05 June, 2014

Male sprinters and fighting females: Morphology and anti-predator strategies in Bark scorpions

Sexual dimorphism is present in many scorpion species. In many species, the males are more slender and having longer tails, while the females are larger and heavier with shorter tails. Bradley Carlson and co-workers have tested if these differences in morphology have an impact on the genders anti-predator strategies in the errant scorpion Centruroides vittatus (Say, 1821) (Buthidae).

Males sprinted faster than females, while females were more aggressive and used their stinger significantly more often than males. The larger and heavier females (burdened by being pregnant 80% of the year) can not run away from potential predators and have in stead evolved a higher aggression and sting use as the main defense. Males, unburdened by a load of developing embryos in the their abdomen, have developed morphological traits enhancing their running abilities as the main defensive behavior.

This article is also mentioned in the ScienceNews blog under the title "Beware of the pregnant scorpion".

Sexual dimorphism can result from sexual or ecological selective pressures, but the importance of alternative reproductive roles and trait compensation in generating phenotypic differences between the sexes is poorly understood. We evaluated morphological and behavioral sexual dimorphism in striped bark scorpions (Centruroides vittatus). We propose that reproductive roles have driven sexually dimorphic body mass in this species which produces sex differences in locomotor performance. Poor locomotor performance in the females (due to the burden of being gravid) favors compensatory aggression as part of an alternative defensive strategy, while male morphology is coadapted to support a sprinting-based defensive strategy. We tested the effects of sex and morphology on stinging and sprinting performance and characterized overall differences between the sexes in aggressiveness towards simulated threats. Greater body mass was associated with higher sting rates and slower sprinting within sexes, which explained the greater aggression of females (the heavier sex) and, along with longer legs in males, the improved sprint performance in males. These findings suggest females are aggressive to compensate for locomotor costs of reproduction while males possess longer legs to enhance sprinting for predator evasion and mate finding. Sexual dimorphism in the metasoma (‘‘tail’’) was unrelated to stinging and sprinting performance and may best be explained by sexual selection.

Carlson BE, McGinley S, Rowe MP. Meek Males and Fighting Females: Sexually-Dimorphic Antipredator Behavior and Locomotor Performance Is Explained by Morphology in Bark Scorpions (Centruroides vittatus). PLoS One. 2014;9(5):e97648. [Free full text]

Thanks to Matt Simon for informing me about this paper!

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