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Watch out for jumping cholla, Cylindropuntia fulgida. This shrubby and branched cactus anchors – if brought about by touch – its thorny thorns in the flesh of the delinquent. The barbed spines grip so hard that a segment of cacti often breaks with them, leaving the victim with a thorny problem.
It is one of six species of cactus subjected to thorough testing by Stephanie Crofts, a postdoctoral researcher at the University of Illinois, and Philip Anderson, professor of biology animal. The researchers, who are studying the biomechanics of plant and animal perforation, wanted to know how the structure of the spine affects its performance.
They found that the same biomechanical characteristics that allow cholla spines and other barbed spines to easily penetrate the flesh of animals also make thorns more difficult to dislodge. The researchers report their findings in the Acts of the Royal Society B.
"We examine the fundamental mechanisms of a puncture event and how the differences in cactus spines – especially their microstructure – affect the way they perforate and anchor what they perforate," he said. Crofts.
In addition to the cholla, the researchers evaluated the spines of Echinocactus grusonii, the cactus to the barrel of gold; Opuntia fragilis, or fragile prickly pear; Pereskia grandiflora, the pink cactus; Echinopsis terscheckii, the Argentine saguaro; and Opuntia polyacantha, the plain of Barbary of the plains.
Cactus spines can have a variety of functions, including defending the plant against predators, shading and collecting fog water. The spines of the cholla have a reproductive purpose: by clinging to a creature rather unlucky to avoid them, they help the plant to be distributed in several places.
To compare different spines, Crofts and Anderson tested them on skinless chicken breasts, pig shoulders (with the skin) and synthetic elastomers of different densities. They measured the force needed to drill – and remove – each material with each type of spine.
"Before we started the experiments, we examined the spines with a scanning electron microscope," said Crofts. "The barbed spines – like those of the cholla – looked incredibly like porcupine quills studied by other groups."
Like porcupine quills, barbed cactus spines look like shingles, resulting in overlapping layers of barbs. And, like those on porcupine quills, cactus barbels are the perfect size to hook animal muscle fibers, the researchers discovered.
The researchers found that barb-less spines required more work to initiate a fracture. Barbed spines penetrated their targets more easily and required less work. They were also harder to remove from animal tissues.
"To be able to perforate effectively, the cholla's spine must be able to penetrate the target very easily, so a simple brushing is enough," Anderson said. "At the same time, it must be very difficult to remove."
In porcupine quills and barbed cactus spines, barbs act as small, sharp blades that concentrate stress and allow animal tissues to fracture more easily, Anderson said.
"Then the barbs catch on your muscle fibers, which makes it difficult to eliminate them," he said.
The researchers detected reliable performance differences between cactus species. For example, O. polyacantha "It took a lot more work to remove chicken than any other species," the team said. Cholla spines were significantly more difficult to remove from pork tissue. In fact, a single cholla spine could lift a piece of pork half a pound through its skin.
The crazy conclusions go further. The team discovered that barbed spines removed from the chicken breast had been covered with animal tissue. Barbed pork-fleshed spines came out clean, but they lacked some grit. These, presumably, remained in the flesh.
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