Development and aging

Wild chimpanzees (Pan troglodytes schweinfurthii) use tools to access out of reach water

Mackenzie, C. Brodnan, S., Felsche, E., Sabbi, K., Otali, E., Wrangham, R., Rosati, A.G., & Machanda, Z.P. (2025). Wild chimpanzees (Pan troglodytes schweinfurthii) use tools to access out of reach water. American Journal of Primatology. [PDF] [Supplementary] [Publisher’s version]

Abstract
The use of tools to drink water is well‐documented in wild chimpanzees, but the specific function of this behavior is unclear. Here we use a large data set of drinking behaviors spanning 14 years of observation from the Kanyawara community of
chimpanzees living in Kibale National Park, Uganda, to test two possible functions of leaf‐sponges and other drinking tools. On
the one hand, chimpanzees may use tools to access water that is hard to reach, which predicts that chimpanzees will pref-
erentially use tools to drink at tree holes and crevices compared to all other locations. Conversely, chimpanzees may use these
tools to filter stagnant water, in which case they would use tools more often at holes and puddles compared to running water
sources (e.g., streams). We compared both likelihood of using a tool to drink at different locations, as well as overall rates of
drinking, and found chimpanzees in this community most often drink from streams without tools. However, when they do use
tools, they preferentially do so to drink at tree holes. Given known age and sex effects on tool use in chimpanzees, we also
examined demographic variation in drinking tool use to understand the emergence of this behavior. While females use tools
more often than males overall—in part driven by differences in drinking rates at different locations—both males and females
use tools more frequently at tree holes than other locations when they do drink there. Finally, comparisons by age indicate that
this selectivity strengthens over development with older chimpanzees showing a more pronounced effect of using tools more
often at tree holes, suggesting that younger chimpanzees may exhibit exploratory tool use behavior. These results pinpoint the
specific function of tool use during drinking and further suggest that even simple tools may require learning for use in
appropriate contexts.

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Early emergence of metacognition in rhesus monkeys

Huang, Y. & Rosati, A.G. (2025). Early emergence of metacognition in rhesus monkeys. Developmental Science, 28: e70012. [PDF] [Supplementary] [Study Video] [Publisher’s version]

Abstract
Metacognition, or monitoring and controlling one’s knowledge, is a key feature of human cognition. Accumulating evidence shows that foundational forms of metacognition are already present in young infants and then scaffold later-emerging skills.
Although many animals exhibit cognitive processes relevant to metacognition, it is unclear if other species share the developmental
trajectories seen in humans. Here, we examine the emergence of metacognitive information-seeking in rhesus monkeys (Macaca
mulatta). We presented a large sample of semi-free-ranging monkeys, ranging from juvenility to adulthood, with a one-shot task
where they could seek information about a food reward by bending down to peer into a center vantage point in an array of tubes.
In the hidden condition, information-seeking was necessary as no food was visible on the apparatus, whereas in the visible control,
condition information-seeking was not necessary to detect the location of the reward. Monkeys sought information at the center
vantage point more often when it was necessary than in the control condition, and younger monkeys already showed competency
similar to adults. We also tracked additional monkeys who voluntarily chose not to approach to assess monkeys’ ability to actively
infer opportunities for information-seeking, and again found similar performance in juveniles and adults. Finally, we found that
monkeys were overall slower to make metacognitive inferences than to approach known reward, and that younger monkeys were
specifically slower to detect opportunities for information-seeking compared to adults. These results indicate that many features of
mature metacognition are already detectable in young monkeys, paralleling evidence for “core metacognition” in infant humans.

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Flexible information-seeking in chimpanzees

Rosati, A.G., Felsche, E., Cole, M.F., Atencia, R., & Rukundo, J. (2024). Flexible information-seeking in chimpanzees. Cognition, 251: 105898. [PDF] [Supplementary] [Study Videos] [Publisher’s version]

Abstract
Humans can flexibly use metacognition to monitor their own knowledge and strategically acquire new information when needed. While humans can deploy these skills across a variety of contexts, most evidence for metacognition in animals has focused on simple situations, such as seeking out information about the location of food. Here, we examine the flexibility, breadth, and limits of this skill in chimpanzees. We tested semi-free-ranging chimpanzees on a novel task where they could seek information by standing up to peer into different containers. In Study 1, we tested n = 47 chimpanzees to assess if chimpanzees would spontaneously engage in information-seeking without prior experience, as well as to characterize individual variation in this propensity. We found that many chimpanzees engaged in information-seeking with minimal experience, and that younger chimpanzees and females were more likely to do so. In two subsequent studies, we then further tested chimpanzees who initially showed robust information-seeking on new variations of this task, to disentangle the cognitive processing shaping their behaviors. In Study 2, we examined how a subset of n = 12 chimpanzees applied these skills to seek information about the location versus the identity of rewards, and found that chimpanzees were equally adept at seeking out location and identity information. In Study 3, we examined whether a subset of n = 6 chimpanzees could apply these skills to make more efficacious decisions when faced with uncertainty about reward payoffs. Chimpanzees were able to use information-seeking to resolve risk and choose more optimally when faced with uncertain payoffs, although they often also engaged in information-seeking when it was not strictly necessary. These results identify core features of flexible metacognition that chimpanzees share with humans, as well as constraints that may represent key evolutionary shifts in human cognition.

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Individual differences in sociocognitive traits in semi-free-ranging rhesus monkeys (Macaca mulatta)

Diaz, A.A., Hernández-Pacheco, R., & Rosati, A.G. (2025). Individual differences ­­in sociocognitive traits in semi-free-ranging rhesus monkeys (Macaca mulatta). American Journal of Primatology, 87: e23660. [PDF] [Supplementary] [Study Videos] [ Publisher’s version] Abstract

Characterizing individual differences in cognition is crucial for understanding the evolution of cognition as well as to test the biological consequences of different cognitive traits. Here, we harnessed the strengths of a uniquely large, naturally-living primate population at the Cayo Santiago Biological Field Station to characterized individual differences in rhesus monkey performance across two social cognitive tasks. A total of n = 204 semi-free-ranging adult rhesus monkeys participated in a data collection procedure, where we aimed to test individuals on both tasks at two time-points that were one year apart. In the socioemotional responses task, we assessed monkeys’ attention to conspecific photographs with neutral versus negative emotional expressions. We found that monkeys showed overall declines in interest in conspecific photographs with age, but relative increases in attention to threat stimuli specifically, and further that these responses exhibited long-term stability across repeated testing. In the gaze following task we assessed monkeys’ propensity to co-orient with an experimenter. Here, we found no evidence for age-related change in responses, and responses showed only limited repeatability over time. Finally, we found some evidence for common individual variation for performance across the tasks: monkeys that showed greater interest in conspecific photographs were more likely to follow a human’s gaze. These results show how studies of comparative cognitive development and aging can provide insights into the evolution of cognition, and identify core primate social cognitive traits that may be related across and within individuals.

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Age-related physiological dysregulation progresses slowly in semi-free-ranging chimpanzees

Cole, M.F., Barnes, P., Monroe, I., Rukundo, J., Emery Thompson, M., & Rosati, A.G. (2024). Age-related physiological dysregulation progresses slowly in semi-free-ranging chimpanzees. Evolution, Medicine, and Public Health, 12: 129–142. [PDF] [Supplementary] [Publisher’s version] Abstract

Background and objectives: Lifestyle has widespread effects on human health and aging. Prior results from chimpanzees, one of human’s closest evolutionary relatives, indicates that these lifestyle effects may also be shared with other species, as semi-free-ranging chimpanzees fed a naturalistic diet show healthier values in several specific health biomarkers, compared with their sedentary, captive counterparts. Here, we examined how lifestyle factors associated with different environments affect rates of physiological aging in closely related chimpanzees (Pan troglodytes). Methodology: We compared physiological dysregulation, an index of biological aging, in semi-free-ranging chimpanzees in an African sanctuary versus captive chimpanzees in US laboratories. If the rate of aging is accelerated by high-calorie diet and sedentism, we predicted greater age-related dysregulation in the laboratory populations. Conversely, if costs of a wild lifestyle accelerate aging, then semi-free-ranging chimpanzees at the sanctuary, whose environment better approximates the wild, should show greater age-related dysregulation. We further tested whether dysregulation differed based on sex or body system, as in humans. Results: We found that semi-free-ranging chimpanzees showed lower overall dysregulation, as well as lower age-related change in dysregulation, than laboratory chimpanzees. Males experienced lower dysregulation than females in both contexts, and the two populations exhibited distinct aging patterns based on body system. Conclusions and implications: Our results support the conclusion that naturalistic living conditions result in healthier aging in chimpanzees. These data provide support for the proposal that lifestyle effects on human health and aging are conserved from deeper in our evolutionary history.

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Predictions about reward outcomes in rhesus monkeys

Huang, Y., Chang, H., Santos, L.R. & Rosati, A.G. (2024). Predictions about reward outcomes in rhesus monkeys. Behavioral Neuroscience, 138: 43-58

[PDF] [Supplementary] [Study Videos] [Publisher’s version]Abstract
Human infants and nonhuman animals respond to surprising events by looking longer at unexpected than expected situations. These looking responses provide core cognitive evidence that nonverbal minds make predictions about possible outcomes and detect when these predictions fail to match reality. We propose that this phenomenon has crucial parallels with the processes of reward prediction error, indexing the difference between expected and actual reward outcomes. Most work on reward prediction errors to date involves neurobiological techniques that cannot be implemented in many relevant populations, so we developed a novel behavioral task to assess monkeys’ predictions about reward outcomes using looking time responses. In Study 1, we tested how semi-free-ranging monkeys (n = 210) responded to positive error (more rewards than expected), negative error (less rewards than expected), and a number control. We found that monkeys looked longer at a given reward when it was unexpectedly large or small, compared to when the same quantity was expected. In Study 2, we compared responses in the positive error condition in monkeys ranging from infancy to old age (n = 363), to assess lifespan changes in sensitivity to reward predictions. We found that adolescent monkeys showed heightened responses to unexpected rewards, similar to patterns seen in humans, but showed no changes during aging. These results suggest that monkeys’ looking responses can be used to track their predictions about rewards, and that monkeys share some developmental signatures of reward sensitivity with humans, providing a new approach to access cognitive processes underlying reward-based decision making.

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Advancing methods for the biodemography of aging within social contexts

Hernández-Pacheco, R., Steiner, U.K., Rosati, A.G., Tuljapurka, S. (2023). Advancing methods for the biodemography of aging within social contexts. Neuroscience and BioBehavioral Reviews, 153: 105400.

[PDF] [Publisher’s version]Abstract
Several social dimensions including social integration, status, early-life adversity, and their interactions across the life course can predict health, reproduction, and mortality in humans. Accordingly, the social environment plays a fundamental role in the emergence of phenotypes driving the evolution of aging. Recent work placing human social gradients on a biological continuum with other species provides a useful evolutionary context for aging questions, but there is still a need for a unified evolutionary framework linking health and aging within social contexts. Here, we summarize current challenges to understand the role of the social environment in human life courses. Next, we review recent advances in comparative biodemography and propose a biodemographic perspective to address socially driven health phenotype distributions and their evolutionary consequences using a nonhuman primate population. This new comparative approach uses evolutionary demography to address the joint dynamics of populations, social dimensions, phenotypes, and life history parameters. The long-term goal is to advance our understanding of the link between individual social environments, population-level outcomes, and the evolution of aging.

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Distinct developmental trajectories for risky and impulsive decision-making in chimpanzees

Rosati, A.G., Emery Thompson, M., Atencia, R., & Buckholtz, J.W. (2023). Distinct developmental trajectories for risky and impulsive decision-making in chimpanzees. Journal of Experimental Psychology: General, 152: 1551–1564.

[PDF][Supplementary][Publisher’s version] Abstract
Human adolescence is characterized by a suite of changes in decision-making and emotional regulation that promote risky and impulsive behavior. Accumulating evidence suggests that behavioral and physiological shifts seen in human adolescence are shared by some primates, yet it is unclear if the same cognitive mechanisms are recruited. We examined developmental changes in risky choice, intertemporal choice, and emotional responses to decision outcomes in chimpanzees, our closest-living relatives. We found that adolescent chimpanzees were more risk-seeking than adults, as in humans. However, chimpanzees showed no developmental change in intertemporal choice, unlike humans, although younger chimpanzees did exhibit elevated emotional reactivity to waiting compared to adults. Comparisons of cortisol and testosterone indicated robust age-related variation in these biomarkers, and patterns of individual differences in choices, emotional reactivity, and hormones also supported a developmental dissociation between risk and choice impulsivity. These results show that some but not all core features of human adolescent decision-making are shared with chimpanzees.

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Sensitivity to line-of-sight in tolerant versus despotic macaques (Macaca sylvanus and Macaca mulatta)

Bettle, R. & Rosati, A.G. (2022) Sensitivity to line-of-sight in tolerant versus despotic macaques (Macaca sylvanus and Macaca mulatta). Journal of Comparative Psychology, 136: 93–104.

[PDF] [Supplementary] [Study Video] [Publisher’s version] [Commentary] Abstract
Complex social life is considered important to the evolution of cognition in primates. One key aspect of primate social interactions concerns the degree of competition that individuals face in their social group. To examine how social tolerance versus competition shapes social cognition, we experimentally assessed capacities for flexible gaze-following in more tolerant Barbary macaques (Macaca sylvanus) and compared to previous data from despotic rhesus macaques (Macaca mulatta). Monkeys experienced one of two possible conditions. In the barrier condition, they observed an actor look upwards into an overheard barrier, so they could not directly see the target of the actor’s gaze without reorienting. In the no barrier condition, they observed an actor look upwards without a barrier blocking her line-of-sight, so they could observe the target of the actor’s gaze by also looking upwards. Both species (N = 58 Barbary macaques, 64 rhesus macaques) could flexibly modulate their gaze responses to account for the demonstrator’s line of sight, looking up more often when no barrier was present, and this flexible modulation declined with age in both species. However, neither species preferentially approached to look inside the barrier when their view of the target location was obscured, although rhesus macaques approached more overall. This pattern suggests that both tolerant and despotic macaques exhibit similar capacities to track other’s line of sight and do not preferentially reorient their bodies to observe what an actor looks at in this situation. This contrasts with other work indicating that competitive primates are especially adept at some aspects of theory of mind. Thus, it is important to understand both the similarities and differences in the social–cognitive abilities of primates with different social styles.

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Children show economic trust for both ingroup and outgroup partners

Grueneisen, S., Rosati, A.G., Warneken, F. (2021) Children show economic trust for both ingroup and outgroup partners. Cognitive Development, 59: 101077

[PDF] [Publisher’s version] Abstract
Trust is a critical aspect of human cooperation, allowing individuals to overcome the risks posed by such interactions because of others’ presumed cooperative inclinations. Adults sometimes mitigate these risks by preferentially trusting members of their own social group, yet it is currently unclear if the early emergence of children’s trust in others’ cooperative tendencies is affected by their intergroup psychology. Here we tested whether group membership impacts two key aspects of trust-based cooperation in young children – their trust in others’ willingness to reciprocate an investment (assessed using the Investment Game, Study 1), and their trust in others’ generosity (assessed using the Faith Game, Study 2). In both studies, children assigned to novel and otherwise arbitrary groups demonstrated general preferences for ingroup members on several measures. However, group membership did not influence their decisions about economic trust. In Study 1, 4- and 6-year-old children showed high levels of trust in both ingroup and outgroup members’ tendency to reciprocate an investment. In Study 2, 6- to 7-year-old children similarly showed high levels of trust in ingroup and outgroup members’ generosity, and they did so regardless of whether their group membership was a matter of common knowledge between themselves and the trustee. These findings show that young children’s preferences for ingroup members do not result in bias due to shared group membership when making economic trust decisions. Rather, children tend to exhibit trust in the cooperativeness of others regardless of group membership.

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The primate origins of human social cognition

Bettle, R. & Rosati, A.G. (2021). The primate origins of human social cognition. Language Learning and Development, 17: 96-127

[PDF] [Publisher’s version] Abstract
The ability to understand the mental states of other individuals is central to human social behavior, yet some theory of mind capacities are shared with other species. Comparisons of theory of mind skills across humans and other primates can provide a critical test of the cognitive prerequisites necessary for different theory of mind skills to emerge. A fundamental difference between humans and non-humans is language: while language may scaffold some developing theory of mind skills in humans, other species do not have similar capacities for or immersion in language. Comparative work can therefore provide a new line of evidence to test the role of language in the emergence of complex social cognition. Here we first provide an overview of the evidence for shared aspects of theory of mind in other primates, and then examine the evidence for apparently human-unique aspects of theory of mind that may be linked to language. We finally contrast different evolutionary processes, such as competition and cooperation, that may have been important for primate social cognition versus human-specific forms of theory of mind. We argue that this evolutionary perspective can help adjudicate between different proposals on the link between human-specific forms of social cognition and language.

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Variation in primate decision-making under uncertainty and the roots of human economic behaviour

De Petrillo, F. & Rosati, A.G. (2021). Variation in primate decision-making under uncertainty and the roots of human economic behaviour. Philosophical Transactions of the Royal Society B, 376: 20190671

[PDF] [Supplementary] [Publisher’s version] Abstract
Uncertainty is a ubiquitous component of human economic behaviour, yet people can vary in their preferences for risk across populations, individuals, and different points in time. As uncertainty also characterizes many aspects of animal decision-making, comparative research can help evaluate different potential mechanisms that generates this variation, including the role of biological differences or maturational change versus cultural learning, as well as identify human-unique components of economic decision-making. Here we examine decision-making under risk across primates, our closest relatives. We first review theoretical approaches and current methods for understanding decision-making in animals. We then assess current evidence for variation in animal preferences between species and populations; between individuals based on personality, sex, and age; and finally, between different contexts and individual states. We then use this primate data to evaluate the processes that can shape human decision-making strategies and identify the primate foundations of human economic behaviour.

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The evolutionary origins of natural pedagogy: Rhesus monkeys show sustained attention following nonsocial cues versus social communicative signals

Bettle, R. & Rosati, A.G. (2021). The evolutionary origins of natural pedagogy: Rhesus monkeys show sustained attention following nonsocial cues versus social communicative signals. Developmental Science, 24: e12987.

[PDF] [Supplementary] [Videos] [Publisher’s version] Abstract
The natural pedagogy hypothesis proposes that human infants preferentially attend to communicative signals from others, facilitating rapid cultural learning. In this view, sensitivity to such signals are a uniquely human adaptation and as such nonhuman animals should not produce or utilize these communicative signals. We test these evolutionary predictions by examining sensitivity to communicative cues in 206 rhesus monkeys (Macaca mulatta) using an expectancy looking time task modeled on prior work with infants. Monkeys observed a human actor who either made eye contact and vocalized to the monkey (social cue), or waved a fruit in front of her face and produced a tapping sound (nonsocial cue). The actor then either looked at an object (referential look) or looked towards empty space (look away). We found that, unlike human infants in analogous situations, rhesus monkeys looked longer at events following nonsocial cues, regardless of the demonstrator’s subsequent looking behavior. Moreover, younger and older monkeys showed similar patterns of responses across development. These results provide support for the natural pedagogy hypothesis, while also highlighting evolutionary changes in human sensitivity to communicative signals.

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Social selectivity in aging wild chimpanzees

Rosati, A.G., Hagberg, L., Enigk, D.K., Otali, E., Emery Thompson, M., Muller, M.N., Wrangham, R.W. & Machanda, Z.P. (2020), Social selectivity in aging wild chimpanzees. Science, 370: 473-476.

[PDF] [Supplementary] [Publisher’s Version] [Commentary] Abstract

Humans prioritize close, positive relationships during aging, and socioemotional selectivity theory proposes that this shift causally depends on capacities for thinking about personal future time horizons. To examine this theory, we tested for key elements of human social aging in longitudinal data on wild chimpanzees. Aging male chimpanzees have more mutual friendships characterized by high, equitable investment, whereas younger males have more one-sided relationships. Older males are more likely to be alone, but they also socialize more with important social partners. Further, males show a relative shift from more agonistic interactions to more positive, affiliative interactions over their life span. Our findings indicate that social selectivity can emerge in the absence of complex future-oriented cognition, and they provide an evolutionary context for patterns of social aging in humans.

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Healthy cardiovascular biomarkers across the lifespan in wild-born chimpanzees (Pan troglodytes)

Cole, M.F., Cantwell, A., Rukundo, J. Ajarova, L., Fernandez-Navarro, S., Atencia, R. & Rosati, A.G. (2020). Healthy cardiovascular biomarkers across the lifespan in wild-born chimpanzees (Pan troglodytes).Philosophical Transactions of the Royal Society B, 375: 20190609.

[PDF] [Supplementary] [Publisher’s version] Abstract
Chimpanzees (Pan troglodytes) are a crucial model for understanding the evolution of human health and longevity. Cardiovascular disease is a major source of mortality during aging in humans and therefore a key issue for comparative research. Current data indicates that compared to humans, chimpanzees have proatherogenic blood lipid profiles, an important risk factor for cardiovascular disease in humans. However, most work to date on chimpanzee lipids come from laboratory-living populations where lifestyles diverge from a wild context. Here we examined cardiovascular profiles in chimpanzees living in African sanctuaries, who semi-free-range in large forested enclosures, consume a naturalistic diet, and generally experience conditions more similar to a wild chimpanzee lifestyle. We measured blood lipids, body weight, and body fat in 75 sanctuary chimpanzees and compared them to publicly-available data from laboratory-living chimpanzees from the Primate Aging Database. We found that semi-free-ranging chimpanzees exhibited lower body weight and lower levels of lipids that are risk factors for human cardiovascular disease, and that some of these disparities increased with age. Our findings support the hypothesis that lifestyle can shape health indices in chimpanzees, similar to effects observed across human populations, and contribute to an emerging understanding of human cardiovascular health in evolutionary context.

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