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Cactus spine shapes determine how they stab victims

Tests in hunks of meat revealed that some spines simply poke, while others hitch a ride

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6:00am, November 27, 2018
cactus spine poking a finger

THE SPINE WHO LOVED ME  Scientists are working to understand how cactus spines, like those from this brittle prickly-pear cactus (Opuntia fragilis), puncture their victims so easily.

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Scientists have unraveled some of the mechanical mysteries behind the pokes and prods of cacti.

Like porcupine quills, the barbed spines of some cactus species easily puncture their prey but are difficult to remove. Smooth spines, however, puncture flesh easily and are removed just as readily, researchers report in the Nov. 21 Proceedings of the Royal Society B.

That variation is probably reflective of plants’ different ecological needs, says study coauthor Philip Anderson, an evolutionary biomechanist at the University of Illinois at Urbana-Champaign.

The team tested the spine strength of six different cactus species by thrusting the spines into different substances, from synthetic polymers to butcher meats. The researchers measured the force and pressure it took to poke into the substances, and the difficulty in removing the spiky structures.

Spines of Opuntia polyacantha and Cylindropuntia fulgida, for instance, are covered in microscopic barbs that make the spines easy to insert but difficult to remove from a pork shoulder and skinless chicken breast, becoming tangled in the fibrous tissues during experiments. Barbed C. fulgida spines can get so deeply embedded that, as their target pulls away, it tears off chunks of cactus that can be dropped to grow again at other locations.

 jumping cholla cactus and meat being poked
But smooth spines, such as those from Echinocactus grusonii, had a tough time implanting themselves in the polymers, and easily slid in and out of the chicken and pork samples. This finding suggests that these spines’ function is defensive, aimed at warding animals away from the plant, the researchers say.

Cacti have “a really cool diversity of spine morphologies that likely first evolved as herbivore deterrents,” says study coauthor Stephanie Crofts, also an evolutionary biomechanist at the University of Illinois at Urbana-Champaign. But the spines have since been “co-opted for a crazy number of secondary uses,” she says, noting some can even draw water from fog or aid in regulating the plants’ temperature.

Citations

S.B. Crofts and P.S.L. Anderson. The influence of cactus spine surface on puncture performance and anchoring ability is tuned for ecology. Proceedings of the Royal Society B. Vol. 285, November 21, 2018. doi:10.1098/rspb.2018.2280.

Further Reading

A. Cunningham. Cactus goo purifies water. Science News. Vol. 168, September 17, 2005, p. 190.

B. Harder. Before the Booze: Cactus extract dulls hangovers. Science News. Vol. 166, July 3, 2004, p. 4.

J. Rehmeyer. Mathematical lives of plants. Science News. Vol. 172, July 21, 2007, p. 42.

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