Sting use, risk assessment and control of venom used in Parabuthus transvaalicus

One of the conclusions from my master thesis many years ago was that sting use is probably costly for scorpions and because of this they will only use their stinger if the prey is large and/or the prey resists capture. My study was based on an assumption that use of venom in scorpions was costly and also that scorpions were able to control both the use of the stinger and of the venom. And now it seems that my assumptions were correct :)

Since then, it has been showed that venom use is costly, both in terms of metabolic and ecological costs (Nisani et al, 2007; Nisani, 2008). It has also been demonstrated that some scorpions have two types of venom with different composition (and different "production costs") (Inceoglu et al., 2003).

Now Zia Nisani and Wlliam Hayes have shown that Parabuthus transvaalicus Purcell, 1899 (Buthidae) are able to regulate venom use at three levels:

• They can choose between a dry and a wet sting (use of venom).
• When using when om they can control the volum of venom expended, delivering more venom under hight threat and less volum under low threat conditions.
• They can vary the composition of the venom injected, either using "cheap" prevenom or the more potent protein rich venom.

All of the above results support the hypothesis that scorpions display a venom optimization strategy by regulating venom expenditure based on the level of perceived threat.

This is a great paper that I reccomend to all that are interested in scorpion behavior and scorpion's adaption to their environment!

Abstract:
Venom is a metabolically expensive commodity that animals should use judiciously. The venommetering hypothesis proposes that venomous animals control, or meter, the quantity of venom they deploy during predatory or defensive situations.We sought to clarify experimentally whether the buthid scorpion Parabuthus transvaalicus can regulate defensive venom expenditure based on perceived risk. Scorpions were tested under two threat conditions by inducing them to sting repeatedly a parafilmcovered cup. The high-threat condition involved five sting presentations at 5 s intervals, and the lowthreat condition comprised five sting presentations at 5 min intervals. Venom metering appeared to be modulated at three levels: wet versus dry sting, composition of venom injected and volume of venom delivered. Scorpions delivered dry stings more often under the low-threat condition, but in both conditions were more likely to employ wet stings as the threat persisted. Venom appearance changed during successive stings from clear (potassium-rich ‘prevenom’), to opalescent, to milky (protein-rich ‘venom’), with biochemical analyses (protein assay and MALDI-TOF) confirming the compositional changes. However, progression through this sequence depended on quantity of venom expended, with milky secretion appearing only after the limited quantity of clear secretion was exhausted (i.e. composition and volume covaried). Scorpions injected approximately two-fold more venom per sting during the high-threat compared to the low-threat condition, with milky venom appearing more quickly within the sequence of five stings for the high-threat condition. Thus, these scorpions perceive risk and regulate venom expenditure during stinging according to level of threat, providing further support for the venommetering hypothesis.

Reference:
Nisani Z, Hayes WK. Defensive stinging by Parabuthus transvaalicus scorpions: risk assessment and venom metering. Anim Behav. 2011;81(3):627-33. [Subscription required for fulltext]

Other references mentioned:
Inceoglu B, Lango J, Jing J, Chen L, Doymaz F, Pessah IN, et al. One scorpion, two venoms: Prevenom of Parabuthus transvaalicus acts as an alternative type of venom with distinct mechanism of action. Proc Natl Acad Sci U S A. 2003;100(3):922-7.

Nisani Z, Dunbar SG, Hayes WK. Cost of venom regeneration in Parabuthus transvaalicus (Arachnida: Buthidae). Comp Biochem Physiol A Mol Integr Physiol. 2007 Jun;147(2):509-13.

Nisani Z. Behavioral and physiological ecology of scorpion venom expenditure: stinging, spraying, and venom regeneration: Loma Linda University; 2008 (Ph.D. thesis).