Paternity of offspring in multiply–mated, female crickets: the effect of nuptial food gifts and the advantage of mating first

Abstract

The spermatophore transferred by male decorated crickets ( Gryllodes sigillatus ) includes a large gelatinous mass, the spermatophylax, that is consumed by the female after mating. This nuptial gift preoccupies the female while sperm are discharged from the remaining portion of the spermatophore, the sperm ampulla, into her reproductive tract. There is considerable variation in the mass of the spermatophylax, and about half of all males produce spermatophylaxes that are too small to ensure complete sperm transfer. We tested two hypotheses concerning the maintenance of this variation: (i) males trade–off investment in spermatophylaxes against copulation frequency; and (ii) males synthesize the largest spermatophylaxes of which they are physiologically capable. Males synthesizing large and small food gifts were permitted multiple mating opportunities with the same females, and allozyme markers were used to establish the paternity of offspring. There was a significant advantage to those males that mated first irrespective of gift size. This advantage probably arose, in part, because the sperm of first males would have had exclusive access to female9s eggs during the first 24 hours of oviposition, and underscores the benefits of matings with virgin females. The paternity of ‘small–gift’ males increased with gift mass, but there was no such increase in ‘large–gift’ males. This difference probably stems from the relationship between gift mass and sperm transfer: most of the gifts of the large–gift males would have been above the threshold needed to achieve complete inseminations, whereas those of small–gift males would have been below the threshold. Within mating–order positions, there was no significant difference in the paternity of large–gift and small–gift males, a result seemingly consistent with the ‘trade–off’ hypothesis. However, there was no correlation between spermatophylax mass and male mating frequency, so that the mechanism by which small–gift males offset their fertilization disadvantage remains unknown.