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Prickly Trees of Australia: Please don’t stroke them

The tradition that surrounds Australia’s gigantic prickly trees, of the genus Dendrocnid, is perhaps as dubious as it is vast. Tales abound with nightmarish encounters with the hypodermic hairs of its leaves injecting a toxin that drives men mad and prompted horses to dash off the cliffs.

Some of these stories are centuries old and cannot be verified. But as Edward Gilding can attest, these legends contain at least a hint of truth: the sheer agony of being stabbed by the fine, fluffy hairs that adorn Dendrocnid’s leaves and stems. The trees, which can grow taller than 100 feet, are found throughout the rainforests of eastern Australia, where they are known to haunt hikers.

“It’s like having a nail stuck in your flesh,” said Dr. Gilding, a biologist at the University of Queensland and a self-described stinger.

Tree hair sting also has immense staying power, distributing distress in waves for hours or days. Some anecdotes have reported intermittent pain lasting months; some particularly nasty bites even landed people in the hospital.

The painful power of Dendrocnid plants has plagued researchers for decades. Trees harm people so often that many of their habitats are marked with warning signs, warning unwary visitors to “watch out for the stinging tree”. People who frequent these forests sometimes bring respirators, heavy gloves, and a handful of antihistamines.

But even scientists pushed enough to inject themselves with extracts made from tree toxins have been unable to figure out the source of the sting, said Irina Vetter, a pain researcher at the University of Queensland and author of the new study.

Those experiments, which are ethically gray, can no longer be conducted, said Dr. Vetter. But she, Dr. Gilding, and their colleagues were still able to separate the chemical components of the toxin from two Dendrocnid species and create synthetic versions of the compounds in the laboratory. A very small protein found in both plants caused the mice to lick and pinch where it was injected. Released onto nerve cells, the molecule flipped the triggered cells into an “active” position, forcing them to send out a deluge of signals.

The researchers named the tiny pain-causing molecules gympietides, in homage to gympie-gympie, the word for stinging tree in the language of the Gubbi Gubbi people, a group of indigenous Australians.

Dr. Vetter was surprised to discover that the gimpietidae bore a remarkable resemblance to the toxins produced by poisonous spiders and cone snails, which use chemicals to incapacitate their unfortunate prey.

“These are three widely divergent groups of organisms – spiders, cone snails and now these trees – that produce a very similar toxin,” said Shabnam Mohammadi, a toxin researcher at the University of Nebraska-Lincoln who was not involved in the. study.

It is an extraordinary example, he added, of several branches of the tree of life converging on the same solution.

Researchers aren’t sure how the toxin benefits Dendrocnid trees. Perhaps it serves as a kind of chemical armor to ward off hungry herbivores, Dr. Vetter said. But some animals, like beetles and pademelons – small relatives of kangaroos – seem to happily nibble on Dendrocnid foliage, prickly thorns and all.

Dr Chiu and Dr Mohammadi both said they suspect gimpietidae aren’t the only factors that make Dendrocnid toxin so hard to take, especially given the plants’ bizarre and persistent side effects. Some of Dr. Vetter’s previous struggles with trees resulted in chest pain and discomfort in extremities, among other symptoms.

“I think they have barely scratched the surface of what these plants contain,” said Dr Mohammadi.

Until more of those mysterious ingredients are identified, Dr. Gilford has recommended avoiding prickly trees. “If you work with the plant, it’s virtually impossible not to be stung,” he said.

This challenge is made more difficult by the plant’s inviting appearance, Dr. Gilford noted. The same hairs that can deliver an incredible dose of pain make the leaves and stems seemingly soft and overjoyed, “like it’s a hairy, friendly green plant you want to rub,” she said.

In case it’s still not clear: don’t.

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