Male Gammarus roeseli provide smaller ejaculates to females infected with vertically transmitted microsporidian parasites

The effects of parasites on the reproduction of their hosts are widespread, but studies investigating the effect of female parasitic status on sperm allocation in males, a form of postcopulatory mate choice, remain scarce. Because males are often sperm limited, strategic sperm investment, in which females of low reproductive value receive fewer sperm, is predicted to occur to maximize long-term male reproductive success. In this study based on pairs collected in natura, we investigated how Gammarus roeseli (Crustacea: Amphipoda) males allocated sperm when paired with females infected with the vertically transmitted, sex ratio-distorting, microsporidian parasites, Nosema granulosis or Dictyocoela sp. Since infected females had similar fecundity to uninfected ones, and offspring of females infected with N. granulosis showed a higher survival rate, we predicted equivalent or even larger sperm investment from males paired with infected females. Contrary to our predictions, males paired with infected females had a lower sperm count before insemination and provided smaller ejaculates than those paired with uninfected females. This pattern suggests either a strategic sperm investment as a function of the female's parasitic status, or that males in good condition had a higher probability of pairing with uninfected females than those in poor condition.