“When it comes to scorpions, the bigger the better”

 

The title is a quote from one of the popular Indiana Jones movies, and most of us that have been working with scorpions for many years are familiar with the rule of thumb when it comes to the potency of scorpion stings: That large scorpions have less potent venom than smaller ones or that scorpions with large pedipalps are less dangerous than scorpions with small, slender claws.

The above is also reflected in prey capture, where larger scorpions with more powerful pedipalps will more often rely on bruth force to subdue prey without using the stinger (and venom), while smaller species with more slender claws are more prone to using the stinger and venom.

Alannah Forde and co-workers have recently published an interesting study comparing LD50 potency values and morphology measures from the literature to see if the above mentioned rule of thumb when it comes to classifying dangerous scorpions is actually true.

And they did actually find that larger scorpions, with more robust chelae, are less potent than small species, with thin chelae. The evolutionary explanations for this is also discussed.

I must warn that even though it seems that Indiana Jones was correct and our commonly used rule of thumb seems to hold, there may be exceptions. Species in the infamous genus Hemiscorpius Peters, 1861 (Hemicorpiidae) are quite large and have robust pedipalps, but they can cause serious morbidity and even death in humans.

Abstract:
Scorpionism is a global health concern, with an estimation of over one million annual envenomation cases. Despite this, little is known regarding the drivers of scorpion venom potency. One widely held view is that smaller scorpions with less-developed chelae possess the most potent venoms. While this perception is often used as a guide for medical intervention, it has yet to be tested in a formal comparative framework. Here, we use a phylogenetic comparative analysis of 36 scorpion species to test whether scorpion venom potency, as measured using LD50, is related to scorpion body size and morphology. We found a positive relationship between LD50 and scorpion total length, supporting the perception that smaller scorpions possess more potent venoms. We also found that, independent of body size, scorpion species with long narrow chelae have higher venom potencies compared to species with more robust chelae. These results not only support the general perception of scorpion morphology and potency, but also the presence of an ecology trade-off with scorpions either selected for well-developed chelae or more potent venoms. Testing the patterns of venom variations in scorpions aids both our ecological understanding and our ability to address the global health burden of scorpionism.

Reference:
Forde A, Jacobsen A, Dugon MM, Healy K. Scorpion Species with Smaller Body Sizes and Narrower Chelae Have the Highest Venom Potency. Toxins. 2022;14(3):219. [Open Access]

Thanks to Jeroen for reminding me about this article!