There are some 1,400 described species of scorpion in the world, and while only 25 of those have proven they can take down a human being with their venom, many more of them can easily injure and kill smaller creatures. Given that, you’d expect scorpions to be important predators in desert food webs, but you might not expect them to be equally important as prey.
Yes, despite the pincers and the stinger and the venom, plenty of animals – among them, centipedes, tarantulas, lizards, owls,shrews and bats – regularly chow down on scorpions. Hemprich’s long-eared bat (Otonycteris hemprichii, at right), found in deserts in northern Africa, the Middle East and south-central Asia, considers scorpions a major food group, along with beetles, centipedes and spiders. In Israel’s Negev Desert, the proportions of these groups in the bat’s diet changes throughout the year, with scorpion fragments found in only 10% of bat droppings in the early spring and in a whopping 70% in the late summer.
That’s a whole lot of scorpions they’re eating, and while some of those are certainly weakly toxic Large-clawed Scorpions, the bats also prey on the Palestine yellow scorpion (Leiurus quinquestriatus, below). Everything you need to know about L. quinquestriatus can be summed up in its nickname, the deathstalker. These scorpions are considered some of the most dangerous in the world and possess a highly toxic venom that contains a grab bag of neurotoxins. Their sting can cause extreme pain, fever, convulsions, paralysis and death (via heart or respiratory failure), even in humans. These scorpions are also “sit-and-wait predators” that hunt by remaining quiet and still and lashing out at unsuspecting prey that wanders too close.
They certainly sound appetizing, but how do the bats deal with prey that present such a challenge to both detecting and disabling them? Carmi Korine, of Ben-Gurion University of the Negev in Midreshet Ben-Gurion, Israel, and colleagues found that Hemprich’s long-eared bat is what’s known as a gleaner, which means it takes prey from surfaces and not right out of the air, as many bats do. Korine and his team hypothesized that, like gleaning Pallid bats that prey on scorpions in North America, Hemprich’s bat might rely on passive gleaning to find its meals, simply listening for the prey to make noise instead of actively using echolocation to detect it based on its echo signature. The researchers figured that, if that was the way the bats hunted, then they would select prey based on 1) body size (bigger scorpions being easier to detect), and 2) toxicity (less venomous scorpions posing less risk of injury).
The team captured eight O. hemprichii individuals, set them up in a room with a scorpion buffet that included both living and dead Large-clawed Scorpions (Scorpio maurus palmatus), Israeli common scorpions (Buthus occitanus israelis) and, of course, deathstalkers.
When the bats took off to search for food, they spent a few minutes circling the room and dropped down directly onto a scorpion once they noticed it. They only went after the live scorpions, ignoring the dead ones and even walking right over motionless live ones if they missed on their initial divebomb, confirming that they glean passively and rely on prey noises.
Once they landed on a scorpion, the bats immediately started biting the scorpions’ heads. The scorpions did not take this lying down and fought back, stinging the bats on the head and face and, in one case, under the eyelid. The bats made no observable attempts to either avoid or disable the stingers and once they had killed the scorpions they often ate the whole thing, including the stinger and poison gland.
This is how the bats hunted all the scorpions. Contrary to the researchers’ predictions, the bats showed no preference among scorpion species based on either on size or toxicity, diving on their prey immediately after detection without any further inspection and going only on limited acoustic information. In 49% of the test sessions the bats actually went after the more poisonous of the available species (and in 24 direct comparisons, the deathstalker was chosen 50% of the time).
The researchers did not have an explanation for the bats’ indifference to the danger their meals pose, but speculated that either scorpions aren’t able to pierce the bat’s skin, or that the bats have at least a partial tolerance to the venom. Given that the bats regularly ate the stingers and venom glands, it seems more likely that their just tough enough to handle what the deathstalkers can dish out.
For other takes on this study, see Michael Marshall’s post at Zoologger and Zen Faulke’s post at Neuro Dojo .
Reference: Holderied M, Korine C, & Moritz T (2010). Hemprich’s long-eared bat (Otonycteris hemprichii) as a predator of scorpions: whispering echolocation, passive gleaning and prey selection. Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology PMID: 21086132
Images: “Otonycteris hemprichii” by Wikimedia Commons user Charlotte Roemer, used under a Creative Commons license.
“Deathstalker near Tzehelim, Israel” by Wikimedia Commons user Yair Goldstof, used under a Creative Commons license