They lurk in the woodshed, the crawl space and the closet you should have cleaned out. Their glossy black bodies are marked with a ruby hourglass, and they’ve pierced many an unwary thumb with their fangs. As hidden menaces go, black widow spiders are not aggressive — you often have to actually pinch or squeeze one to get it to attack — but their venom makes bites extremely painful to humans, and to small animals like mice, fatal.
It’s difficult to believe that anything would want to make a meal of such a venomous arachnid, but in California and other Western states, the alligator lizard snarfs up widows like crunchy black popcorn.
That fascinated Chris Feldman, now a professor of biology at the University of Nevada, Reno, who first read about the lizards snacking on black widows as a graduate student. Although widow venom is harmless when eaten, the lizards are most likely bitten as they subdue their prey. Were they somehow immune?
In a paper published Wednesday in Royal Society Open Science, Dr. Feldman and his colleagues tried to answer this question in experiments involving three lizard species and a tiny racetrack. Their results suggest that over the course of a long history living with the spiders, alligator lizards have evolved a remarkable ability to shrug off widow venom.
In the warm environments where widows thrive in the American West, lizards are their near neighbors. Black widows even eat small lizards, which may get tangled in their webs, Dr. Feldman said. To see whether different species might have evolved protection, he, Vicki Thill, a graduate student, and their collaborators brought alligator lizards and Western fence lizards, both of which eat widows, and the side-blotched lizard, which widows have been known to eat, into the lab. There, they injected the lizards with black widow venom and had them run along a small racetrack to see whether the venom affected their speed.
The side-blotched lizard did slow down, showing that the venom was having some effect. But though some of them had received enough venom to kill five mice, alligator lizards and fence lizards showed no change at all.
“They were pretty much unimpaired,” said Mike Teglas, also a professor at Reno and an author of the paper. “We were pretty excited.”
Next, the researchers examined the muscle tissue in the lizards’ legs. In mammals, black widow venom kills muscle cells and leaves a spreading wake of damage around the bite. Fence lizards and side-blotched lizards had some signs of muscle injury and inflammation. But in alligator lizards, the muscle looked completely untouched. It was as if the venom had never been injected.
That suggests that alligator lizards have evolved a fast-acting way to protect themselves from widows’ venom.
“My guess is that the alligator lizards may have something that circulates in their serum — in their blood — that works right away,” Dr. Feldman said, meaning that some compound could be neutralizing or whisking away the venom before it does any damage.
California kingsnakes, which occasionally eat rattlesnakes, have evolved just such a defense, he points out.
“They have these giant proteins in their blood that bind to proteins in rattlesnake venom that render them useless,” he said.
Further experiments with other lizard species, still unpublished, suggested that whatever protection the alligator lizard has, it is a result of its long association with the spiders — many species that don’t prey on black widows were much more susceptible to the creatures’ venom.
Exploring exactly how the alligator lizard achieves this feat will probably have to wait until its genome is sequenced, which will make it easier to see what it has that related species don’t. In the meantime, Dr. Feldman wonders whether more studies of just how many spiders the lizards eat in the wild can help reveal the origins of the trait.
Dr. Feldman said he was first inspired to study the alligator lizard’s appetites after reading a letter to the journal Science written in 1937 by a herpetologist. The scientist was responding to a proposed plan to control black widows in Southern California by introducing cane toads and suggested the area’s alligator lizards were a better fit.
What was poorly understood at the time was that the toads are voracious invasive species, and poisonous too. They’ve decimated ecosystems where, in the 20th century, they were introduced to help control insects.
Luckily, California avoided that particular scourge. The alligator lizards and their prey live on together, as they have since time out of mind.
