Immune system activation affects male sexual signal and reproductive potential in crickets

Parasite-mediated sexual selection theory posits that individuals (usually females) choose mates by assessing the expression of costly secondary sexual signals, which provide reliable indications of parasite resistance. If these signals are indeed reliable, then immune-compromised males are predicted to exhibit changes in the sexual signal that are discernable by the female. Moreover, the mating pair is predicted to exhibit some reduction in reproductive fitness if the male is immune compromised. Here, we addressed these predictions in the ground cricket, Allonemobius socius, by injecting juvenile males with lipopolysaccarides, which allowed us to activate the immune system without the introduction of a metabolically active pathogen. As a consequence, we were able to disentangle the cost of immune system activation from the cost of infection. We found that immune activation had a long-term effect on male calling song and the males' ability to provide paternal resources, which can constrain male and female reproductive potential. We also found that song interpulse interval varied significantly with the male's immune treatment and may therefore provide choosy females with a way to avoid mating with immune-compromised males. In short, our data support the parasite-mediated theory of sexual selection, suggesting that female's gain direct benefits by mating with males who are immune competent.