Development and aging

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.

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.

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.

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.

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.