State-dependent risky choices in primates: Variation in energy budget does not affect tufted capuchin monkeys' (Sapajus spp.) risky choices.

Economic models predict that rational decision makers' choices between a constant, "safe" option and a variable, "risky" option leading, on average, to the same payoff, should be random. However, a wealth of research has revealed that, when faced with risky decisions, both human and nonhuman animals deviate from economic rationality. According to the risk-sensitivity theory, individuals should prefer a safe option when they are in a positive energy state and a risky option when they are in a negative energy state. The abundance/risk hypothesis proposes that individuals should prefer risky options when diet quality exceeds their nutritional requirements. We tested how energy budget affects decision making under risk by presenting 22 capuchins belonging to two colonies (IT: N = 12, US: N = 10) with a risky choice task. Capuchins had to choose between a constant option (always four food items) and a variable option (one or seven food items with a 50% probability) in two conditions. In the Low-energy condition capuchins were tested before their main meal, whereas in the High-energy condition they were tested following a high-caloric meal. In neither colony did we find a significant difference between conditions, suggesting that the energy budget did not affect risk preferences. However, we found differences between colonies in their general response to risky choices: US capuchins were more risk-prone after selecting a safe option than a risky option and after selecting a bad (one food item) than a good (seven food items) risky option, whereas this did not hold true in IT capuchins. Furthermore, in the IT colony, males were more risk-prone under the High-energy condition compared to the Low-energy condition. Subtle differences in individual characteristics, management conditions, or stochastic founder effects may be implied, with relevant consequences for the outcomes of research on risky decision-making across laboratories.

Chimpanzees (Pan troglodytes) do not develop contingent reciprocity in an experimental task.

Chimpanzees provide help to unrelated individuals in a broad range of situations. The pattern of helping within pairs suggests that contingent reciprocity may have been an important mechanism in the evolution of altruism in chimpanzees. However, correlational analyses of the cumulative pattern of interactions over time do not demonstrate that helping is contingent upon previous acts of altruism, as required by the theory of reciprocal altruism. Experimental studies provide a controlled approach to examine the importance of contingency in helping interactions. In this study, we evaluated whether chimpanzees would be more likely to provide food to a social partner from their home group if their partner had previously provided food for them. The chimpanzees manipulated a barpull apparatus in which actors could deliver rewards either to themselves and their partners or only to themselves. Our findings indicate that the chimpanzees' responses were not consistently influenced by the behavior of their partners in previous rounds. Only one of the 11 dyads that we tested demonstrated positive reciprocity. We conclude that contingent reciprocity does not spontaneously arise in experimental settings, despite the fact that patterns of behavior in the field indicate that individuals cooperate preferentially with reciprocating partners.