Evolution of mate guarding intensity in crickets as a possible adaptation to sexual conflict over sperm transfer

Mate guarding is a common behavioral adaptation in males to ensure mating or to hinder current partners from remating with rival males. In the decorated cricket, Gryllodes sigillatus, males transfer a spermatophore to females at mating, comprising a sperm-containing ampulla enveloped in a large gelatinous mass, the spermatophylax, that serves as a nuptial food gift for the female. Males guard the female while she feeds on the nuptial gift and while sperm are evacuated from the ampulla into her reproductive tract. Mate guarding behavior functions to prevent competitors from mating with the female but may also represent a behavioral counter-adaptation to the propensity of females to cease nuptial feeding and to terminate sperm transfer prematurely. Using an experimental evolution approach, we tested the prediction that males would guard females more vigorously in response to an experimentally intensified sexual conflict. We recorded the intensity of male mate guarding from replicate lines reared for 25 generations at either an elevated or a reduced intensity of sexual conflict by imposing male- and female-biased adult sex ratios each generation, respectively. We observed the evolution of higher intensity mate guarding behavior in one of the four male-biased selection lines, but in none of the female-biased lines; previous work has revealed that the males in this specific line also produce less manipulative food gifts, suggesting the existence of alternative mating strategies. Additionally, we found a positive correlation between the behavioral investment in mate guarding and the duration of the sperm transfer from the ampulla. As such, this provides a possible mechanism through which more intense mate guarding could increase male fitness.