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Human language expands meaning through the structured combination of sounds, but such mechanisms remain rare in nonhuman animals, raising questions about their evolution. Shifts in meaning from single calls to combinations appear across a range of combinations and contexts in apes, but current evidence from other species is mainly restricted to alarm contexts. To address this, we applied a quantitative, whole-repertoire approach to assess meaningful combinatorial capacities in sooty mangabeys (Cercocebus atys), a forest-dwelling monkey. We recorded 1751 vocal utterances from two groups in the Taï National Park, Ivory Coast. Using context of production as a proxy for potential meaning, we focused on two mechanisms: (1) bigrams—sequences of two different calls, and (2) iteration—reoccurrence of the same call type interspersed with others. Euclidean distance analyses suggested order-sensitive meaning in female bigrams combining ‘grunt’ and ‘twitter’ calls. Whereas single ‘grunts’, ‘twitters’ and the bigram ‘grunt_twitter’ occurred frequently during feeding, ‘twitter_grunt’ occurred predominantly during infant-directed affiliations. Female iterative sequences in which ‘grunt’ and ‘twitter’ calls recurred (e.g. ‘twitter_grunt_twitter’) also showed context-specific shifts, indicating a possible role for iteration in meaning generation. These findings suggest that meaningful combinatorial capacities can extend beyond great apes and alarm contexts to socially benign interactions in monkeys. However, repertoire-wide use of meaning-shifting bigrams remains unconfirmed outside hominids.
Charitable giving is a cornerstone of social welfare, and others’ donations can promote individual giving. However, the mechanisms through which social information shapes donation behavior, and how individuals differ in their responsiveness, remain poorly understood. Here, we investigated how the statistical properties of others’ donations, specifically their mean and variability, shape individual giving, and how this was affected by individual differences. Across 4 pre-registered experiments (total N = 1,491), participants made donations before and after observing sequential donations from diverse others whose mean and variability were systematically manipulated. We found that observing generous average donations robustly increased individual giving, while observing stingy donations decreased it. Moreover, the variance in social information did not modulate this shift. In parallel, exposure to others’ donations consistently reduced the variability of individual donations, with stronger reductions when others’ donations were more consistent (versus more variable). These effects also generalized to novel donation contexts. At the individual level, changes in donations following social exposure were consistently positively associated with psychopathic traits. Computational modeling further revealed that people with higher psychopathic traits were more susceptible to social information, assigning greater weight to observed donations when making their second decisions. These results were replicated across all experiments, despite variations in donation magnitudes, population type, variability discrepancy, and incentive structures. Together, our findings demonstrate that the statistical structure of social information - its magnitude and variability - shapes the corresponding features of individual donations, and identify psychopathy as a key factor underlying heterogeneity in social influence.
OBJECTIVES:In human foraging societies, hunting skill is often interpreted as a signal of male quality linked to reproductive success through his ability to provision his family and community. Similarly, in some bird and insect species, males offer mates food to indicate their quality as a provider. Among nonhuman primates, however, the relationship between meat sharing and reproductive success is underexplored, leaving the evolutionary origins of this phenomenon unresolved. MATERIALS AND METHODS:Guinea baboons (Papio papio) are a promising model to investigate whether meat sharing signals male quality, since females choose their mates, have been shown to rely on males to catch and transfer prey, and respond to male foraging skill. We combined records of 109 meat-eating events with 9 years of behavioral data to test whether males who more frequently acquired and transferred meat to females had more females in their social units for longer. RESULTS:Contrary to our predictions, we found no evidence that females preferred males who acquired or transferred meat more frequently, suggesting that meat acquisition is not a signal of male quality in Guinea baboons. DISCUSSION:One explanation may be the relatively low frequency of meat-eating events. Another is that females are less dependent on males for meat than previously reported. Our results revealed that nearly half (41%) of female meat intake originated from sources besides her unit male, including the first documented cases of female prey capture (11% of events) in this species. Thus, females likely apply other, more pertinent criteria in mate choice.
Accurately attributing belief to others is a milestone of social cognition and serves as an indicator of intact cognitive functioning. However, tracking all others’ beliefs in complex social interactions is often inefficient or even impossible. We propose that in such cases, attributing one’s own belief to others (i.e., egocentric belief attribution) may serve as a functional approximation, enabling individuals to interact with others successfully. In this study, we employed computational modeling with behavioral measures to investigate this approach in social exchange in a novel group decision-making task across three experiments. We found that, beyond the well-documented reciprocity, participants exhibited consistent alternating behavior, characterized by the switching between potential recipients. This alternation was not driven by fairness concerns but reflected a strategic balance between maintaining stable partnerships and exploring alternatives. Crucially, a reinforcement learning model incorporating egocentric belief attribution (eToM) consistently outperformed all alternative models. These findings highlight that egocentric belief attribution is not a cognitive deficit, but a core mechanism supporting adaptive decision-making in complex social environments. ### Competing Interest Statement The authors have declared no competing interest.
Post-decisional information search is a behavioral indicator of epistemic curiosity, the intrinsic desire to acquire knowledge for its own sake. Under certain conditions, post-decisional information search can also reveal counterfactual curiosity—the drive to understand what might have occurred had a different choice been made. In two preregistered studies, we investigated whether chimpanzees and human children (a) engage in post-decisional information search, reflecting epistemic curiosity, and (b) whether their search increases under conditions reflecting counterfactual curiosity (when they had control over their decision or when the outcome of their choice was unfavorable). In Study 1, chimpanzees and children chose between two boxes containing hidden rewards and were then given the opportunity to peek into the unchosen box. Both chimpanzees and 4-to-5-year-old children engaged in post-decisional information search, with children's search increasing with age. Children also showed increased search following negative outcomes. In Study 2, participants chose between two accessible boxes while a third was inaccessible. After their choice, both chimpanzees and children preferred to peek into the remaining box that had been accessible to them. These findings provide the first comparative evidence that chimpanzees and children exhibit epistemic curiosity and, at least to some extent, counterfactual curiosity.
