Investigating reactions of squirrel monkeys (Saimiri sciureus) towards unequal food distributions in a tray-pulling paradigm

Female squirrel monkeys' (Saimiri boliviensis) responses to inequity in a group context; testing a link between cooperation and inequity responses

Primates of several species respond negatively to receiving less preferred rewards than a partner for completing the same task (inequity responses), either rejecting rewards or refusing to participate in the task when disadvantaged. This has been linked to cooperation, with species that cooperate frequently refusing to participate in inequity tasks (the 'cooperation hypothesis'). However, inequity is a social response, and previous research has involved dyads, precluding studying the effects of additional social partners. While dyads allow for tighter control in experimental settings, dyadic interactions in nature do not take place in a social vacuum, so understanding the role of the social context is needed to verify that the pattern of results supports the cooperation hypothesis. Here we focus on Bolivian squirrel monkeys, Saimiri boliviensis, a highly social species that does not generally cooperate and has not responded to inequity in previous dyadic research, although they do respond to receiving a lower reward than they expected. In the current study, we provide a more nuanced test by studying female Bolivian squirrel monkeys, the demographic most likely to cooperate in both field and laboratory contexts, in a more socially relevant group setting. For some reward values, females responded in both the inequity condition, rejecting less preferred rewards when they were disadvantaged relative to their social group, and a contrast condition, wherein all animals received a lower reward than they expected, making it difficult to disentangle contrast from inequity. As in capuchin monkeys, refusals increased when monkeys were to receive low-value rewards compared to medium-value rewards. These results suggest that the relationship between cooperation and inequity responses may be more nuanced than previously suggested, with demographic, social context and reward value potentially influencing outcomes even within species.

Seasonal Change in Activity Rhythms and Time Budgets of Tibetan Macaques

Activity rhythms and time budgets are important to understand behavioral variability and adaptation in primates because animals normally use a behavioral adjustment as a preferential choice in response to environmental changes. Therefore, we observed a group of un-provisioned Tibetan macaques (Macaca thibetana) in Tianhu Mountain County Nature Reserve, Mount Huangshan, Southern Anhui, China. We used the instantaneous scan sampling method to collect behavioral data on their activity rhythms and time budget. The results showed that Tibetan macaques have two foraging peaks (9:00–10:00 and 14:00–15:00) and a resting peak at 12:00–13:00. They spent 31.96% resting, 28.59% foraging, 26.96% moving, 6.90% grooming, and 5.59% other. The total time of foraging and moving per month significantly and positively correlated with fruits and buds and negatively correlated with leaves. Different sexes and age groups demonstrated different activity time budgets, with adult males, adult females, and young macaques spending most of their time resting, grooming, and playing, respectively. We elucidated the effects of different environmental conditions on Tibetan macaques and their behavioral adaptation strategies.

Western lowland gorillas (Gorilla gorilla gorilla) do not show an aversion to inequity in a token exchange task

Although individuals in some species refuse foods they normally accept if their partner receives a more preferred one, this is not true across all species. The cooperation hypothesis proposes that this species-level variability evolved because inequity aversion is a mechanism to identify situations in which cooperation is not paying off, and that species regularly observed cooperating should be more likely to be averse to inequity. To rule out other potential explanations of inequity aversion, we need to test the converse as well: species rarely observed cooperating, especially those phylogenetically close to more cooperative species, should be less likely to be inequity averse. To this end, we tested eight zoo-housed Western lowland gorillas (Gorilla gorilla gorilla) on a token exchange task in which subjects received either the same food reward or a less-preferred reward for the same or more effort than their partner, recording both refusals to participate in the exchange and refusals to accept the reward. Supporting the cooperation hypothesis, even with procedural differences across sessions, gorillas were significantly more likely to refuse in all conditions in which they received a low-value food reward after completing an exchange, regardless of what their partner received, suggesting that gorillas were not inequity averse, but instead would not work for a low-value reward. Additionally, gorillas were more likely to refuse later in the session; while the pattern of refusals remained unchanged after accounting for this, this suggests that species should be tested on as many trials as is practical.