In many animal systems, females exhibit a localized immune response to insemination that helps defend against sexually transmitted disease. However, this response may also kill sperm, reducing a male's reproductive potential. If males could suppress this response, they may be able to increase their sperm's representation in the female's reproductive tract, thereby increasing their fitness. Here we address the hypothesis that, under conditions of sperm competition, males interfere with female immunity. To test our hypothesis, we manipulated levels of female mating frequency (single vs. multiply mated) and seminal diversity (monandrous vs. polyandrous) in the cricket, Allonemobius socius and measured female immune response. As mating frequency increased, female hemocyte load decreased, indicating a general reproductive cost. As seminal diversity increased, phenoloxidase (PO) activity (in vitro measure of 'potential' macroparasitic defense) increased and encapsulation ability (in vivo measure of 'realized' macroparasitic defense) decreased in polyandrous females. These results suggest that males may manipulate female immunity by interrupting the pro-PO cascade, which begins with the activation of PO and ends in the encapsulation of invading foreign bodies. In other words, female immune function may serve as a battleground over which a sexual conflict is fought.
- Sperm competition