2004; Vale et al., 2017; Kendal et al., 2018), meaning some behaviors are adopted and others are not. Factors that determine whether a behavior is transmitted throughout a group include features of the demonstrator (Horner et al., 2010; Kendal et al., 2015; Wood et al., 2016) and the benefit of the behavior (Vale et al., 2017). Experimentally examining why certain behaviors are adopted by chimpanzees aids our understanding of chimpanzee social learning and cultural evolution. And, in turn, how and why their culture differs from that of humans.

Social learning research can be divided into two broad categories: mechanisms (Hoppitt & Laland, 2013) and biases (Laland, 2004). Mechanisms explain how social learning is facilitated. Transmission biases are the processes by which individuals decide when and from whom to learn (Henrich & McElreath, 2003). Both are essential to developing a comprehensive understanding of social learning. However, there is a lack of research into transmission biases as they relate to human and primate cultural capacities, especially studies that consider simultaneous conflicting social learning strategies. That is, decisions whereby individuals are presented with two options that are adaptive in one way and maladaptive in another, with both options containing opposing information.

Transmission Biases

Transmission biases (also called social learning strategies) are evolved cognitive strategies that allow individuals to acquire information in the most efficient and effective manner (Kendal et al., 2009; Stubbersfield, 2022). Therefore, when learning socially, individuals should not simply copy indiscriminately. Instead, they should employ transmission biases that allow them to select the best conspecific, or group of conspecifics, to copy and to do so at the most appropriate time (Watson et al., 2018). Copying indiscriminately could result in adopting potentially maladaptive behaviors (Heyes, 2016) or absorbing outdated or erroneous information (Rendell et al., 2011). This was demonstrated in a seminal experiment with humans that revealed participants would select obviously incorrect answers when other individuals in the room did the same (Asch, 1951), so learning should take place in a directed manner to achieve the best outcome (Kendal et al., 2009).

Learners may first consider their internal state to determine if they should learn socially or asocially. This includes situations in which an individual is uncertain (Kendal et al., 2015; Wood et al., 2016; Kendal et al., 2018; Mörchen et al., 2023; Nakamichi, 2024), unable to complete a task due to a lack of knowledge (van Leeuwen et al., 2023; Garcia-Nisa et al., 2023), or entering a novel environment (Luncz & Boesch, 2014; Watson et al., 2018). However, chimpanzees are generally behaviorally conservative and normative (Whiten et al., 2005; Schlingloff & Moor, 2017; Vale et al., 2017; Fitzpatrick, 2020). Therefore, they may choose to disregard social learning opportunities, even if those opportunities are potentially beneficial (Hopper et al., 2011; but see van Leeuwen et al., 2013). For example, chimpanzees who learned a stick technique to gain a food reward did not switch to an innovative rattling technique, despite it being more efficient (Hrubesch et al., 2008). Other intrinsic factors, such as age (Wood et al., 2013), personality (Carter et al., 2014; Rawlings et al., 2022), and sex (Choleris & Kavaliers, 1999), can also modify whether one copies and who one copies.

Individuals also conform to the majority during some instances of learning. For example, both chimpanzees and children are more likely to copy a behavior performed once by three individuals than by one individual three times (Haun et al., 2012). Group consensus is employed in several behavioral domains in chimpanzees (Luncz & Boesch, 2014; van Leeuwen et al., 2014) which can result in group-wide differences among wild chimpanzees in behaviors such as leaf-swallowing (Huffman et al., 2010). This is supported by a token-reward task where chimpanzees ignored a novel efficient seeded behavior to conform to a previously learned and group-wide behavior (Hopper et al., 2011). This may be a result of neophobia and conservatism and shows how chimpanzees may value conformity over positive behavioral outcomes. However, a different token exchange task demonstrated that captive chimpanzees can forego the behavior of the majority if an alternative behavior offers a higher yield (van Leeuwen et al., 2013).

Copying a higher yield is a form of payoff bias, whereby behaviors are adopted because they provide a higher yield, preferred outcome, or more efficient solution than an alternative option (Kendal et al., 2009; Mesoudi, 2011; Hong, 2022). This is a highly beneficial strategy, but cognitively demanding, as it requires constant surveillance of conspecifics to note who is the most successful (Camacho-Alpízar & Guillette, 2023). This bias influences water sponging techniques among wild chimpanzees, where moss sponging arose as an alternative to leaf sponging as the moss sponges absorbed more liquid (Lamon et al., 2018). Similar findings have been recorded in captive chimpanzees in a straw-sucking task (Yamamoto et al., 2013), a foraging apparatus task (Davis et al., 2016) and a token-reward task (van Leeuwen et al., 2013).

While most research suggests that large differences in efficiency or yield are required to produce behavioral change (Davis et al., 2016), even small payoff increases can sometimes elicit behavioral changes in chimpanzees and gorillas (Jacobson & Hopper, 2019). An eye-tracking experiment demonstrated that chimpanzees and bonobos direct their attention preferentially toward videos of human demonstrators using more efficient tools (such as a straw) as opposed to a less efficient tool (a dipping stick). This effect was reduced in individuals who already had experience with the less efficient tool. Video demonstrations did not result in the adoption of the high payoff behavior, however, regardless of visual attention (Piao et al., 2025).

When compared to human children, the payoff strategies employed by chimpanzees demonstrate an over-reliance on personal experience over socially derived information (Vale et al., 2017). Once chimpanzees have learned one behavior from a demonstrator or become proficient through individual learning, they are reluctant to modify their behavior to achieve a greater reward (Hrubesch et al., 2009; Hopper et al., 2011) or avoid ineffective techniques (Price et al., 2009; Bonnie et al., 2012). This means that behavioral conservatism may inhibit payoff bias in chimpanzees. Even in humans, pay-off biases can be underused (Mesoudi et al., 2011), which is one reason why innovative high-yielding behaviors may not propagate across the whole group, although this may also differ on an individual basis (Berdugo et al., 2024).

Another factor influencing a lack of high-payoff behavioral spread is the features of the demonstrator (model-based biases). Chimpanzees prefer to copy dominant individuals (Kendal et al., 2015). Chimpanzees and bonobos also direct more visual attention toward individuals of the dominant sex, particularly when those individuals are familiar (Lewis et al., 2021). This pattern likely reflects prestige bias, the tendency to attend to and copy higher-ranking individuals who are perceived as more competent or successful (Horner et al., 2010). Comparable prestige-based learning has been documented in humans (Jiménez & Mesoudi, 2019; Brand et al., 2021; Berl et al., 2021), although some studies suggest that its expression may vary across cultural contexts (Reyes-Garcia et al., 2008; Chellappoo, 2021). Rank, therefore, may serve as a proxy for success (Henrich & Gil-White, 2001). This bias may also exist as a form of mirroring, whereby learners will copy high-ranking individuals to increase their rank by behaving in a certain manner. Similarly, when placed into novel environments, adults will defer to prestigious individuals to learn, although subsequent environmental shifts did not result in more use of prestige bias, and success biases were also used (Atkisson et al., 2012). Further, the context in which social learning is being employed can also impact the type of strategy used, with adults preferentially using payoff biases as opposed to conformity or prestige in the context of social dilemmas (Watson et al., 2021).

These model biases are particularly relevant among chimpanzees, since juveniles and lower-ranking individuals tend to be the most innovative individuals (Reader & Laland, 2001; Bandini & Harrison, 2020). Given their age and rank status, they are not frequently copied by conspecifics (Kendal et al., 2015), resulting in a lack of transmission of new behaviors to older or higher-ranking group members. Despite this, subordinate chimpanzees have been shown to transmit novel behaviors if there is no alternative demonstrator, indicating low rank does not necessarily impede observation and behavioral transmission (Watson et al., 2017). Nevertheless, rank bias has been suggested to be a key factor in the spread of socio-cultural behaviors in wild chimpanzees (Hobaiter et al., 2014; Kalan & Boesch, 2018). However, for certain behaviors like the adoption of moss over leaves for water sponging, other researchers have suggested that the payoff of the behavior is equally driving its transmission and maintenance (Lamon et al., 2018).

Present study

Prior studies have shown that chimpanzees prefer to copy higher-ranking individuals (Kendal et al., 2016; Horner et al., 2010) and that they can recognize and copy higher payoff behaviors (van Leeuwen et al., 2013; Lamon et al., 2018; Davis et al., 2016; Yamamoto et al., 2013). However, most research on transmission biases in chimpanzees and other primates has focused on the employment of single strategies (e.g., Haun et al., 2012) or multiple complementary strategies like high rank, older age, and superior knowledge (Horner et al., 2010) in the decision-making process. A handful of studies have considered how more than one SLS may interact with each other in some primate taxa (Barrett et al., 2017; Bono et al., 2018; Canteloup et al., 2020). Additionally, humans and other taxa make social learning decisions by combining and choosing between simultaneously available relevant strategies (Hong, 2022). There is a critical gap in our understanding of how chimpanzees make social learning decisions when faced with two conflicting options.

This study provided subject chimpanzees with the option to copy either a high- or low-ranking chimpanzee demonstrator in a token reward task. Unlike previous experiments, a variable was added whereby each demonstrator performed a behavior of differing payoffs (gaining a piece of preferred pineapple or non-preferred carrot). In the main experimental group (Group 1), the low-ranking individual demonstrated a high payoff behavioral variation, and in the alternative experimental group (Group 2), the high-ranking individual demonstrated a high payoff behavioral variation. It was expected that subjects in the Group 2 would select the beneficial behavioral option significantly more frequently. However, it was also expected that in both groups, payoff may override rank bias since the study took place in a controlled environment where payoff differences were pronounced. Therefore, we expected Group 1 to select the high payoff option more frequently than the low payoff option, but not as frequently as in Group 2.

Subjects

Subjects were a subset of the chimpanzees residing at Ngamba Island Sanctuary in Lake Victoria, Uganda. Most of the chimpanzees at Ngamba were rescued from the pet trade and circuses and cannot return to the wild. They have access to ninety-nine hectares of forest as well as an indoor holding facility where most of the chimpanzees sleep at night (Fig. 1). Chimpanzees are free to remain in the forest overnight if they wish, and all chimpanzees are provisioned with extra food at four daily feedings. There are fifty-four chimpanzees at Ngamba who make up one community. Thirty-five individuals (twenty female) participated in the study. Two individuals were used as models, and seven individuals were used as a control group. The remaining individuals were split into two experimental groups (each n = 13). Participant ages ranged from 17 to 40 years old (mean 28; median 25). Two chimpanzees participated with their dependent infants, but the infants were not included as subjects.

This study took place in August and September 2024. Trials were carried out in the holding facility where chimpanzees were never deprived of water. Food provisioned in this study was supplementary to the chimpanzees’ daily intake. Chimpanzees were called by name to participate every morning but did so voluntarily. If a chimpanzee refused to enter the experiment room, they could enter the forest or remain in their resting room. Chimpanzees spent no longer than four hours each day in the experiment or resting rooms before joining the rest of the group in the forest.

Tokens and token preference pre-trial testing

The main paradigm of this study entailed chimpanzees passing tokens to the researcher in exchange for food (see Brosnan & de Waal, 2004; Horner et al., 2010; Hopper et al., 2011; Vale et al., 2017 for similar designs). Tokens were tubes made of PVC. One was black with a thin green pinstripe, and the other was solid blue (Fig. 2). The black token had a diameter of ~ 2.5cm and was ~ 16cm in length. The blue token had a diameter of ~ 2 cm and was ~ 14 cm in length. Both tube types could be easily passed through gaps in the holding facility bars.

To ensure chimpanzees did not intrinsically prefer either token, 7 control individuals were presented one of each token in dichotomous choice tests and given a peanut when they returned a token to a researcher. Each chimpanzee made 100 exchanges in this test in exchange for peanuts. blue tokens were selected on average 51% of the time (359 out of 700; Wilcoxon p = 0.67; Supplementary Fig. 1), meaning there was no significant preference for either token.

Food Preference Tests

Rewards for this study were pineapple and carrot, used as high and low payoff rewards, respectively. Pineapple and carrot are both eaten by the chimpanzees at Ngamba, and pineapple is considered a higher value item as determined through food grunts and occasional rejection of carrot when other foods are present. However, to ensure that one food was preferred over the other, chimpanzees were individually presented with pineapple and carrot simultaneously on a sliding table in a pre-experiment trial. They were given the dichotomous choice of the two foods 20 times. The percentage of times the chimpanzees chose pineapple was then noted to determine that pineapple was preferred overall. The group selected pineapple 84% of the time (Wilcoxon p < 0.001; Fig. 3). None of the chimpanzees selected carrots more than they selected pineapple. However, there were variations in the number of carrots and pineapples selected, so individual preferences were also recorded and used as a variable in subsequent analyses.

Demonstrator training

The demonstrators selected for this task were females of differing ranks (low and mid). The specific females selected, Medina (low) and Pasa (mid; designated as ‘high’ in analysis for ease of comprehension), were chosen on recommendation by keepers and because they were intelligent individuals who quickly understood the task. Since chimpanzees are male-dominant (Klinkova et al., 2005), selecting a very high-ranking individual would have created an additional variable of sex.

Chimpanzees at Ngamba often accept food in exchange for an item they have taken and sometimes take items such as food bowls to later exchange with care staff for food. Nevertheless, a training session took place for the demonstrators where they exchanged tokens for a food reward. Tokens were placed in a container at one end of the enclosure, and the experimenter sat at the other end with a bowl of carrots, pineapple, and peanuts. Demonstrators could collect tokens to exchange for food rewards. Both demonstrators exchanged a token within one minute of entering the experiment room. Only the relevant token was made available to demonstrators during the trials in which they were demonstrating. Once demonstrators exchanged tokens 30 times each, they were considered trained (Horner et al., 2010). This took place over one training session per demonstrator.

Main procedure

Subjects were split into two experimental groups. Group 1 saw a low-ranking demonstrator using a token that resulted in a high payoff reward (pineapple) and a high-ranking demonstrator using a token to receive a low payoff reward (carrot). Group 2 saw the low-ranking demonstrator receive the low payoff reward and the high-ranking demonstrator receive a high payoff reward. Individual subjects always saw the same token correspond to the same food reward.

The procedure had two phases. During phase 1, demonstrators presented their respective exchange type one after another to the subject who was placed in an adjacent room with no tokens to access themselves. Demonstrators spent 5 minutes each passing tokens to the researcher, gaining the appropriate reward for that token as the subject observed. The first viewed demonstrator was alternated between subjects (n = 12, 14 split across experimental groups).

During phase 2, the same demonstrators were shown again to the subject, again for 5 minutes each. This time, the subject was able to collect tokens from two containers to exchange with the experimenter at the same time as the demonstrator. Phase 2 was repeated three times on subsequent days, split into phases 2a, 2b, and 2c.

Data Collection and Analysis

Videos were recorded on a Panasonic HC-V250 camcorder. Videos were analyzed by the researcher to record a) the number of exchanges from each demonstrator observed by participants, b) the number of exchanges of each token made by participants, c) the first exchanged token in each trial, and d) instances of subjects stealing from the demonstrator. Observations were defined as exchanges made by the demonstrator when the subject was oriented toward them.

Statistical analyses were conducted in R (4.5.0) and R Studio (2025.05.0 + 496) on a MacBook Air (BigSur 15.3.2). For descriptive purposes we first ran basic non-parametric tests on subject-level summary measures. We used one-sample Wilcoxon signed-rank tests to test deviation from chance of HP token selection frequency (mu = 50). To account for repeated measures and individual differences we fit Bayesian logistic mixed-effects models from the package ‘brms’ (v2.22.0; Bürkner, 2017). Chimpanzee ID was included as the random intercept to account for repeated measures. The three separate model outcomes were: 1. Proportion of HP tokens selected overall (binomial family), 2. Proportion of HP token first selected in each trial (bernoulli family) and 3. Proportion of observations of the demonstrator (binomial family). For each outcome, several models varying in complexity were produced and compared using the package ‘loo’ (v2.8.0; Vehtari et al., 2017). ‘Trial number’ was retained in Brms 1 and 2 despite the loo analysis indicating it didn’t improve model fit. We determined it was important to analyze any potential effect of individual learning over time. Across models, Rhat values were 1.00, indicating good convergence. Posterior checks also revealed no problems.

Token Exchanges Between Groups

2215 token exchanges took place during the experimental sessions. 1075 of these were in Group 1, and 1140 were in Group 2. On average, Group 1 exchanged 82 tokens per chimpanzee, and Group 2 exchanged 87 tokens per chimpanzee, excluding tokens that were stolen directly from demonstrator chimpanzees (Figure. 4a).

Group 1 exchanged an HP token 55% (598/1075) of the time, and Group 2 exchanged an HP token 90% (1033/1140) of the time. The percentage frequency of HP token choice was positive and significant only in Group 2 (Wilcox Signed Rank Group 1 p = 0.977; Group 2 p < 0.001).

Subjects in Group 1 showed no statistically significant preference towards either token in their first token choice in each trial (36/73; Wilcoxon p = 0.837). However, chimpanzees in Group 2 selected the HP option significantly more frequently as their first exchange (60/73; Wilcox p < 0.001).

Observations of Demonstrators Between Groups

In Group 1, the high-payoff low-ranking demonstrator was observed exchanging tokens significantly more frequently than the high-ranking demonstrator (Wilcoxon p < 0.001). In Group 2, the low-payoff low-ranking demonstrator was observed exchanging tokens more frequently but not significantly so (Wilcoxon p = 0.17). Subjects in both groups combined observed the lower-ranked individual significantly more frequently (Wilcoxon p = 0.002).

Bayesian Models

Brm1 analyzed the effect of variables of group membership (Group 1 or 2), subject sex, subject rank, subject food preference, trial phase, and number of exchanges observed on the likelihood of selecting the HP token more frequently than the LP token. All Rhat values were 1.00, indicating convergence. Membership in Group 2 showed a positive association, estimated at 3.16 (95% CrI [1.98, 4.47]). Being male also increased the likelihood of HP selection, with a coefficient estimation of 3.11 (95% CrI [1.81, 4.47]; Figure. 6a). The second low-payoff trial phase had a moderate negative effect on the likelihood of selecting the HP tokens, with an estimate of -0.54 (95% CrI [-0.96, -0.13]). The other variables, including personal food preference, did not show notable coefficients.

Brm2 analyzed the effect of group membership, subject sex, rank, and trial phase on predicted proportions of selecting the HP token first in each trial (Figure. 6b). All Rhat values were 1.00, indicating convergence. Membership in Group 2 showed a positive association again, with a coefficient estimate of 2.43 (95% CrI [0.51, 4.86]). Being male also showed a positive association, estimated at 3.10 (95% CrI [0.71, 6.05]). The trial phase and other variables did not produce notable effects.

Brm3 analyzed the effect of group membership, subject sex, and rank on the proportion of observations of the low-ranking demonstrator, Medina (Figure. 6c). All Rhat values were 1.00, indicating good convergence. A simple model first analyzed whether subjects were more likely to observe Medina overall, which they were, with an estimate of 0.44 (95% CrI [0.33, 0.56]). In the full model, membership in Group 2 produced weak evidence for a negative association to observations of Medina, with an estimate of -0.44 (95% CrI [-0.85, -0.06]). There is an uncertain effect of rank. Being mid-ranked slightly positively influenced observations of Medina, with an estimate of 0.52 (95% CrI [-0.01, 1.04]).

This study investigated how chimpanzees utilize social learning strategies to make decisions when confronted with conflicting information. A small number of studies have considered how simultaneous social learning strategies are employed in chimpanzees (e.g., Horner et al., 2010; Kendal et al., 2015). Fewer studies of primate social learning have provided subjects with conflicting choices (van Leeuwen & Call, 2017; Bono et al., 2018; Canteloup et al., 2020). Research into the complexities of social learning strategies can elucidate why certain beneficial behaviors do not propagate across chimpanzee groups (Watson et al., 2017). In Group 1, chimpanzees could observe and copy a demonstrator who was either high-ranking but obtaining a low payoff or was low-ranking but obtaining a high payoff. Chimpanzees did not select one option more frequently than the other, implying the two conflicting biases may balance each other out to produce no significant preference. In the alternative experiment group, Group 2, the demonstrator roles were reversed. Subjects in Group 2 copied the high-payoff option significantly more frequently than the low-payoff option. The difference in frequency of selection of the high-payoff option between experiment groups was substantial, including for the frequency of high payoff as the first token that was selected.

This result replicates research documenting a rank bias in chimpanzees (Horner et al., 2010; Hobaiter et al., 2014; van Leeuwen et al., 2014; Kendal et al., 2015). Rank bias is adaptive since higher-ranked individuals are assumed to be more knowledgeable or otherwise better-suited demonstrators (Brand et al., 2021). In Group 1, a higher payoff version of behavior was not an adequate incentive to completely abandon an intrinsic bias toward copying individuals of higher ranks, even when the higher-ranked individual was not of particularly high rank among the wider group at Ngamba. However, the results also give credence to the findings of Watson and colleagues (2018), who suggested that subordinate chimpanzees can transmit novel behaviors, but in conditions where there is no competing behavior (Bonnie et al., 2007).

Payoff bias is equally adaptive since direct observation of a more beneficial behavior would result in a better outcome for the copier (van Leeuwen et al., 2013). The fact that subjects in Group 1 copied the low-ranking chimpanzee around 50% of the time shows that they do recognize high payoffs (Buttelmann et al., 2013; Yamamoto et al., 2013; Davis et al., 2016; Lamon et al., 2018; Jacobson & Hopper, 2019). The benefit of a behavior may also influence the likelihood that it will propagate across a group in opposition to other biases (van Leeuwen & Call, 2017), something previously shown to be inhibited by conformity and conservatism in chimpanzees (Hopper et al., 2011). Van Leeuwen et al. (2013) found that chimpanzees can overcome another transmission bias to select a high-payoff behavioral option, but this was not the case in this study. This may indicate that rank bias is very strong in chimpanzees. However, chimpanzees could easily collect both token types, so the pressure to select the high payoff option may not have been strong enough to override rank bias. Future experiments could create a set up where individuals can only select one option so this effect could be studied in more depth.

By providing a competing behavior, and hence a conflicting decision, this study reveals how the spread of beneficial alternative behaviors may be inhibited by the presence of high-ranking individuals, even when that beneficial behavior is demonstrated clearly. By extension, this may explain why cumulative culture, the ability to ratchet and improve upon former behaviors (Gunasekaram et al., 2024), is not as widespread or complex in chimpanzees as it is in humans (Dean et al., 2014). For cumulative culture to exist, innovations must occur, and individuals must be able to modify their behaviors to use more efficient alternatives (Davis et al., 2016). Previous studies have shown that it is generally young and low-ranking individuals who most frequently innovate (Reader & Laland, 2001; Bandini & Harrison, 2020). If group members are copying higher-ranking individuals even when better alternative behaviors have been created, then those alternatives may not circulate within the group. This, combined with conservatism and conformity (Hrubesch et al., 2009; Price et al., 2009; Hopper et al., 2011; Bonnie et al., 2012), may be the reason why cumulative culture in chimpanzees is less complex compared to that of humans (Marshall-Pescini & Whiten, 2008).

When considering the observational differences between demonstrators, an effect was only notable in Group 1, where Medina was performing the high-payoff behavior. We expected that Pasa would be viewed significantly more frequently in Group 2, as she is higher-ranking (Kendal et al., 2015; Horner et al., 2010) and performed the high-payoff behavior (Vale et al., 2017). However, this was not the case. Observation levels in this study may not have been directly related to subsequent token choices. Some eye-tracking experiments (e.g., Lewis et al., 2021) found that observation was related to subsequent behavioral choices. Nevertheless, Piao et al. (2025) discovered that chimpanzees and bonobos did not subsequently learn a high-payoff technique even after viewing a video demonstration of that technique. Their study also found that observations of familiar techniques were less frequent than those of unfamiliar ones, so familiarity with token-exchange paradigms may have resulted in fewer observations overall. However, these studies used videos, while this study used live demonstrators, which would result in participant responses that much closely mirror real social learning instances.

Chimpanzees may be over-relying on personal previous experience or individual learning (Vale et al., 2017; Bandini and Tennie, 2017; Bandini and Tennie, 2019) instead of relying on demonstrator observation. But this seems unlikely since Groups 1 and 2 differed in token choice levels. Individual learning would also not explain why one chimpanzee was observed more frequently than the other in both groups.

Since she was lower-ranking, Medina may have been easier to intimidate than Pasa (Fultz et al., 2022), which may explain why participants observed her more frequently. Some individuals were able to fit their hands through a space in the enclosure to knock a token out of the hands of the demonstrator or to pick up tokens that the demonstrator dropped. Stealing has been observed previously in captive chimpanzees (Horner et al., 2010; Gonçalves, 2015) and 7 participants engaged in stealing in this study. Chimpanzees only stole from the low-ranking demonstrator regardless of experiment group. 22 instances of stealing were recorded overall and 17 of these instances were in Group 1 when the token stolen was high payoff (Supplementary Fig. 2).

For one subject who frequently affiliated with the low-ranking demonstrator, the demonstrator dropped or held tokens near the gap in the enclosure bars to facilitate bouts of stealing by this subject. This could provide an example of prosociality, where the demonstrator was relinquishing a token to a companion (Melis et al., 2010), or it could be an example of expected future reciprocity (de Waal, 1989; Silk et al., 2013; Knofe et al., 2024). As a demonstrator, Medina also received a large amount of pineapple during the experiment, so providing a token to a companion would have been a low-risk behavior.

Our Bayesian models provided strong evidence that membership in Group 2 had a positive association with the selection of HP tokens. Intrinsic factors also influenced the likelihood that subjects would choose the high-payoff token. Across groups, being male resulted in a higher likelihood of using the high-payoff token. Males may be more driven by food rewards than females (Fahy et al., 2013), or more prone to risk-taking (Haux et al., 2023). Also, some studies have suggested that males perform better than females in imitative tasks (Custance et al., 1995; Choleris & Kavaliers, 1999, but see Lonsdorf et al., 2004). For observation levels, the effects were not as pronounced, but those in Group 1 were more likely to watch the low-ranking demonstrator, and there was a weak effect of mid-ranking individuals being more likely to observe the low-ranking demonstrator.

Whilst the effects of individual differences are not pronounced in this study, they indicate that future researchers should consider how individual differences may influence social learning strategies in chimpanzees and other taxa, especially individual traits that were not measured here, like personality (Carter et al., 2014). In human adults, the role of individual differences plays a part in decision-making, with humans demonstrating individual preferences for specific SLS regardless of context (Molleman et al., 2014). Additionally, children develop individual strategies of either competency or majority bias and remain consistent in themselves (Burdett et al., 2016). Further, chimpanzees exhibit individual differences in social and asocial puzzle solving (Chalmeau and Gallo, 1993; Hopper et al., 2014) and handling of social dilemmas (Brosnan et al., 2015). Behavioral contagion is also mediated by factors like social closeness and age, depending on the behavioral context (Sandars et al., 2024).

This study was conducted in captivity, so an interesting avenue for future research would be to conduct this study ino a wild population. The impact of differentiated payoff might be more pronounced in a population where low-ranking individuals may not be in view of participants as frequently due to large group sizes (Neal Webb et al., 2019) or the nature of a fission-fusion group dynamic (Lehmann & Boesch, 2004). An open diffusion experiment could also be a fruitful avenue for future research. Unfortunately, an open diffusion experiment was not feasible in the enclosure where the experiment took place, or in the wider forest at Ngamba. Future studies should consider this methodology to see whether the results of an open diffusion test would be the same as the results of this study. In addition, future studies, especially controlled experiments, should also attempt to include more than two transmission biases to analyze the effect of several strategies on decision-making processes, especially three-way interactions. Future studies should focus on more holistic studies of social learning strategies in chimpanzees and on finding ways to bring these controlled studies to wild populations.

This study showed that both rank and payoff influence behavioral adoption via social learning among chimpanzees. When faced with conflicting decisions, chimpanzees employed both adaptive strategies. But when demonstrators possessed two beneficial traits, subjects copied that individual significantly more frequently.

Ethical approval.

Research was undertaken with approval from Ngamba Island Chimpanzee Sanctuary, the Uganda Wildlife Authority, and the Uganda National Council for Science and Technology.

Competing interests.

The authors declare no competing interests.

Author Contribution

E.P. and C.B.S conceptualized the study. E.P. wrote the main manuscript text with revisions provided by C.B.S. E.P. carried out the statistical analysis and prepared the figures.

Acknowledgement

The authors would like to thank Dr Titus Mukungu and all the keepers and staff at Ngamba Island Sanctuary. Without their help and care, this research could not have been conducted. We would also like to thank Bruce Rawlings for his guidance and valuable feedback on this paper.

Data Availability

The datasets generated from this study can be found at the Dataset repository [https://doi.org/10.7910/DVN/NSPSX2]. The main R code used for this study is also available at [https://github.com/eloisepe/Rank-and-payoff-biases-influence-subject-choices]. Additional R code is available upon request.

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Rank and payoff biases influence subject choices in a foraging task among sanctuary chimpanzees (Pan troglodytes)

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Transmission Biases

Present study

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Tokens and token preference pre-trial testing

Food Preference Tests

Demonstrator training

Main procedure

Data Collection and Analysis

Results

Token Exchanges Between Groups

Observations of Demonstrators Between Groups

Bayesian Models

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Acknowledgement

Data Availability

References

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