Differences across taxa in nuptial gift size correlate with differences in sperm number and ejaculate volume in bushcrickets (Orthoptera: Tettigoniidae)

Abstract

The spermatophores of bushcrickets consist of two parts: an ampulla which contains the sperm and a spermatophylax which the female eats following mating. There are two different, though not mutually exclusive, hypotheses concerning the selective pressures important in the evolutionary enlargement of the spermatophylax in bushcrickets. The paternal investment hypothesis proposes that elaboration of spermatophylax size has proceeded through selection for male nutritional investment in offspring. The ejaculate protection hypothesis, on the other hand, proposes that the evolutionary enlargement of the spermatophylax has proceeded through selection on males to ensure complete ejaculate transfer. The latter hypothesis predicts that evolutionary changes in spermatophylax size should correlate positively with evolutionary changes in ampulla size (i.e. ejaculate volume) and sperm number. Here we present the results of a comparative study designed to test this prediction. Measurements of spermatophylax mass, ampulla mass and male body mass were taken for 43 species of European bushcrickets. Measurements of sperm number were taken for 31 of these species. These data were analysed using the independent comparisons method. As predicted by the ejaculate protection hypothesis, a positive relationship was found, across taxa, between contrasts in spermatophylax mass and contrasts in both ampulla mass and sperm number, controlling for male body weight.