Spermatophore size variation in the bush-cricket genus Poecilimon : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Palmerston North, New Zealand

Abstract

During mating, male bush-crickets transfer a costly nuptial gift to the female to consume while the ejaculate is transferred into her. The nuptial gift functions primarily as ejaculate protection, although in some larger spermatophore-producing species the gift functions additionally as paternal investment. While costly, production of large spermatophores may increase male fitness by providing a way in which males outcompete conspecific male sperm competition and female control over mating. For females, the nuptial gift may provide nutrients that increase her fecundity or allow greater fitness; however, larger gifts may also reduce a female’s mating optima. A large variation in spermatophore size exists among bush-crickets; traditionally this is attributed to environmental and physiological differences. However, interspecific size variation may also be due to behaviour or common ancestry. Few studies have documented the evolutionary ecology of spermatophore size variation while accounting for environmental variation and relatedness. Controlling for body mass, common ancestry, and diet, my thesis is a study of the variations in spermatophore size of the genus Poecilimon. I investigate aspects of operational sex ratio, reproductive effort, mating effort, paternal investment, ejaculate protection, sperm competition, mate choice, sexual conflict and reproductive fitness. I gathered previously unpublished data and extracted data from the literature to make comparative analyses among 33 Poecilimon taxa. For specific focal comparisons, I further intensively studied five taxa in the field that vary markedly in spermatophore size. First, I observed that variation in Poecilimon spermatophore size is as wide as that of the entire bush-cricket family (Tettigoniidae), and thus can be viewed as the ideal model system for investigating gift size variations across tettigoniids. Furthermore, using a phylogenetically independent contrast analysis I showed that evolutionary history has been of little importance in preventing changes in spermatophore size. I present evidence that both ejaculate protection and paternal investment are behind the evolution of larger spermatophore investments within Poecilimon. However, potential increases in spermatophore size are predicted to be selected against by female opportunities to increase fitness through multiple mating. In contrast, in a small spermatophore-producing species I found female mate choice for young, virgin males that are likely to transfer greater sperm volumes than previously mated males. In this small spermatophore-producing species I found selection for larger spermatophores. Theory predicts further restrictions to nuptial gift production, as a trade-off between alternative reproductive efforts. However, I found increases in paternal assurance enhanced by transferring larger spermatophores may allow for increased selection to advertise expensive gifts; because spermatophore size and investment in mate attraction are coupled, it appears there is no trade-off between these expensive mating efforts. Moreover, I found that spermatophore size within Poecilimon is correlated with a risk-shift in pair-formation protocol between taxa whereby stationary males that call and wait for females to approach are able to produce larger spermatophores than males that approach calling females. Sexual conflict has been predicted to influence spermatophore size variation because dosedependent manipulations of gift size on female polyandry occur in most insects, yet I found large spermatophore-producing Poecilimon taxa to have a larger per mating fitness increase than small spermatophore-producing taxa. Furthermore, I observed no direct cost of spermatophore size on female fitness. In fact, independent of the spermatophore size received per mating, females of different taxa typically receive similar volumes of spermatophore over their lifetime. Spermatophore size variation across Poecilimon reflects predictable within-species adjustments that males make to each spermatophore component in response to environmental constraints, ejaculate protection, paternal investment, and female selection as conditional strategies to maximize reproductive fitness.