Social inhibition and behavioural flexibility when the context changes: a comparison across six primate species

Early-life group size influences response inhibition, but not the learning of it, in Japanese quails

In complex social environments, animals benefit from suppressing inappropriate responses (known as Response Inhibition) to avoid conflicts and maintain group cohesion. Recent research suggests that an individual's early-life social environment can shape their response inhibition. However, these findings have mostly been correlational, and results vary across species. Furthermore, the role of learning is often overlooked when measuring response inhibition, despite its potential importance to understanding group differences. We investigated the effect of early-life group size, a key determinant of social complexity, on response inhibition in Japanese quails (Coturnix japonica), whilst taking the role of learning into account. Quails (n = 120) were raised in either small groups of five or large groups of 15 individuals. Response inhibition was assessed using the cylinder task. Up to ten trials were administered to assess whether the birds' responses changed with increasing experience of the task. Among the quails that completed ten trials, we found that those raised in large groups consistently spent less time pecking the cylinder than those raised in small groups. The quails' responses were also influenced by their body condition, food motivation and sex. Importantly, the quails learned to inhibit their responses - successful trials increased, and time spent pecking the cylinder decreased, across ten trials. However, learning rates did not differ between the treatment groups. These findings suggest that early-life social group size promotes the development of response inhibition in quails, but not their learning of it, during the cylinder task.

The ability to inhibit impulses is related to social behavior in long-tailed macaques

Performance in cognitive tasks has been linked to differences in species' social organization, yet to understand its function its relationship to within-species variation in behavior should also be explored. One important cognitive capacity, the ability to inhibit impulses, is typically better in egalitarian than despotic primate species and in primate species with strong fission-fusion dynamics. A different line of research indicates that a high ability to inhibit impulses is related to less aggressive behavior and more socio-positive behavior. However, within species the relationship between performance on cognitive inhibition tasks and variation in social behavior remains to be explored. Here we investigate how performance in a typical inhibition task in cognitive research is related to aggressive and socio-positive behavior in despotic long-tailed macaques. Twenty individuals living in two naturalistic mixed-sex groups were tested with the Plexiglass Hole Task. Aggressive behavior and three types of socio-positive behavior (neutral/friendly approaches, socio-positive signaling, and grooming others) among group members were measured. Individuals differed in their ability to inhibit impulses. Individuals that were not good at inhibiting impulses showed higher rates of aggressive behavior, but also more socio-positive signals, whereas inhibition was not related to neutral/friendly approaches and grooming. These results confirm the positive link between impulsiveness and aggression. In addition, the results indicate that some social-positive behavior may be enhanced when inhibition is limited. In this species, benefits potentially derived from aggression and socio-positive signals match a low ability to inhibit impulses, suggesting that a low ability to inhibit impulses may actually be advantageous. To understand differences between species in cognitive skills, understanding the benefits of variation in a cognitive capacity within a species is crucial.

Variation in communicative complexity in relation to social structure and organization in non-human primates

Communicative complexity relates to social complexity, as individuals in more complex social systems either use more signals or more complex signals than individuals living in less complex ones. Taking the individual group member's perspective, here we examine communicative complexity in relation to social complexity, which arises from two components of social systems: social structure and social organization. We review the concepts of social relationships and social complexity and evaluate their implications for communicative and cognitive complexity using examples from primate species. We focus on spider monkeys (Ateles geoffroyi), as their social organization is characterized by flexibility in grouping dynamics and they use a variety of communicative signals. We conclude that no simple relationship exists among social complexity, communicative complexity and cognitive complexity, with social complexity not necessarily implying cognitive complexity, and communicative and cognitive complexity being independently linked to social complexity. To better understand the commonly implied link between social complexity and cognitive complexity it is crucial to recognize the complementary role of communicative complexity. A more elaborated communicative toolkit provides the needed flexibility to deal with dynamic and multifaceted social relationships and high variation in fission-fusion dynamics. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

The Evolution of Cognitive Control in Lemurs

Cognitive control, or executive function, is a key feature of human cognition, allowing individuals to plan, acquire new information, or adopt new strategies when the circumstances change. Yet it is unclear which factors promote the evolution of more sophisticated executive-function abilities such as those possessed by humans. Examining cognitive control in nonhuman primates, our closest relatives, can help to identify these evolutionary processes. Here, we developed a novel battery to experimentally measure multiple aspects of cognitive control in primates: temporal discounting, motor inhibition, short-term memory, reversal learning, novelty responses, and persistence. We tested lemur species with targeted, independent variation in both ecological and social features (ruffed lemurs, Coquerel's sifakas, ring-tailed lemurs, and mongoose lemurs; N = 39) and found that ecological rather than social characteristics best predicted patterns of cognitive control across these species. This highlights the importance of integrating cognitive data with species' natural history to understand the origins of complex cognition.

Individual differences in co-representation in three monkey species (Callithrix jacchus, Sapajus apella and Macaca tonkeana) in the joint Simon task: the role of social factors and inhibitory control

Behavioral coordination is involved in many forms of primate interactions. Co-representation is the simultaneous mental representation of one’s own and the partner’s task and actions. It often underlies behavioral coordination and cooperation success. In humans, the dyadic social context can modulate co-representation. Here, we first investigated whether individual differences in co-representation in the joint Simon task in capuchin monkeys and Tonkean macaques can be explained by social factors, namely dyadic grooming and sociality index, rank difference and eigenvector centrality. These factors did not predict variation in co-representation. However, in this specific task, co-representation reduces rather than facilitates joint performance. Automatic co-representation therefore needs to be inhibited or suppressed to maximize cooperation success. We therefore also investigated whether general inhibitory control (detour-reaching) would predict co-representation in the joint Simon task in Tonkean macaques, brown capuchin and marmoset monkeys. Inhibitory control did neither explain individual differences nor species differences, since marmosets were most successful in their joint performance despite scoring lowest on inhibitory control. These results suggest that the animals’ ability to resolve conflicts between self and other representation to increase cooperation success in this task is gradually learned due to frequent exposure during shared infant care, rather than determined by strong general inhibitory control. Further, we conclude that the joint Simon task, while useful to detect co-representation non-invasively, is less suitable for identifying the factors explaining individual differences and thus a more fruitful approach to identify these factors is to design tasks in which co-representation favors, rather than hinders cooperation success.

Heterogeneity of performances in several inhibitory control tasks: male rhesus macaques are more easily distracted than females

Inhibitory control, the ability to override a dominant response, is crucial in many aspects of everyday life. In animal studies, striking individual variations are often largely ignored and their causes rarely considered. Hence, our aims were to systematically investigate individual variability in inhibitory control, to replicate the most common causes of individual variation (age, sex and rank) and to determine if these factors had a consistent effect on three main components of inhibitory control (inhibition of a distraction, inhibition of an action, inhibition of a cognitive set). We tested 21 rhesus macaques (Macaca mulatta) in a battery of validated touchscreen tasks. We first found individual variations in all inhibitory control performances. We then demonstrated that males had poorer performances to inhibit a distraction and that middle-aged individuals exhibited poorer performance in the inhibition of a cognitive set. Hence, the factors of age and sex were not consistently associated with the main components of inhibitory control, suggesting a multi-faceted structure. The rank of the subjects did not influence any inhibitory control performances. This study adopts a novel approach for animal behaviour studies and gives new insight into the individual variability of inhibitory control which is crucial to understand its evolutionary underpinnings.

Linking ecology and cognition: does ecological specialisation predict cognitive test performance?

Variation in cognitive abilities is thought to be linked to variation in brain size, which varies across species with either social factors (Social Intelligence Hypothesis) or ecological challenges (Ecological Intelligence Hypothesis). However, the nature of the ecological processes invoked by the Ecological Intelligence Hypothesis, like adaptations to certain habitat characteristics or dietary requirements, remains relatively poorly known. Here, we review comparative studies that experimentally investigated interspecific variation in cognitive performance in relation to a species’ degree of ecological specialisation. Overall, the relevant literature was biased towards studies of mammals and birds as well as studies focusing on ecological challenges related to diet. We separated ecological challenges into those related to searching for food, accessing a food item and memorising food locations. We found interspecific variation in cognitive performance that can be explained by adaptations to different foraging styles. Species-specific adaptations to certain ecological conditions, like food patch distribution, characteristics of food items or seasonality also broadly predicted variation in cognitive abilities. A species’ innovative problem-solving and spatial processing ability, for example, could be explained by its use of specific foraging techniques or search strategies, respectively. Further, habitat generalists were more likely to outperform habitat specialists. Hence, we found evidence that ecological adaptations and cognitive performance are linked and that the classification concept of ecological specialisation can explain variation in cognitive performance only with regard to habitat, but not dietary specialisation.

Comparative performance of orangutans (Pongo spp.), gorillas (Gorilla gorilla gorilla), and drills (Mandrillus leucophaeus), in an ephemeral foraging task

A goal of the comparative approach is to test a variety of species on the same task. Here, we examined whether the factors that helped capuchin monkeys improve their performance in a dichotomous choice task would generalize to three other primate species: orangutans, gorillas, and drill monkeys. In this task, subjects have access to two options, each resulting in an identical food, but one (the ephemeral option) is only available if it is chosen first, whereas the other one (the permanent option) is always available. Therefore, the food-maximizing solution is to choose the ephemeral option first, followed by the permanent option for an additional reward. On the original version (plate task), the options were discriminated by the color and pattern of the plates holding the food, while on two subsequent versions we used altered cues that we predicted would improve performance: (1) the color of the foods themselves (color task), which we hypothesized was relevant to primates, who choose foods rather than substrates on which foods are found when foraging, and (2) patterned cups covering the foods (cup task), which we hypothesized would help primates avoid the prepotent response associated with visible food. Like capuchins, all three species initially failed to solve the plate task. However, while orangutans improved their performance from the plate to the color task, they did not for the cup task, and only a few gorillas and no drills succeeded in either task. Unfortunately, our ability to interpret these data was obscured by differences in the subjects' level of experience with cognitive testing and practical constraints that precluded the use of completely identical procedures across species. Nonetheless, we consider what these results can tell us, and discuss the value of conducting studies across multiple sites despite unavoidable differences.

Problem solving flexibility across early development

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We tested cognitive flexibility in 2-, 3, and 4-year-old children.

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Children were presented with a novel task previously used with nonhuman primates.

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All children spontaneously solved the task; most (83.61%) used an efficient method.

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Children responded flexibly when task demands changed.

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4-year-olds were significantly more efficient than 2-year-olds.

Cognitive flexibility allows individuals to adapt to novel situations. However, this ability appears to develop slowly over the first few years of life, mediated by task complexity and opacity. We used a physically simple novel task, previously tested with nonhuman primates, to explore the development of flexible problem solving in 2-, 3-, and 4-year-old children from a developmental and comparative perspective. The task goal was to remove barriers (straws) from a clear tube to release a ball. The location of the ball, and therefore the number of straws necessary to retrieve it, varied across two test phases (four of five straws and two of five straws, respectively). In Test Phase 1, all children retrieved the ball in Trial 1 and 83.61% used the most efficient method (removing only straws below the ball). Across Phase 1 trials, 4-year-olds were significantly more efficient than 2-year-olds, and solve latency decreased for all age groups. Test Phase 2 altered the location of the ball, allowing us to explore whether children could flexibly adopt a more efficient solution when their original (now inefficient) solution remained available. In Phase 2, significantly more 4-year-olds than 2-year-olds were efficient; the older children showed greater competency with the task and were more flexible to changing task demands than the younger children. Interestingly, no age group was as flexible in Phase 2 as previously tested nonhuman primates, potentially related to their relatively reduced task exploration in Phase 1. Therefore, this causally clear task revealed changes in cognitive flexibility across both early childhood and species.

Understanding Language Evolution: Beyond Pan-Centrism

Language does not fossilize but this does not mean that the language's evolutionary timeline is lost forever. Great apes provide a window back in time on our last prelinguistic ancestor's communication and cognition. Phylogeny and cladistics implicitly conjure Pan (chimpanzees, bonobos) as a superior (often the only) model for language evolution compared with earlier diverging lineages, Gorilla and Pongo (orangutans). Here, in reviewing the literature, it is shown that Pan do not surpass other great apes along genetic, cognitive, ecologic, or vocal traits that are putatively paramount for language onset and evolution. Instead, revived herein is the idea that only by abandoning single-species models and learning about the variation among great apes, there might be a chance to retrieve lost fragments of the evolutionary timeline of language.

Fertility as a constraint on group size in African great Apes

Gorillas and chimpanzees live in social groups of very different size and structure. Here I test the hypothesis that this difference might reflect the way fertility maps onto group demography as it does in other Catarrhines. For both genera, birth rates and the number of surviving offspring per female are quadratic (or ∩-shaped) functions of the number of adult females in the group, and this is independent of environmental effects. The rate at which fertility declines ultimately imposes a constraint on the size of social groups that can be maintained in both taxa. The differences in group size between the two genera seem to reflect a contrast in the way females buffer themselves against this cost. Gorillas do this by using males as bodyguards, whereas chimpanzees exploit fission–fusion sociality to do so. The latter allows chimpanzees to live in much larger groups without paying a fertility cost (albeit at a cognitive cost).

Establishing an infrastructure for collaboration in primate cognition research

Inferring the evolutionary history of cognitive abilities requires large and diverse samples. However, such samples are often beyond the reach of individual researchers or institutions, and studies are often limited to small numbers of species. Consequently, methodological and site-specific-differences across studies can limit comparisons between species. Here we introduce the ManyPrimates project, which addresses these challenges by providing a large-scale collaborative framework for comparative studies in primate cognition. To demonstrate the viability of the project we conducted a case study of short-term memory. In this initial study, we were able to include 176 individuals from 12 primate species housed at 11 sites across Africa, Asia, North America and Europe. All subjects were tested in a delayed-response task using consistent methodology across sites. Individuals could access food rewards by remembering the position of the hidden reward after a 0, 15, or 30-second delay. Overall, individuals performed better with shorter delays, as predicted by previous studies. Phylogenetic analysis revealed a strong phylogenetic signal for short-term memory. Although, with only 12 species, the validity of this analysis is limited, our initial results demonstrate the feasibility of a large, collaborative open-science project. We present the ManyPrimates project as an exciting opportunity to address open questions in primate cognition and behaviour with large, diverse datasets.

Capuchin and rhesus monkeys but not humans show cognitive flexibility in an optional-switch task

Learned rules help us accurately solve many problems, but by blindly following a strategy, we sometimes fail to find more efficient alternatives. Previous research found that humans are more susceptible to this "cognitive set" bias than other primates in a nonverbal computer task. We modified the task to test one hypothesis for this difference, that working memory influences the advantage of taking a shortcut. During training, 60 humans, 7 rhesus macaques, and 22 capuchin monkeys learned to select three icons in sequence. They then completed 96 baseline trials, in which only this learned rule could be used, and 96 probe trials, in which they could also immediately select the final icon. Rhesus and capuchin monkeys took this shortcut significantly more often than humans. Humans used the shortcut more in this new, easier task than in previous work, but started using it significantly later than the monkeys. Some participants of each species also used an intermediate strategy; they began the learned rule but switched to the shortcut after selecting the first item in the sequence. We suggest that these species differences arise from differences in rule encoding and in the relative efficiency of exploiting a familiar strategy versus exploring alternatives.

How does cognition shape social relationships?

The requirements of living in social groups, and forming and maintaining social relationships are hypothesized to be one of the major drivers behind the evolution of cognitive abilities. Most empirical studies investigating the relationships between sociality and cognition compare cognitive performance between species living in systems that differ in social complexity. In this review, we ask whether and how individuals benefit from cognitive skills in their social interactions. Cognitive abilities, such as perception, attention, learning, memory, and inhibitory control, aid in forming and maintaining social relationships. We investigate whether there is evidence that individual variation in these abilities influences individual variation in social relationships. We then consider the evolutionary consequences of the interaction between sociality and cognitive ability to address whether bi-directional relationships exist between the two, such that cognition can both shape and be shaped by social interactions and the social environment. In doing so, we suggest that social network analysis is emerging as a powerful tool that can be used to test for directional causal relationships between sociality and cognition. Overall, our review highlights the importance of investigating individual variation in cognition to understand how it shapes the patterns of social relationships.This article is part of the theme issue 'Causes and consequences of individual differences in cognitive abilities'.

Time-space-displaced responses in the orangutan vocal system

One of the defining features of language is displaced reference-the capacity to transmit information about something that is not present or about a past or future event. It is very rare in nature and has not been shown in any nonhuman primate, confounding, as such, any understanding of its precursors and evolution in the human lineage. Here, we describe a vocal phenomenon in a wild great ape with unparalleled affinities with displaced reference. When exposed to predator models, Sumatran orangutan mothers temporarily suppressed alarm calls up to 20 min until the model was out of sight. Subjects delayed their vocal responses in function of perceived danger for themselves, but four major predictions for stress-based mechanisms were not met. Conversely, vocal delay was also a function of perceived danger for another-an infant-suggesting high-order cognition. Our findings suggest that displaced reference in language is likely to have originally piggybacked on akin behaviors in an ancestral hominid.