Leaders of war: modelling the evolution of conflict among heterogeneous groups.

War, in human and animal societies, can be extremely costly but can also offer significant benefits to the victorious group. We might expect groups to go into battle when the potential benefits of victory (V) outweigh the costs of escalated conflict (C); however, V and C are unlikely to be distributed evenly in heterogeneous groups. For example, some leaders who make the decision to go to war may monopolize the benefits at little cost to themselves ('exploitative' leaders). By contrast, other leaders may willingly pay increased costs, above and beyond their share of V ('heroic' leaders). We investigated conflict initiation and conflict participation in an ecological model where single-leader-multiple-follower groups came into conflict over natural resources. We found that small group size, low migration rate and frequent interaction between groups increased intergroup competition and the evolution of 'exploitative' leadership, while converse patterns favoured increased intragroup competition and the emergence of 'heroic' leaders. We also found evidence of an alternative leader/follower 'shared effort' outcome. Parameters that favoured high contributing 'heroic' leaders, and low contributing followers, facilitated transitions to more peaceful outcomes. We outline and discuss the key testable predictions of our model for empiricists studying intergroup conflict in humans and animals. This article is part of the theme issue 'Intergroup conflict across taxa'.

Vaccination and altruism under the COVID-19 pandemic.

OBJECTIVES: The goal of this study is to examine the role of altruism and sensitivity to public shame in individuals' willingness to pay for a COVID-19 vaccine. STUDY DESIGN: We apply expected utility theory to predict the role of individuals' altruism and public shame in the willingness to pay for vaccines. Subsequently, we test the prediction by using a unique cross-sectional survey. METHODS: We use online survey data collected from those aged 30-49 in Japan between June 18th and 25th, 2020 (n = 1686). The selection of respondents follows quota sampling with regard to age group, gender, and prefecture of residence. We employ an ordinary least square (OLS) model to regress respondents' willingness to pay for a hypothetically-effective vaccine for COVID-19 on binary indicators of altruism and sensitivity to public shame, as well as socio-demographic characteristics. RESULTS: The willingness to pay for the vaccine is higher among those with stronger altruistic concerns and sensitivity to shaming. CONCLUSION: Voluntary vaccinations may be inefficient, because the uptake of vaccines could be low for selfish individuals who often violate social distancing requirements. To improve the efficiency of vaccine uptake, some interventions, such as nudges and a vaccine passport, may be needed.

Social distancing as a public good under the COVID-19 pandemic.

OBJECTIVES: The purpose of this study is to show that social distancing is a public good under the COVID-19 pandemic. STUDY DESIGN: We apply economic theory to analyse a cross-sectional survey. METHODS: Economic theory is complemented with empirical evidence. An online survey of those aged 30-49 years in Japan (n = 2177) was conducted between April 28 and May 7. Respondents were selected by quota sampling with regard to age group, gender and prefecture of residence. Our main figure shows the proportion of people who increased/did not change/decreased social distancing, relative to the level of altruism and sensitivity to public shaming. The results of OLS and logit models are shown in Supplementary Materials. RESULTS: Social distancing is a public good under the COVID-19 pandemic for which the free-rider problem is particularly severe. Altruism and social norms are crucial factors in overcoming this problem. Using an original survey, we show that people with higher altruistic concerns and sensitivity to shaming are more likely to follow social distancing measures. CONCLUSIONS: Altruism and social norms are important for reducing the economic cost of the pandemic.

Intergroup encounters in Verreaux's sifakas (Propithecus verreauxi): who fights and why?

ABSTRACT: Individuals living in groups have to achieve collective action for successful territorial defense. Because conflicts between neighboring groups always involve risks and costs, individuals must base their decision to participate in a given conflict on an evaluation of the trade-off between potential costs and benefits. Since group members may differ in motivation to engage in group encounters, they exhibit different levels of participation in conflicts. In this study, we investigated factors influencing participation in intergroup encounters in Verreaux's sifakas (Propithecus verreauxi), a group-living primate from Madagascar. Over a period of 12 months, we studied eight adjacent sifaka groups in Kirindy Forest. We observed 71 encounters between known neighboring groups in which adult females and males participated equally as often. No individual participated in every encounter, and non-participation occurred more often in larger groups. Females participated less often in encounters when they had dependent infants, presumably to reduce the risk of infanticide. Male participation was influenced by social status: dominant males participated in most encounters, whereas males with fewer opportunities to reproduce participated less often, hence male participation is influenced by the incentive of maintaining access to females. The number of actively participating individuals in the opponent group positively influenced the participation in both sexes. Thus, sifakas seem to decide joining a given encounter opportunistically, most likely based on a combination of individual incentives and the actual circumstance of each encounter, suggesting that the complexity in intergroup relationships appears to be the product of decisions made by each individual group member. SIGNIFICANCE STATEMENT: Cooperation among group-living animals is often challenged by collective action problems resulting from individual differences in interests in contributing to collective behaviors. Intergroup encounters involve distinguished costs and benefits for each individual despite being in the same social group. Therefore, encounters between groups offer a good opportunity to investigate individual participation in collective action. In this study, we investigate the influence of different incentives on individual participation in intergroup encounters in wild Malagasy primate, Verreaux's sifakas. We propose a novel approach that takes into account the variable circumstances of each conflict, such as the number of individuals fighting in both groups as a predictor for participation. We believe that our study not only provides novel data on wild sifakas, but it also offers new perspectives for the interpretation of intergroup relationships in other taxa.

Males matter: Increased home range size is associated with the number of resident males after controlling for ecological factors in wild Assamese macaques.

OBJECTIVES: There is increasing evidence of male resource defense during intergroup encounters in non-human primates. Only few studies showed a reproductive benefit of having more males in a group, and evidence only comes from territorial species, or from species with relatively small male group sizes where males are less prone to suffer from collective action problems. We investigated the effect of male group size on home range size and female reproductive success in a non-territorial species with male dispersal and large male group sizes. METHODS: We studied one wild group of Assamese macaques (Macaca assamensis) by following them almost daily (June 2006-September 2012) and collected spatial, behavioral, climate and spatiotemporal data on food plants. RESULTS: Among ecological factors, decreasing rainfall and a statistical interaction between food abundance and distribution were positively related to home range size. After controlling for ecological predictors, we found that male group size but not overall group size had a significant positive effect on full and core home range size. A simple correlation analysis suggests that such an increase in home range area, presumably increasing access to food resources, can be associated with increased female fecundity measured as the proportion or the number of females conceiving in a given year. DISCUSSION: Within the limitations of this study, the results suggest that male resource defense could be a strategy benefitting both sexes if male reproductive skew was low and many males benefited from increased female fertility.

Communal range defence in primates as a public goods dilemma.

Classic socio-ecological theory holds that the occurrence of aggressive range defence is primarily driven by ecological incentives, most notably by the economic defendability of an area or the resources it contains. While this ecological cost-benefit framework has great explanatory power in solitary or pair-living species, comparative work on group-living primates has always found economic defendability to be a necessary, but not sufficient condition to account for the distribution of effective range defence across the taxon. This mismatch between theory and observation has recently been ascribed to a collective action problem among group members in, what is more informatively viewed as, a public goods dilemma: mounting effective defence of a communal range against intrusions by outgroup conspecifics. We here further develop this framework, and report on analyses at three levels of biological organization: across species, across populations within a single lineage and across groups and individuals within a single population. We find that communal range defence in primates very rarely involves collective action sensu stricto and that it is best interpreted as the outcome of opportunistic and strategic individual-level decisions. Whether the public good of a defended communal range is produced by solitary, joint or collective action is thus the outcome of the interplay between the unique characteristics of each individual, local and current socio-ecological conditions, and fundamental life-history traits of the species.