Researchers led by Ashlee Rowe in the US have reported that little grasshopper mice are resistant to the painful effects of the highly toxic venom of bark scorpions, thanks to a genetic quirk. Bark scorpion stings can kill other mammals of similar size, yet it seems to act as a painkiller in grasshopper mice, rather than a toxic poison. Writing in the journal Science, the researchers went into the desert to collect mice and scorpions, testing the effects of injecting either scorpion toxin or salt water placebo into the animal's paws. But rather than licking their paws more in response to the toxin – the usual pain response for these mice – they licked them less than when injected with salt water.
By looking at the gene sequences for the receptor molecules that respond to the toxin, the researchers discovered a single crucial difference between the receptors in the grasshopper mice and those in other mammals that do feel pain from the scorpion's stings. The change is enough to turn the toxin from a pain-causer to a painkiller. The scientists still don't know why the grasshopper mice don't get poisoned by the stings, but they do hope their discovery could lead to ways to better develop painkillers for humans, rather than mice.