Mechanisms of equality and inequality in mammalian societies

Abstract

The extent of (in)equality is highly diverse across species of social mammals, but we have a poor understanding of the factors that produce or inhibit equitable social organizations. Here, we adopt a comparative evolutionary perspective to test whether the evolution of social dominance hierarchies, a measure of social inequality in animals, exhibits phylogenetic conservatism and whether interspecific variation in these traits can be explained by sex, age or captivity. We find that hierarchy steepness and directional consistency evolve rapidly without any apparent constraint from evolutionary history. Given this extraordinary variability, we next consider multiple factors that have evolved to mitigate social inequality. Social networks, coalitionary support and knowledge transfer advantage to privilege some individuals over others. Nutritional access and prenatal stressors can impact the development of offspring, generating health disparities with intergenerational consequences. Intergenerational transfer of material resources (e.g. stone tools, food stashes, territories) advantage those who receive. Nonetheless, many of the same social species that experience unequal access to food (survival) and mates (reproduction) engage in levelling mechanisms such as food sharing, adoption, revolutionary coalitions, forgiveness and inequity aversion. Taken together, mammals rely upon a suite of mechanisms of (in)equality to balance the costs and benefits of group living. This article is part of the theme issue ‘Evolutionary ecology of inequality’.

Toward an evolutionary ecology of (in)equality

“…nepotistic inheritance) to reinforce intergenerational legacies of inequality [43]. Matrilineal inheritance structures profoundly influence resource access, survival and reproduction in non-human animals [7], but matrilineal human societies, such as that of Mosuo, possess striking similarities in how power and access is transferred among maternal lines [27]. Like humans, other mammals also possess countering mechanisms such as inequity aversion, peacekeeping, forgiveness and sharing food with non-kin [44][45][46][47][48] to reduce inequality [7].…”

“…Matrilineal inheritance structures profoundly influence resource access, survival and reproduction in non-human animals [7], but matrilineal human societies, such as that of Mosuo, possess striking similarities in how power and access is transferred among maternal lines [27]. Like humans, other mammals also possess countering mechanisms such as inequity aversion, peacekeeping, forgiveness and sharing food with non-kin [44][45][46][47][48] to reduce inequality [7]. Moreover, variation in dominance structures across mammals exhibits minimal phylogenetic constraints, revealing greater flexibility in this social trait than previously assumed [7].…”

“…Both theoretical and empirical work implicates income inequality (measured by the Gini index) as fostering low levels of trust and high levels of violence and mortality, even holding average income constant in humans [75][76][77][78]. In non-human animals, biologists often measure inequality in terms of hierarchy strength, which influences an individual's priority of access to resources that contribute to variation in reproductive success and survival [5][6][7]. These measures allow for comparisons among societal structures to help identify which ecological conditions and historical factors contribute to more or less equal societies.…”

“…B 378: 20220287 structured variation we define as inequality [89]. The varied forms and dynamics this can take are amply covered in various papers in this issue [7,11,13,20,21,31,32].…”

“…We define wealth as attributes or possessions that contribute to well-being or fitness. Forms of wealth can be material (resources, such as food or territory), relational (social networks) or embodied (knowledge, skill) [4][5][6][7]. Note that in this view, power or social influence is viewed as a consequence of underlying wealth inequalities, although more power can also contribute to subsequent wealth accumulation.…”