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
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Date
2010
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Massey University
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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 I gathered previously unpublished data and extracted data from the literature to make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.
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Keywords
Poecilimon, Spermatophore size variation, Tettigoniidae, Nuptial gifts, Bush-crickets, Mating