In the arid deserts of the Southwestern United States, there exists a humble-looking cactus by the name of Cylindropuntia fulgida. With its stout stature and knobby tentacles, C. fulgida is a sight to behold—but best admired from afar.
At the slightest provocation, the spiny stems of this cactus will detach and lodge themselves in whatever’s nearby, be it fur, clothes, or flesh—earning it the nickname “jumping cholla.” It’s actually a bit of an exaggeration: This plant is not going to physically leap out and attack you. But its trigger-happy strategy is not one to toy with unless you’re eager for a world of hurt.
The prickliest part of the problem are the tiny, backward-facing barbs that decorate the tips of jumping cholla spines. These barbs not only enable the spines to impale flesh, but also make extraction excruciating. According to a new study, once anchored, the barbs cling so tightly that a single inch-long spine can lift a half-pound slab of pork.
“This work really brought home for us how tenacious these spines are,” says study author Stephanie Crofts, a biomechanist at the University of Illinois at Urbana-Champaign. “It blew us away.”
Thousands of species of cacti exist, spanning all sorts of shapes, sizes, and statures; even their spines (which are actually a type of leaf) are no stranger to diversity. Some spines serve as a blatant deterrent to hungry herbivores, but others, like those from cacti in the Opuntioideae group, which include both the prickly pear and jumping cholla, are thought to play a different role. They help the plant latch onto hapless passersby, giving them a shot at dispersing into new territory like the sharpest of stowaways. To up their staying power, these spines bear ornaments unlike anything you’d ever want to adorn your Christmas tree: microscopic barbs that enable the needly structures to stick and stay stuck. But until now, no one has quantified the difference these embellishments really make.
That’s where Crofts and her advisor, Philip Anderson, an evolutionary biomechanist at the University of Illinois at Urbana-Champaign, come in. Crofts’ work focuses on the power of puncture—work that entails a lot of jabbing, piercing, and perforating. With their naturally prickly arsenal of weaponry, cacti stood out to Crofts as a natural subject for her research.
In Crofts’ and Anderson’s new work, published today in the journal Proceedings of the Royal Society B, they collected five spines each from six species of cacti. Three of them—Pereskia grandifolia, or rose cactus; Echinopsis terscheckii, or Argentine saguaro; and Echinocactus grusonii, or golden barrel cactus—bore unbarbed spines, while the others—Opuntia fragilis, or brittle prickly pear; Opuntia polyacantha, or plains prickly pear; and, of course, Cylindropuntia fulgida, or jumping cholla—had barbed spines.
Cactus cuttings encased in newspaper clippings, were shipped to the University of Illinois by the Huntington Botanical Gardens in California. Crofts bravely unwrapped the contents by hand and clipped the spines from each specimen. “It was an exciting few days when we had loose cacti rolling around,” Crofts recalls. “I definitely ended up having to use the tweezers on my Swiss Army knife.” (Crofts graciously thanks the makers of the multitool in the acknowledgments of her publication.)
To test the puncturing prowess of each set of spines, Crofts next selected her victims: skinless chicken breasts; raw, skin-on pork shoulders; and a silicon-based polymer. Chicken breast is soft, relatively unprotected muscle—something that might easily mimic the inside of a hungry animal’s mouth, Crofts explains. Pig tissue, on the other hand, would have a similar texture and toughness of human flesh (if anything, pig’s a bit thicker). Lastly, the polymer has an amorphous, elastic texture, and is devoid of the collagen and muscle fibers present in animal flesh.
When Crofts got to stabbing, she found that barbed spines were easier to embed into both animal tissue and the polymer. It seems a bit counterintuitive at first, Crofts explains, but the barbs are actually a huge asset to the spines’ otherwise needly structure: Each concentrates pressure onto a small area, reducing the amount of force necessary to grind through its unfortunate target—much like slicing with a serrated knife.
Extricating the spines, however, was quite a different story. After barbs deploy within tissue, withdrawal goes against the grain, making the spines a nightmare to extract. And the more fibrous the tissue, the more there is for the barbs to snag on. Because of this, removal of barbed spines from the jello-y polymer made for smooth sailing, but the chicken and pig weren’t quite so lucky.
The pork shoulder in particular proved to be astoundingly stubborn. In one trial, Crofts found that a lone, inch-long spine snipped from a jumping cholla was enough to hoist a half-pound cut of pig into the air. Only a couple millimeters of the spine had penetrated the tissue—just enough to clear the layer of skin—but there the pork dangled, as if completely weightless.
“It was this big old chunk of meat just hanging there, a suspended chunk of pig,” Crofts says. “We physically had to pull the pig off the spine.”
And after jumping cholla spines like these were wrested from their porcine prisons, Crofts discovered the ordeal had stripped a handful of barbs from their tips. They didn’t penetrate so well after that.
Of course, cacti are far from the only plants prone to puncture—or even the only organisms. Throughout their work, the researchers were struck by the resemblance of barbed cactus spines to an unexpected kindred spirit: North American porcupine quills. Six years ago, a study led by biomedical engineer Jeffrey Karp of Harvard Medical School conducted a similar series of experiments on these quills; when Crofts compared the two sets of results side by side, she was astounded by what she refers to as a “weird cross-kingdom convergence.” Though porcupine quills are much bigger and primarily serve as a defense mechanism, both structures penetrate flesh with ease and stay firmly put—a dangerous double whammy of entry and retention.
“This is really exciting,” says Martha Muñoz, an evolutionary biologist at Virginia Polytechnic Institute and State University who didn’t participate in the new finding. “Here, we see similarities in the puncture properties in organisms as different as cacti and porcupines. It’s hard to get more distant than that.”
However, Crofts and Anderson’s work didn’t directly compare the two structures—something that would probably illuminate the similarities and differences between them more clearly, points out Julie Messier, a plant ecologist at the University of Waterloo who was not involved in the new research. Still, Karp says it’s “wonderful” that this new work has hinted that the benefits of barbs transcend the branches of the tree of life.
In fact, shortly after Karp’s group published their initial findings on porcupines, he and his son Joshua also took note of the spiky resemblance between porcupine and cactus—so much so that the younger Karp, then 10 years old, pitted the two head-to-head in his next science fair project, entitled, “The Secret of the Barb: Porcupine Quills vs. Cactus Needles.” In one experiment, Joshua drove quills and spines into raw chicken to see how much flesh the barbs retained upon withdrawal; in another, he pitted the structures tip-to-tip and made them duel to the death.
The results were mixed: The porcupine quill took a bigger chunk out of the chicken—but a cactus spine conclusively skewered a porcupine quill. For now, the jury’s still out on the victor of this jabby joust.
Still, one thing’s for sure, according to Crofts: “I do not ever want to brush up against a jumping cholla,” she says. “They do what they do very well. But I don’t want to be what they’re puncturing.”