Infection stress and mate assessment in wolf-spiders: female mate choice varies with sensory mode of male courtship
Description
Fighting infections is energetically costly and can result in resource trade-offs with other life history processes, including reproduction. Studies investigating parasite-mediated sexual-selection have sometimes overlooked the effect of pathogen-induced effects on female choosiness, focusing on costs to the more obvious male secondary sex traits, which include courtship behavior, and subsequent responses of healthy females to infected males. This experimental study examined the trade-offs between infection stress and female mate choice in a well-studied wolf spider. Females are choosey, opting for males displaying condition-dependent traits indicating high mate quality (larger/ more symmetrical tufts of foreleg bristles; production of higher peak amplitude vibratory signals). Female S. ocreata were tested to determine whether recent infection by Pseudomonas aeruginosa, a common bacterium in their habitat, impacts choosiness when presented with video and/or vibratory playback of courting male stimuli with manipulated leg tuft size and vibration amplitude. Three sensory modes were tested (visual, vibratory, or both-multimodal) and three treatment groups were utilized, one infected group and two controls— testing for effects of sham control treatment. The additional unmanipulated control group was later excluded once it was determined that the sham control treatment did not affect behavior. Females were presented with a choice between video or vibration playback of courtship signals of high- and low-quality males (large vs. small foreleg tufts; high vs low peak amplitude of vibratory signals). Female behavior was scored using the number of receptivity displays and time spent visiting either of the two screens within the 5-minute trials. Results show that female receptivity behavior varied based on sensory mode of male courtship and infection treatment. In the visual sensory mode, control females showed significantly greater choosiness than their infected counterparts. These results suggest that infection stress may be impacting the visual sensory pathway more than vibration perception.
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Spiders were collected in the field at the Cincinnati Nature Center, Rowe Woods facility (Clermont County, OH, USA) in Spring 2021. Spiders were raised to maturity under controlled lab conditions (13:11 light:dark cycle, stable temperature 24±1°C, and humidity >50%) in individual opaque plastic Solo deli containers (9 cm diam., 5 cm ht.), fed two 1/8-inch crickets (Acheta domesticus) twice per week and given water ad libitum. Females were selected for trials at times of peak receptivity (between two and three weeks post final molt – Uetz & Norton 2007) and randomly assigned a treatment group: infected or control. One day before trials, all spiders were deprived of water for 24 hours prior to presentation of a treatment droplet. Treatment droplets were made up of either 1μl sterile water containing 600 colony forming units (CFUs) of bacteria (Infected) or 1μl of pure sterile water (Control). The bacterial pathogen utilized was Pseudomonas aeruginosa suspended in sterile water, prepared using the 2 McFarland turbidity solution for comparison. Choice trials were conducted 2-3 hours post consumption of the water droplet to allow sufficient rehydration before observation. 3 sensory condition treatments were used to test female preference: unimodal-visual, unimodal-vibratory, and multimodal- visual and vibratory simultaneously. Each sensory condition presented a choice between high- and low-quality male courtship stimuli; signal components that were manipulated for quality were tuft size (area) and peak amplitude of vibratory percussive component An arena of 20 cm diameter and a height of 7 cm set on top of poster board and a block of granite was arranged with opposing stimuli situated on opposite sides of the arena, high quality vs poor quality. Video playback of courting male stimuli was presented to females on iPod Touch® units that differed only in male foreleg tuft size, based on confidence limits of natural distributions. Vibratory stimuli from male courtship representing positive and negative confidence limits of amplitude from natural populations were played to females using piezoelectric disc benders attached flush with the poster board substrate of the arena and covered and directly contacting parchment paper so that spiders could perceive vibration throughout. Accurate peak percussion amplitudes were calibrated using a PDF-100 laser Doppler vibrometer (LDV) measuring the male courtship vibrations being played-back by piezoelectric disc benders. Trials lasted 5 minutes and were recorded using a Sony Camcorder, and videos were later scored for behaviors, including latency to orient to stimulus, number of approaches to stimulus, time spent in front of stimulus, and number of receptivity displays performed in front of stimulus. Each individual was weighed the day of the trial and digitally photographed less than 3 weeks after the trial date. Photographs were later used to measure width of the cephalothorax using Image J.