Unpacking chimpanzee (Pan troglodytes) patch use: Do individuals respond to food patches as predicted by the marginal value theorem?

The marginal value theorem is an optimal foraging model that predicts how efficient foragers should respond to both their ecological and social environments when foraging in food patches, and it has strongly influenced hypotheses for primate behavior. Nevertheless, experimental tests of the marginal value theorem have been rare in primates and observational studies have provided conflicting support. As a step towards filling this gap, we test whether the foraging decisions of captive chimpanzees (Pan troglodytes) adhere to the assumptions and qualitative predictions of the marginal value theorem. We presented 12 adult chimpanzees with a two-patch foraging environment consisting of both low-quality (i.e., low-food density) and high-quality (i.e., high-food density) patches and examined the effect of patch quality on their search behavior, foraging duration, marginal capture rate, and its proxy measures: giving-up density and giving-up time. Chimpanzees foraged longer in high-quality patches, as predicted. In contrast to predictions, they did not depress high-quality patches as thoroughly as low-quality patches. Furthermore, since chimpanzees searched in a manner that fell between systematic and random, their intake rates did not decline at a steady rate over time, especially in high-quality patches, violating an assumption of the marginal value theorem. Our study provides evidence that chimpanzees are sensitive to their rate of energy intake and that their foraging durations correlate with patch quality, supporting many assumptions underlying primate foraging and social behavior. However, our results question whether the marginal value theorem is a constructive model of chimpanzee foraging behavior, and we suggest a Bayesian foraging framework (i.e., combining past foraging experiences with current patch sampling information) as a potential alternative. More work is needed to build an understanding of the proximate mechanisms underlying primate foraging decisions, especially in more complex socioecological environments.

Individual and ecological heterogeneity promote complex communication in social vertebrate group decisions.

To receive the benefits of social living, individuals must make effective group decisions that enable them to achieve behavioural coordination and maintain cohesion. However, heterogeneity in the physical and social environments surrounding group decision-making contexts can increase the level of difficulty social organisms face in making decisions. Groups that live in variable physical environments (high ecological heterogeneity) can experience barriers to information transfer and increased levels of ecological uncertainty. In addition, in groups with large phenotypic variation (high individual heterogeneity), individuals can have substantial conflicts of interest regarding the timing and nature of activities, making it difficult for them to coordinate their behaviours or reach a consensus. In such cases, active communication can increase individuals' abilities to achieve coordination, such as by facilitating the transfer and aggregation of information about the environment or individual behavioural preferences. Here, we review the role of communication in vertebrate group decision-making and its relationship to heterogeneity in the ecological and social environment surrounding group decision-making contexts. We propose that complex communication has evolved to facilitate decision-making in specific socio-ecological contexts, and we provide a framework for studying this topic and testing related hypotheses as part of future research in this area. This article is part of the theme issue 'The power of sound: unravelling how acoustic communication shapes group dynamics'.

Dynamics of collective motion across time and species.

Most studies of collective animal behaviour rely on short-term observations, and comparisons of collective behaviour across different species and contexts are rare. We therefore have a limited understanding of intra- and interspecific variation in collective behaviour over time, which is crucial if we are to understand the ecological and evolutionary processes that shape collective behaviour. Here, we study the collective motion of four species: shoals of stickleback fish (Gasterosteus aculeatus), flocks of homing pigeons (Columba livia), a herd of goats (Capra aegagrus hircus) and a troop of chacma baboons (Papio ursinus). First, we describe how local patterns (inter-neighbour distances and positions), and group patterns (group shape, speed and polarization) during collective motion differ across each system. Based on these, we place data from each species within a 'swarm space', affording comparisons and generating predictions about the collective motion across species and contexts. We encourage researchers to add their own data to update the 'swarm space' for future comparative work. Second, we investigate intraspecific variation in collective motion over time and provide guidance for researchers on when observations made over different time scales can result in confident inferences regarding species collective motion. This article is part of a discussion meeting issue 'Collective behaviour through time'.

Playbacks of food-associated calls attract chimpanzees towards known food patches in a captive setting.

Food-associated calls have received much research attention due to their potential to refer to discovered food in a word-like manner. Studies have found that in many species, food-associated calls attract receivers to the food patch, suggesting these calls play roles in food sharing, cooperation and competition. Additionally, in various species, these calls play a role that has received much less attention: mediating social interactions among foragers that are already nearby or within the food patch, independently of whether they attract outside foragers. In order to increase understanding of the function of the chimpanzee (Pan troglodytes) food-associated rough grunt, we conducted captive playback studies testing whether rough grunt playbacks attract, repel or have no effect on the proximity of foragers already familiarized with the presence of food. We tested how acoustic playbacks of rough grunts (or control calls) from one of two known, identical feeding sites affected receivers' approach and feeding behaviors. More often than expected, participants first approached the feeding site from which rough grunts, but not control calls, were broadcast. However, neither condition increased the likelihood that participants fed first from a given site. Our results support the hypothesis that rough grunts elicit an approach response in receivers, while providing no evidence that they repel. In addition, our study provides evidence that receivers may approach rough grunts even if they do not intend to feed. We discuss the information rough grunts may convey to receivers beyond information about discovered food and the potential benefits signalers may gain from this calling behavior.