Simultaneous failure of two sex-allocation invariants: implications for sex-ratio variation within and between populations.

António M M Rodrigues,Andy Gardner

doi:10.1098/rspb.2015.0570

Abstract

Local mate competition (LMC) occurs when male relatives compete for mating opportunities, and this may favour the evolution of female-biased sex allocation. LMC theory is among the most well developed and empirically supported topics in behavioural ecology, clarifies links between kin selection, group selection and game theory, and provides among the best quantitative evidence for Darwinian adaptation in the natural world. Two striking invariants arise from this body of work: the number of sons produced by each female is independent of both female fecundity and also the rate of female dispersal. Both of these invariants have stimulated a great deal of theoretical and empirical research. Here, we show that both of these invariants break down when variation in female fecundity and limited female dispersal are considered in conjunction. Specifically, limited dispersal of females following mating leads to local resource competition (LRC) between female relatives for breeding opportunities, and the daughters of high-fecundity mothers experience such LRC more strongly than do those of low-fecundity mothers. Accordingly, high-fecundity mothers are favoured to invest relatively more in sons, while low-fecundity mothers are favoured to invest relatively more in daughters, and the overall sex ratio of the population sex ratio becomes more female biased as a result.

Highlights

Local mate competition (LMC) occurs when male relatives compete for mating opportunities, and this may favour the evolution of female-biased sex allocation [1]
We develop a kin-selection model of sex allocation in the context of a population that is both class-structured—in terms of individuals being separated into males versus females, and breeding females being separated into high- and low-fecundity mothers—and genetically structured—as a consequence of limited dispersal of females between patches from generation to generation
Employing a neighbour-modulated fitness approach to kin selection [46 –52], we find that the condition for natural selection to favour an increase in the sex allocation of high-fecundity mothers is cm 1 nm rS

Summary

Introduction

Local mate competition (LMC) occurs when male relatives compete for mating opportunities, and this may favour the evolution of female-biased sex allocation [1]. The third term in each condition is the LMC effect, which arises as a consequence of greater investment into sons leading to more competition among related males for fewer mating opportunities with females, to the extent that mating occurs among relatives [2].

Results

Conclusion