Bacterial infection and mate choice in a wolf spider
Description
Infection and parasitism are serious risks that can influence condition-indicating ornamental male traits and subsequently inform female mate choice. This paper summarizes recent experiments investigating the relationship between infection stress, male courtship, female choice and parasite avoidance behaviors. We studied the Brush-legged Wolf Spider, Schizocosa ocreata, whose mating behavior, consisting of complex male courtship and female mate choice, highlights complex evolutionary conflict between the sexes at adulthood. Male and female S. ocreata were experimentally infected with the bacterial pathogen Pseudomonas aeruginosa and placed together in triad mating trials wherein one female—either infected or not (control) - was paired with two males—one infected and one control (uninfected). Results showed that control (uninfected) females chose to mate significantly more often with control (uninfected) males than infected males. Infected females, on the other hand, chose infected and control males with equal frequency, suggesting a potential cost of being infected for females—reduced infection avoidance. Male courtship display rates (“bounces”) were higher overall in the group of control (uninfected) males. Furthermore, control males presented with an infected focal female displayed significantly reduced rates of courtship when compared to a control (uninfected) focal female. These results confirm that infection stress has observable impacts on mating success and behavior, but also suggests that the costs of infection may be more subtle than expected and present themselves in ways that are not as dramatic as might be predicted.
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Experimental infection: Bacteria stocks were kept in Copan Cryovials at -80C and grown on Luria broth media (1.0% Tryptone, 0.5% yeast extract, 1.0%NaCl, 1.5%agar). All plates containing bacteria were cultured prior to treatment and suspended in a sterile water solution with the concentration of 600 Colony forming units of bacteria per microliter determined by McFarland turbidity standards. Treatment occurred two-weeks post-maturity; individuals chosen for study were deprived of water for 24 hours to induce thirst. Then, they were presented with a 1 microliter droplet of either sterile water (control group) or water containing 600 Colony Forming Units (CFUs) of bacteria (infected group). Approximately 24-hours after treatment, each female (either infected or uninfected/control) was paired with two males: one infected and one control. Individuals were randomly chosen for triad pairings, while ensuring approximately equivalent size of males to avoid any size bias. The mating arena was constructed using a 20cm diameter cylinder with a circular piece of filter paper lining the bottom as in previous studies (Meyer and Uetz 2019). Females were allowed to explore the arena for 5-10 minutes and lay silk prior to trial start. When trials began, the female was corralled under a vial and in quick succession the two males were introduced within view of the female. Once both males began courtship displays or 3 minutes passed, the female was released from her corral and able to interact directly with both males and allowed to interact for up to 1-hour. Trials ended when successful mating occurred or after 1-hour of interaction, allowing for variable trial lengths if mating was the outcome. Videos of mating trials were scored for behaviors of both the male and female subject in each trial. Each male was scored independently for courtship behaviors (latency to court, number of jerky taps and leg waves), and aggressive behaviors (lunge, grab/grapple, forced mount). Initial analyses of mating outcome were performed on a set of pooled data including trials completed across multiple collection seasons (Fall 2018, Spring 2019, Fall 2020). Behavioral analyses were completed on the Fall 2020 season subset only. Analysis aimed to examine differences between the number and duration of courtship displays of infected and control males in each trial. Behaviors were not normally distributed so nonparametric Wilcoxon rank sums were used to compare across male infection status and Kruskal Wallis tests were used for comparisons across focal female infection status.