A Conceptual Framework for Non-Kin Food Sharing: Timing and Currency of Benefits

Many animal species, from arthropods to apes, share food. This paper presents a new framework that categorizes nonkin food sharing according to two axes: (1) the interval between sharing and receiving the benefits of sharing, and (2) the currency units in which benefits accrue to the sharer (especially food versus nonfood). Sharers can obtain immediate benefits from increased foraging efficiency, predation avoidance, mate provisioning, or manipulative mutualism. Reciprocity, trade, status enhancement and group augmentation can delay benefits. When benefits are delayed or when food is exchanged for nonfood benefits, maintaining sharing can become more difficult because animals face discounting and currency conversion problems. Explanations that involve delayed or nonfood benefits may require specialized adaptations to account for timing and currency-exchange problems. The immediate, selfish fitness benefits that a sharer may gain through by-product or manipulative mutualism, however, apply to various food-sharing situations across many species and may provide a simpler, more general explanation of sharing

A Conceptual Framework for Non-Kin Food Sharing: Timing and Currency of Benefits

Many animal species, from arthropods to apes, share food. This paper presents a new framework that categorizes nonkin food sharing according to two axes: (1) the interval between sharing and receiving the benefits of sharing, and (2) the currency units in which benefits accrue to the sharer (especially food versus nonfood). Sharers can obtain immediate benefits from increased foraging efficiency, predation avoidance, mate provisioning, or manipulative mutualism. Reciprocity, trade, status enhancement and group augmentation can delay benefits. When benefits are delayed or when food is exchanged for nonfood benefits, maintaining sharing can become more difficult because animals face discounting and currency conversion problems. Explanations that involve delayed or nonfood benefits may require specialized adaptations to account for timing and currency-exchange problems. The immediate, selfish fitness benefits that a sharer may gain through by-product or manipulative mutualism, however, apply to various food-sharing situations across many species and may provide a simpler, more general explanation of sharing

Long-term inter-sexual association patterns among wild chimpanzees

The widely accepted socio-ecological model of primate sociality assumes that males and female chimpanzees do not exhibit differentiated social relationships. However, despite anecdotal evidence to the contrary, this assumption has never been explicitly tested. We used 14 years of data from the Kanyawara community in Kibale National Park, Uganda to describe inter-sexual association patterns among these chimpanzees. We calculated a composite index using temporal and spatial association data to characterize the relationships between data to characterize the relationships between 336 male-females dyads. We considered any dyad with a composite index greater than one standard deviation above the mean to be strongly associated. We found that: (1) while the majority of male-female dyads were not strongly associated, a subset of dyads showed greater than average association across several two-year time periods; (2) all but one of the maternal kin dyads (either mother-son or brother-sister) had differentiated relationships; and (3) the association preferences of some dyads remained consistent despite changes in the reproductive condition of the female over time. We used generalized linear models to determine the effect of reproductive state, rank and seasonality on patterning these long-term associations. Our finding that chimpanzees exhibit differentiated inter-sexual association patterns will have far-reaching effects on studies of other forms of male-female interaction such as aggression, and further our understanding of the evolution of human pair-bonding.AnthropologyHuman Evolutionary Biolog

A Direct Comparison of Scan and Focal Sampling Methods for Measuring Wild Chimpanzee Feeding Behaviour

Focal sampling is the most accurate method for measuring primate activity budgets, but is sometimes impractical. An alternative is scan sampling, in which the behaviour of the group is recorded at regular intervals. The simplest technique is to record whether at least one animal is engaged in the behaviour of interest. By direct comparison with focal data collected simultaneously on the same population, we assess the validity of this simple group-level sampling method for studying chimpanzee (Pan troglodytes schweinfurthii) feeding behaviour. In a 13-month study at Kanyawara, Kibale National Park, Uganda, group-level scan sampling provided statistically similar measures of broad diet composition to those produced by focal data, despite considerable seasonal variation. Monthly means of the percentage of time spent consuming non-fig fruit calculated from group-level scan sampling were highly correlated with those from focal sampling. This validates previous methodology used to identify periods of high energy availability. However, group-level scans tended to overestimate the percentage of observation time spent feeding, particularly for adult males. We conclude that this method of group-level scan sampling provides valuable data for characterising broad diet choice in chimpanzees and other species, but may be of limited use for estimating individual feeding time.Human Evolutionary Biolog

No Evidence of Short-Term Exchange of Meat for Sex among Chimpanzees

The meat-for-sex hypothesis posits that male chimpanzees (Pan troglodytes) trade meat with estrous females in exchange for short-term mating access. This notion is widely cited in the anthropological literature and has been used to construct scenarios about human evolution. Here we review the theoretical and empirical basis for the meat-for-sex hypothesis. We argue that chimpanzee behavioral ecology does not favor the evolution of such exchanges because 1) female chimpanzees show low mate selectivity and require little or no material incentive to mate, violating existing models of meat-for-sex exchange; and 2) meat-for-sex exchanges are unlikely to provide reproductive benefits to either partner. We also present new analyses of 28 years of data from two East African chimpanzee study sites (Gombe National Park, Tanzania; Kanyawara, Kibale National Park, Uganda) and discuss the results of previously published studies. In at least three chimpanzee communities, 1) the presence of sexually receptive females did not increase hunting probability; 2) males did not share preferentially with sexually receptive females and 3) sharing with females did not increase a male’s short-term mating success. We acknowledge that systematic meat-sharing by male chimpanzees in expectation of or as reward for immediate copulations might be discovered in future studies of chimpanzees. However current data indicate that such exchanges are so rare in studied populations, and are so different in nature from exchanges among humans, that with respect to chimpanzees, sexual bartering in humans should be regarded as a derived trait with no known antecedents in the behavior of wild chimpanzees.Human Evolutionary Biolog

Reflections of the Social Environment in Chimpanzee Memory: Applying Rational Analysis Beyond Humans

In cognitive science, the rational analysis framework allows modelling of how physical and social environments impose information-processing demands onto cognitive systems. In humans, for example, past social contact among individuals predicts their future contact with linear and power functions. These features of the human environment constrain the optimal way to remember information and probably shape how memory records are retained and retrieved. We offer a primer on how biologists can apply rational analysis to study animal behaviour. Using chimpanzees (Pan troglodytes) as a case study, we modelled 19 years of observational data on their social contact patterns. Much like humans, the frequency of past encounters in chimpanzees linearly predicted future encounters, and the recency of past encounters predicted future encounters with a power function. Consistent with the rational analyses carried out for human memory, these findings suggest that chimpanzee memory performance should reflect those environmental regularities. In re-analysing existing chimpanzee memory data, we found that chimpanzee memory patterns mirrored their social contact patterns. Our findings hint that human and chimpanzee memory systems may have evolved to solve similar information-processing problems. Overall, rational analysis offers novel theoretical and methodological avenues for the comparative study of cognition

Reflections of the Social Environment in Chimpanzee Memory: Applying Rational Analysis Beyond Humans

In cognitive science, the rational analysis framework allows modelling of how physical and social environments impose information-processing demands onto cognitive systems. In humans, for example, past social contact among individuals predicts their future contact with linear and power functions. These features of the human environment constrain the optimal way to remember information and probably shape how memory records are retained and retrieved. We offer a primer on how biologists can apply rational analysis to study animal behaviour. Using chimpanzees (Pan troglodytes) as a case study, we modelled 19 years of observational data on their social contact patterns. Much like humans, the frequency of past encounters in chimpanzees linearly predicted future encounters, and the recency of past encounters predicted future encounters with a power function. Consistent with the rational analyses carried out for human memory, these findings suggest that chimpanzee memory performance should reflect those environmental regularities. In re-analysing existing chimpanzee memory data, we found that chimpanzee memory patterns mirrored their social contact patterns. Our findings hint that human and chimpanzee memory systems may have evolved to solve similar information-processing problems. Overall, rational analysis offers novel theoretical and methodological avenues for the comparative study of cognition

Social bonds provide multiple pathways to reproductive success in wild male chimpanzees

In most male mammals, fitness is strongly shaped by competitive access to mates, a non-shareable resource. How, then, did selection favor the evolution of cooperative social bonds? We used behavioral and genetic data on wild chimpanzees (Pan troglodytes schweinfurthii) in Gombe National Park, Tanzania, to study the mechanisms by which male-male social bonds increase reproductive success. Social bonds increased fitness in several ways: first, subordinate males that formed strong bonds with the alpha male had higher siring success. Independently, males with larger networks of strong bonds had higher siring success. In the short term, bonds predicted coalition formation and centrality in the coalition network, suggesting that males benefit from being potential allies to numerous male rivals. In the long term, male ties influenced fitness via improved dominance rank for males that attain alpha status. Together, these results suggest that male bonds evolved in chimpanzees by affording both short- and long-term pathways to reproductive success

Correlates of individual participation in boundary patrols by male chimpanzees

Group territory defense poses a collective action problem: individuals can free-ride, benefiting without paying the costs. Individual heterogeneity has been proposed to solve such problems, as individuals high in reproductive success, rank, fighting ability, or motivation may benefit from defending territories even if others free-ride. To test this hypothesis, we analyzed 30 years of data from chimpanzees (Pan troglodytes) in the Kasekela community, Gombe National Park, Tanzania (1978-2007). We examined the extent to which individual participation in patrols varied according to correlates of reproductive success (mating rate, rank, age), fighting ability (hunting), motivation (scores from personality ratings), costs of defecting (the number of adult males in the community), and gregariousness (sighting frequency). In contrast to expectations from collective action theory, males participated in patrols at consistently high rates (mean ± S.D. = 74.5 ± 11.1% of patrols, n=23 males). The best predictors of patrol participation were sighting frequency, age, and hunting participation. Current and former alpha males did not participate at a higher rate than males that never achieved alpha status. These findings suggest that the temptation to free-ride is low, and that a mutualistic mechanism such as group augmentation may better explain individual participation in group territorial behavior.README File: README_MASSARO_2022_DATA_updated04mar2022.txt R-code for data analysis: CodeforCorrelatesofBoundaryPatrols.R Datasets: PPdata.xlsx, PatrolsandPeriph.xlsx, PP5yearPlots.xlsx, WholeStudyPatrolRate.xlsx We do not provide access to the raw data used in some of these analyses, as this raw data represent a substantial fraction of the long-term data from Gombe, which are not publicly available at this time due to multiple ongoing studies, but are available from the corresponding author on reasonable requestAttached data files include summary data collected at Gombe National Park (1998-2007). For detailed methodology, please see the associated manuscript