Female Choice or Male Sex Drive? The Advantages of Male Body Size during Mating in Drosophila Melanogaster.

Santosh Jagadeeshan,Debarti Chakrabarti,Rama S Singh,Ushma Shah

doi:10.1371/journal.pone.0144672

Abstract

The mating success of larger male Drosophila melanogaster in the laboratory and the wild has been traditionally been explained by female choice, even though the reasons are generally hard to reconcile. Female choice can explain this success by virtue of females taking less time to mate with preferred males, but so can the more aggressive or persistent courtships efforts of large males. Since mating is a negotiation between the two sexes, the behaviors of both are likely to interact and influence mating outcomes. Using a series of assays, we explored these negotiations by testing for the relative influence of male behaviors and its effect on influencing female courtship arousal threshold, which is the time taken for females to accept copulation. Our results show that large males indeed have higher copulation success compared to smaller males. Competition between two males or an increasing number of males had no influence on female sexual arousal threshold;—females therefore may have a relatively fixed ‘arousal threshold’ that must be reached before they are ready to mate, and larger males appear to be able to manipulate this threshold sooner. On the other hand, the females’ physiological and behavioral state drastically influences mating; once females have crossed the courtship arousal threshold they take less time to mate and mate indiscriminately with large and small males. Mating quicker with larger males may be misconstrued to be due to female choice; our results suggest that the mating advantage of larger males may be more a result of heightened male activity and relatively less of female choice. Body size per se may not be a trait under selection by female choice, but size likely amplifies male activity and signal outputs in courtship, allowing them to influence female arousal threshold faster.

Highlights

Acts of mating involve interactions between two individuals, and as such, all actions associated with mating may not necessarily fulfill the dictates of natural selection, i.e. that all acts be of positive effects and good to both partners
Whereas much of the sexual selection literature has dichotomized the two processes, in reality, male-male competition and female choice may best represent the extremes of a continuum of consequences involving negotiations where male-male competition and female choice interact in various ways [3,4,5,6]
We found no evidence to support that the presence of a second male of any size behind a barrier (L|L: F, S|S: F, S|L: F, L|S: F) had an impact on courtship arousal threshold (CAT); i.e., CAT remained the same as in the first experiment–larger males took less time to court and copulate with females (CAT = = L > S, p < 0.05), (Fig 1B, S2 Table)

Summary

Introduction

Acts of mating involve interactions between two individuals, and as such, all actions associated with mating may not necessarily fulfill the dictates of natural selection, i.e. that all acts be of positive effects and good to both partners. It is worth noting that much of Darwin’s explanations were largely based on the roles that males played in order to succeed in their efforts for the possession of females, and how females may respond to these efforts [1,2] Male behaviors drew his attention to characterize two mechanisms by which males win in the struggle for reproduction: one, where the winners of malemale combat gain access to mate with females; and the other where male-male competition occurs (through a variety of displays or song) to gain the attention of the female, and females may ‘choose’ their mates (pp 88–91 in [1]). We term the abilities of males related to courtship and mating, i.e., their aggressive and/or persistent efforts to secure matings either by ‘charm’ and/or ‘coercion’, as male sex drive This would be equivalent of what Darwin called ‘eagerness’ of males in order to describe the persistent efforts and strategies used by males to succeed in mating [2]

Objectives

Methods

Results