Mechanisms of equality and inequality in mammalian societies

Social structure and the evolutionary ecology of inequality

From rising disparities in income to limited socio-political representation for minority groups, inequality is a topic of perennial interest for contemporary society. Research in the evolutionary sciences has started to investigate how social structure allows inequality to evolve, but is developing in silo from existing work in the social and cognitive sciences. I synthesise these literatures to present a theoretical framework of how and why cultural and ecological conditions can create social structure that either produces or constrains inequality. According to this framework, such conditions dictate the costs and benefits of cooperation that shape individuals' social preferences and resulting behaviours. These behaviours aggregate to produce distinct structures of a society's social networks, which generate different levels of inequality observed across societies.

Competitive social feedback amplifies the role of early life contingency in male mice

Contingency (or "luck") in early life plays an important role in shaping individuals' development. By comparing the developmental trajectories of functionally genetically identical free-living mice who either experienced high levels of resource competition (males) or did not (females), we show that competition magnifies early contingency. Male resource competition results in a feedback loop that magnifies the importance of early contingency and pushes individuals onto divergent, self-reinforcing life trajectories, while the same process appears absent in females. Our results indicate that the strength of sexual selection may be self-limiting, and they highlight the potential for contingency to lead to differences in life outcomes, even in the absence of any underlying differences in ability ("merit").

Vole hunting: novel predatory and carnivorous behavior by California ground squirrels

Dietary flexibility allows animals to respond adaptively to food pulses in the environment. Here we document the novel emergence of widespread hunting of California voles and carnivorous feeding behavior by California ground squirrels. Over two months in the twelfth year of a long-term study on the squirrel population, we document 74 events of juvenile and adult ground squirrels of both sexes depredating, consuming, and/or competing over vole prey. Our video footage, photographic evidence, and direct observations of marked individual squirrels provide insights into the ecological circumstances favoring behavioral flexibility in foraging associated with a decadal peak in vole abundance. Digital video images related to the article are available at http://www.momo-p.com/showdetail-e.php?movieid=momo241126ob01a.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10164-024-00832-6.

Selective social tolerance drives differentiated relationships among wild female chimpanzees

Strong, affiliative bonds often function to facilitate social competition through cooperative defence of resources, but the benefits of social bonds may be low when direct competition is less intense or less beneficial. In such cases, one possible outcome is that relationships are weak and undifferentiated. Alternatively, negotiating stable, selectively tolerant relationships may be a strategy to mitigate the costs and risks of sharing space when direct competition is undesirable. We investigated dyadic social tolerance among wild adult female chimpanzees, who engage in low rates of affiliation and aggression amongst one another. While females associate with one another at different rates, these patterns could reflect shared patterns of behaviour (e.g., ranging) rather than social preference or variation in relationship quality. We first determined whether patterns of dyadic spatial association (five-meter proximity) were differentiated and stable over time. To assess whether dyadic spatial association reflected preference and variation in social tolerance, we tested whether spatial association was actively maintained by waiting and following behaviour, and associated with decreased aggression and increased cofeeding. Spatial associations were differentiated, and stronger associations were more stable. Frequent associates used following and waiting behaviour to actively maintain associations. Association positively predicted time cofeeding and negatively predicted aggression. These patterns were true among related and unrelated dyads. Among unrelated females, dyads with stronger associations maintained proximity more mutually. This study highlights social tolerance as a stable relationship attribute that can predict and explain patterns of behaviour and social network structure, distinct from, or in the absence of, affiliation.

Sex-specific competitive social feedback amplifies the role of early life contingency in male mice

Contingency (or 'luck') in early life plays an important role in shaping individuals' development. When individuals live within larger societies, social experiences may cause the importance of early contingencies to be magnified or dampened. Here we test the hypothesis that competition magnifies the importance of early contingency in a sex-specific manner by comparing the developmental trajectories of genetically identical, free-living mice who either experienced high levels of territorial competition (males) or did not (females). We show that male territoriality results in a competitive feedback loop that magnifies the importance of early contingency and pushes individuals onto divergent, self-reinforcing life trajectories, while the same process appears absent in females. Our results indicate that the strength of sexual selection may be self-limiting, as within-sex competition increases the importance of early life contingency, thereby reducing the ability of selection to lead to evolution. They also demonstrate the potential for contingency to lead to dramatic differences in life outcomes, even in the absence of any underlying differences in ability ('merit').

Toward an evolutionary ecology of (in)equality

Inequality is increasingly recognized as a major problem in contemporary society. The causes and consequences of inequality in wealth and power have long been central concerns in the social sciences, whereas comparable research in biology has focused on dominance and reproductive skew. This theme issue builds on these existing research traditions, exploring ways they might enrich each other, with evolutionary ecology as a possibly unifying framework. Contributors investigate ways in which inequality is resisted or avoided and developed or imposed in societies of past and contemporary humans, as well as a variety of social mammals. Particular attention is paid to systematic, socially driven inequality in wealth (defined broadly) and the effects this has on differential power, health, survival and reproduction. Analyses include field studies, simulations, archaeological and ethnographic case studies, and analytical models. The results reveal similarities and divergences between human and non-human patterns in wealth, power and social dynamics. We draw on these insights to present a unifying conceptual framework for analysing the evolutionary ecology of (in)equality, with the hope of both understanding the past and improving our collective future. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Identifying key socioecological factors influencing the expression of egalitarianism and inequality among foragers

Understanding how resource characteristics influence variability in social and material inequality among foraging populations is a prominent area of research. However, obtaining cross-comparative data from which to evaluate theoretically informed resource characteristic factors has proved difficult, particularly for investigating interactions of characteristics. Therefore, we develop an agent-based model to evaluate how five key characteristics of primary resources (predictability, heterogeneity, abundance, economy of scale and monopolizability) structure pay-offs and explore how they interact to favour both egalitarianism and inequality. Using iterated simulations from 243 unique combinations of resource characteristics analysed with an ensemble machine-learning approach, we find the predictability and heterogeneity of key resources have the greatest influence on selection for egalitarian and nonegalitarian outcomes. These results help explain the prevalence of egalitarianism among foraging populations, as many groups probably relied on resources that were both relatively less predictable and more homogeneously distributed. The results also help explain rare forager inequality, as comparison with ethnographic and archaeological examples suggests the instances of inequality track strongly with reliance on resources that were predictable and heterogeneously distributed. Future work quantifying comparable measures of these two variables, in particular, may be able to identify additional instances of forager inequality. This article is part of the theme issue 'Evolutionary ecology of inequality'.