Sex ratio effects on reproductive strategies in humans.

Ryan Schacht,Monique Borgerhoff Mulder

doi:10.1098/rsos.140402

Ryan Schacht, Monique Borgerhoff Mulder

Open Access

https://doi.org/10.1098/rsos.140402

Copy DOI

Abstract

Characterizations of coy females and ardent males are rooted in models of sexual selection that are increasingly outdated. Evolutionary feedbacks can strongly influence the sex roles and subsequent patterns of sex differentiated investment in mating effort, with a key component being the adult sex ratio (ASR). Using data from eight Makushi communities of southern Guyana, characterized by varying ASRs contingent on migration, we show that even within a single ethnic group, male mating effort varies in predictable ways with the ASR. At male-biased sex ratios, men's and women's investment in mating effort are indistinguishable; only when men are in the minority are they more inclined towards short-term, low investment relationships than women. Our results support the behavioural ecological tenet that reproductive strategies are predictable and contingent on varying situational factors.

Highlights

Recent advances in the evolutionary analysis of reproductive strategies suggest that the adult sex ratio (ASR) [1,2,3], together with certainty of parentage [4,5] and intrasexual variability in quality [6] are key to generating predictions of sex-differentiated mating and reproductive behaviour
We focus on investment in mating effort, which we measure using the sociosexual orientation inventory (SOI) [52,53]
Sex = male logASR × sex = male inference, we demonstrate that at male-biased sex ratios, men’s and women’s willingness to engage in uncommitted sex are indistinguishable and that when men are in the minority they are more inclined towards indiscriminate mating than when in the majority

Summary

Introduction

Recent advances in the evolutionary analysis of reproductive strategies suggest that the adult sex ratio (ASR) [1,2,3], together with certainty of parentage [4,5] and intrasexual variability in quality [6] are key to generating predictions of sex-differentiated mating and reproductive behaviour. We use natural variation in ASR among different communities in an indigenous Amerindian population in Guyana to explore how one particular aspect of human reproductive effort, investment in mating (a key aspect of reproductive strategy insofar as effort invested in mating trades off directly with parental effort; [7,8], but see [9]), reflects an environmental gradient in the relative abundance of males and females.

Methods

Results

Conclusion