Reproductive inequality in humans and other mammals

Abstract

To address claims of human exceptionalism, we determine where humans fit within the greater mammalian distribution of reproductive inequality. We show that humans exhibit lower reproductive skew (i.e., inequality in the number of surviving offspring) among males and smaller sex differences in reproductive skew than most other mammals, while nevertheless falling within the mammalian range. Additionally, female reproductive skew is higher in polygynous human populations than in polygynous nonhumans mammals on average. This patterning of skew can be attributed in part to the prevalence of monogamy in humans compared to the predominance of polygyny in nonhuman mammals, to the limited degree of polygyny in the human societies that practice it, and to the importance of unequally held rival resources to women’s fitness. The muted reproductive inequality observed in humans appears to be linked to several unusual characteristics of our species—including high levels of cooperation among males, high dependence on unequally held rival resources, complementarities between maternal and paternal investment, as well as social and legal institutions that enforce monogamous norms.

Explaining variation in the kinship composition of mammal groups

“…Here, I am concerned with kinship among offspring born into a group and, as such, the number of juveniles per adult ( θ ), male dispersal, and female dispersal are disregarded even though these factors will play a role in determining kinship composition among adults (e.g., female dispersal and male philopatry will be expected to result in higher relatedness among adult male groupmates than among adult female groupmates ( Rousset 2004 ; Johnstone and Cant 2008 ; Dyble and Clutton-Brock 2020 )). Since my intention is to consider variation in kinship composition across mammals in general rather than one mammalian taxa in particular, I consider broad parameter ranges ( Table 1 ) and include, for example, values for the parameters determining female skew ( β and τ f ) that incorporate both the high female skew seen among cooperative breeders and the low female skew seen among plural breeding species including most primates ( Hager and Jones 2009 ; Ross et al 2023 ).…”

Mechanisms of equality and inequality in mammalian societies

“…Social dominance can have important consequences for animals, contributing to unequal access to resources [35] and-in some cases-reproductive inequality (skew) [36][37][38], which varies within and among species (e.g. [39,40]). Because the diversity and evolution of reproductive inequality across humans and other mammals has recently been characterized elsewhere [40] and reproductive skew is discussed extensively in other contributions in this special issue [16,19,41], the first section of this paper focuses on social dominance as a measure of inequality.…”

“…[39,40]). Because the diversity and evolution of reproductive inequality across humans and other mammals has recently been characterized elsewhere [40] and reproductive skew is discussed extensively in other contributions in this special issue [16,19,41], the first section of this paper focuses on social dominance as a measure of inequality. Social dominance reflects how individuals are organized into a dominance hierarchy based on the outcomes of pairwise agonistic interactions (e.g.…”

Toward an evolutionary ecology of (in)equality

“…Although human populations do certainly exhibit reproductive skew [50], extreme forms of reproductive suppression and altruism such as in mole rats [20] and social insects [51] have little human counterpart. One key difference in the human case is attributed to enhanced paternity certainty and resultant paternal investment, resulting in a major expansion of kinship ties and the option of patrilineal as well as matrilineal networks and inheritance pathways [27,28,52,53].…”