30 June, 2021

Sexual dimorphism in Uroplectes lineatus and a tool for testing specific hypotheses of sex differences in scorpions

Striped Lesser-thicktail Scorpion Uroplectes lineatus. Photo: John Visser (C). Used with permission.

As with many other animals, scorpions also shows sexual dimorphism to a greater or lesser degree. The reasons why we see these somtimes large differences between the sexes are less understood. Jacobus Visser and Sjirk Geerts has used the method static allometry to investigate sexual dimorphism in Uroplectes lineatus C. L. Koch, 1844 (Buthidae).

The results indicate that the sex differences in males may have a function during courtship, while the differences seen in females may have a postive effects on reproduction and parental care. It is important to note that the forces that shapes scorpion morphology are complex and studies of more species are necessary to learn more about the evolution of sexual dimorphism in scorpions.

Scorpions exhibit extreme forms of sexual dimorphism, with a number of recent studies highlighting general patterns. Explanations surrounding the potential drivers of these patterns remain speculative, even though static allometry offers a method for testing specific hypotheses. Importantly, a recent study describes a method of reference character choice when investigating sexual dimorphism and static allometry in scorpions. Here, commonly measured morphometric characters are used to investigate patterns of sexual dimorphism and static allometry in the South African scorpion Uroplectes lineatus C. L. Koch, 1844. Several analyses were used to select telson length as the sexually neutral reference character. Sexual body component dimorphism characterises U. lineatus males, while females generally display sexual size dimorphism. Similar patterns of static allometry characterise both sexes, with negative allometry retrieved for most characters, while three characters display positive allometry. For negatively allometry characters, inter-sexual selection likely favours a standard size of body parts in the population to facilitate inter-sexual interaction during courtship. In contrast, positively allometric characters may be under intrasexual selection, following the utility of features during contests. Even so, the differences in allometric slopes between the sexes indicate the possible functions of male features during courtship, while the female morphology is adapted to enhance reproductive output and parental care. Here, we demonstrate that a set of verification analyses may be effective in choosing an appropriate neutral reference character, but the selective forces which shape scorpion morphology are complex, and standardized methods need to be established to allow for robust inferences and inter-study comparability.

Visser JH, Geerts S. Static allometry and sexual dimorphism in the Striped Lesser-thicktail Scorpion Uroplectes lineatus. Arachnology. 2021;18(7):700-7. [Subscription required for full text]

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