Sex Differences in Defensive Behavior and Venom of The Striped Bark ScorpionCentruroides vittatus(Scorpiones: Buthidae)

Abstract

Studies of venom variability have advanced from describing the mechanisms of action and relative potency of medically important toxins to understanding the ecological and evolutionary causes of the variability itself. While most studies have focused on differences in venoms among taxa, populations, or age-classes, there may be intersexual effects as well. Striped bark scorpions (Centruroides vittatus) provide a good model for examining sex differences in venom composition and efficacy, as this species exhibits dramatic sexual dimorphism in both size and defensive behavior; when threatened by an enemy, larger, slower females stand and fight while smaller, fleeter males prefer to run. We here add evidence suggesting that male and female C. vittatus indeed have different defensive propensities; when threatened via an electrical stimulus, females were more likely to sting than were males. We reasoned that intersexual differences in defensive phenotypes would select for venoms with different functions in the two sexes; female venoms should be effective at predator deterrence, whereas male venoms, less utilized defensively, might be better suited to capturing prey or courting females. This rationale led to our predictions that females would inject more venom and/or possess more painful venom than males. We were wrong. While females do inject more venom than males in a defensive sting, females are also larger; when adjusted for body size, male and female C. vittatus commit equal masses of venom in a sting to a potential enemy. Additionally, house mice (Mus musculus) find an injection of male venom more irritating than an equal amount of female venom, likely because male venom contains more of the toxins that induce pain. Taken together, our results suggest that identifying the ultimate causes of venom variability will, as we move beyond adaptive storytelling, be hard-won.

Venom-gland transcriptomics and venom proteomics of the black-back scorpion (Hadrurus spadix) reveal detectability challenges and an unexplored realm of animal toxin diversity

“…Expression levels were less consistent for the proteomically confirmed toxins (Spearman's rank correlation ρ = 0.66, Pearson's correlation coefficient R = 0.49, and R 2 = 0.24) and the homology-only toxins (Spearman's rank correlation ρ = 0.59, Pearson's correlation coefficient R = 0.61, and R 2 = 0.37). Whether this divergence in expression levels represents individual-level variation, sex-related differences (Miller et al, 2016), or noise in the data was not clear. The nontoxin results suggest that the observed differences for the two putative toxin classes were biological rather than technical.…”

Venom Costs and Optimization in Scorpions

“…In addition to trade-offs between the use of venom and pedipalps for prey capture, there may be trade-offs in venom use and mobility when avoiding predation, as faster scorpions appear less likely to sting predators (Carlson et al, 2014;Miller et al, 2016). For example, female Centuroides vittatus scorpions that are heavier and less mobile, are more likely to sting a potential predator than males, which are more likely to sprint to safety (Carlson et al, 2014;Miller et al, 2016). Furthermore, within each sex, sprint speed decreases and sting rate increases with mass, indicating that higher rates of aggression are associated with reduced mobility (Carlson et al, 2014;Miller et al, 2016).…”

“…For example, female Centuroides vittatus scorpions that are heavier and less mobile, are more likely to sting a potential predator than males, which are more likely to sprint to safety (Carlson et al, 2014;Miller et al, 2016). Furthermore, within each sex, sprint speed decreases and sting rate increases with mass, indicating that higher rates of aggression are associated with reduced mobility (Carlson et al, 2014;Miller et al, 2016). Through fleeing, the males and smaller scorpions are not only able to avoid being eaten, but they also save their venom supply for future encounters and do not need to expend energy regenerating toxins, thus reducing ecological and energetic venom costs.…”

Studying Smaller and Neglected Organisms in Modern Evolutionary Venomics Implementing RNASeq (Transcriptomics)—A Critical Guide

“…A separation of genders is always advantageous and it is suggested to eliminate or reduce gender specific bias, which is not yet well studied in neglected venomous organisms. Most insights on gender specific venom variation are gained by analyses of snake and spider venoms [ 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 ].…”