Resource allocation trade-off between sperm quality and immunity in the field cricket, Teleogryllus oceanicus

Mounting an immune response is costly, requiring an animal to draw on its limited nutrient pool at a cost to future growth and reproduction. A trade-off between immunity and reproduction is central to parasite-mediated models of sexual selection. In this study, I examine a prediction of postcopulatory models of parasite-mediated sexual selection that males face a nutrient allocation trade-off between immunity and ejaculate quality. I experimentally induced an antibacterial immune response in juvenile crickets held on either a restricted diet or allowed ad libitum access to food. Immune-challenged crickets took longer to reach adult eclosion, and crickets on a restricted diet had a greatly reduced ability to encapsulate a foreign body. Neither juvenile immune challenge nor diet influenced adult antibacterial activity. Crickets with restricted access to food had reduced sperm viability after upregulation of their antibacterial immunity when juvenile, whereas crickets fed ad libitum did not suffer a cost of reduced sperm viability. This finding provides evidence for a nutrient allocation trade-off between antibacterial immunity and ejaculate quality. However, the genetic correlation between these traits is negative, rendering any indirect genetic benefits from sperm competition unlikely.