Many animals use chemical spraying as a defensive response to aggressors. It is considered a secondary tactic that renders the prey less easily captured, as opposed to a primary tactic, which reduces the likelihood of the prey being detected initially. Spitting cobras, for example, are able to spray venom at a predator located at a distance of up to three meters away via decisive (not reflexive) control of venom glands. Two genera of spiders have also been recorded to spit a mixture of silk, adhesive, and venom to immobilize both prey and predators. Researchers Zia Nisani and William K. Hayes have investigated scorpions of the genus Parabuthus and their venom spraying behavior. In a recently published study, the researchers observed the behavioral context of scorpions’ venom spraying behavior and the possibility that scorpions modulate their venom emission in accordance with threat level.
Nisani and Hayes conducted their experiment in response to a number of theories regarding scorpion venom spraying behavior. One of these, suggested by Newlands (1969), posited that scorpion spraying is a reflex that is controlled by muscles in the metasoma (tail) and the telson (stinger). He suggested that when scorpions are startled, these muscles contract, causing the venom gland to contract and venom to be expelled. The venom-metering or venom-optimization hypotheses (Hayes et al., 2002; Hayes, 2008; and Wigger et al., 2002, Nisani & Hayes, 2015) state that animals modulate their venom expulsion based on risk assessment to optimize the metabolically expensive venom.
In their recent study, Nisani and Hayes administered “low-threat” and “high-threat” stimuli to 16 female adult and juvenile P. transvaalicus scorpions. The low-threat stimulus consisted of direct contact to the metasoma using forceps. The high-threat stimulus was the same as the low-threat stimulus with the addition of a simultaneous puff of air (expelled using compressed gas). The low-threat stimulus is meant to simulate an attack by a single predator while the high-threat stimulus is meant to simulate an attack by multiple predators or a large predator. The researchers measured whether or not the scorpions’ sprayed venom, whether or not the spray was accompanied by movement of the metasoma, telson, or the rest of the body, as well as the duration and velocity of the spray, and the arc (width) of the spray.
The researchers saw that the scorpions were more likely to spray venom when direct contact was accompanied by the airborne stimuli (the high-threat stimulus). The scorpions also sprayed with or without accompanying movement. The duration, velocity, and arc of the sprays varied markedly. These results interestingly suggest that scorpions are able to moderate their venom spraying in response to level of threat. The researchers speculate that this ability is mediated by the trichobothria, hair-like sensory structures that are activated in response to air movement and vibrations. The scorpions also did not spray venom until the metasoma was grasped. Previous experimental evidence has show that grasshopper mice, which prey on scorpions, were more likely to attack scorpions by seizing the metasoma, especially after sustaining stings in previous encounters. Because the venom spraying occurred with and without accompanying movement, it is not likely that venom spraying is a reflex.
This research provides insight into scorpions’ defensive behavior and their ability to decisively modulate their venom expulsion based on threat level. These results support previous hypotheses of venom optimization shown by the scorpions’ spraying or lack of spraying behavior as well as the variance in duration and velocity of sprays. This modulation is not only economically efficient but it also increases their fitness because it increases the likelihood that they will have venom spray onto predators’ soft tissues. This research can be used as a stepping-stone to further investigate modulated defensive behavior in other species and to determine how much control other organisms have over their defensive behaviors.
Nisani, Z & Hayes, WK. (2015) Venom-spraying behaviour of the scorpion Parabuthus transvaalicus (Arachnida: Buthidae). Behavioural Processes 115, 46-52.
Newlands, G. (1969) Scorpion defensive behaviour. African Wild Life 23, 147-153.
Hayes, WK. (2008) The snake venom-metering controversy: levels of analysis, assumptions, and evidence. The Biology of Rattlesnakes, 191-200.
Hayes, WK, Herbert, SS, Rehling GC, Gennaro, J. (2002) Factors that influence venom expenditure in viperids and other snake species during predatory and defensive contexts. Biology of the Vipers, 207-233.
Wigger, E, Kuhn-Nentwig, L, Nentwig, W. (2002) The venom optimisation hypothesis: a spider injects large venom quantities only into difficult prey types. Toxicon 40, 749-752.