Sperm Precedence and the Potential of the Nuptial Gift to Function as Paternal Investment in the Tettigoniid Steropleurus stali Bolivar (Orthoptera: Tettigoniidae: Epphippigerinae)

Abstract

The spermatophores of the majority of species of tettigoniid consist of two parts: an ampulla which contains the sperm and a spermatophylax which the female eats following the end of copulation. One hypothesis for the function of the spermatophylax is that it is maintained by selection for paternal investment. In order for this to be possible, the donating male must fertilize a significant proportion of the eggs which stand to benefit from his spermatophylax nutrients. In order to determine whether or not this was the case, the pattern of sperm precedence was examined in the tettigoniid Steropleurus stali, using the sterile-male technique. The duration of the female's refractory period, the number of eggs laid in this period and the number of mature eggs stored by females were also recorded. The results indicated pronounced last-male sperm precedence (median P 2 = 0.95). Taking into account the large number of mature eggs typically stored by females of this species, together with the mean number of eggs laid in the female's refractory period, it appears that females are likely to have re-mated before laying eggs which could contain nutrients donated by her previous mate. This means that one male's spermatophylax nutrients are likely to be incorporated into eggs which will be fertilized by the sperm of males that subsequently mate with the female. The very large spermatophylax of S. stali is therefore unlikely to be maintained by selection for paternal investment.