Impact of low sperm competition on male reproductive trait allometries in a bush-cricket

Lennart Winkler,Leon M Kirch,Klaus Reinhold,Steven A Ramm

doi:10.1186/s12862-019-1514-0

Abstract

BackgroundStudying reproductive trait allometries can help to understand optimal male investment strategies under sexual selection. In promiscuous mating systems, studies across several taxa suggest that testes allometry is usually positive, presumably due to strong selection on sperm numbers through intense sperm competition. Here, we investigated testes allometry in a bush-cricket species, Metaplastes ornatus, in which females mate promiscuously, but where sperm removal behaviour by males likely drastically reduces realised sperm competition level.ResultsAs hypothesised, we found evidence for negative testes allometry and hence a fundamentally different male investment strategy compared to species under intense sperm competition. In addition, the mean relative testes size of M. ornatus was small compared to other species of bush-crickets. Surprisingly, the spermatophore gland, a potential alternative trait that males could invest in instead of testes, also did not show positive allometry, but was approximately isometric. We further observed the expected pattern of negative allometry for the male morphological structure responsible for sperm removal in this species, the subgenital plate, supporting the one-size-fits-all hypothesis for intromittent genitalia.ConclusionOur findings suggest that the evolution of sperm removal behaviour in M. ornatus was a key adaptation for avoiding sperm competition, with important consequences for reproductive trait allometries. Nevertheless, they also imply that it does not pay for larger males to invest disproportionately in nuptial gift production in this species.

Highlights

Studying reproductive trait allometries can help to understand optimal male investment strategies under sexual selection
Beginning with the testes, we found that the allometric slope of 0.234 was significantly greater than zero (p = 0.0005) (Table 1, Fig. 1a), but significantly smaller than one (p < 0.0001), the slope estimated through Reduced major axis (RMA) was somewhat higher and significantly greater that one
To examine the other major gland responsible for ejaculate production, we investigated allometry of the spermatophore gland

Summary

Introduction

Studying reproductive trait allometries can help to understand optimal male investment strategies under sexual selection. In addition to often mating multiply, female insects, in particular, are often able to store and maintain sperm in specially adapted spermathecae and other reproductive organs, further promoting high levels of sperm competition [5,6,7,8] This has led to a variety of male adaptations and strategies in order to achieve fertilization success, and these can broadly be considered as arising either from selection on mechanisms for pre-emption of stored sperm (from other males) or selection on mechanisms to prevent preemption of (own) stored sperm, i.e. as offensive and defensive sperm competition traits, respectively [1, 5, 9]. Known defensive adaptations include for instance prolonged copulation durations as found in the lovebug Plecia nearctica, which mates for up to 56 h [10]; extensive post-copulatory mate guarding, for example in the firebug Pyrrhocoris apterus [11]; mating plugs that prevent the female from remating like those deposited by male checkerspot butterflies, Euphydryas

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