News in Science

Stinging Trees

Australia is claimed to have the nine most poisonous spiders in the world. It is true that we do have the most venomous snake in the world - the Fierce Snake. And it's also true that we have the world's most painful plant - the aptly-named stinging tree.

In fact, there are six species of stinging tree in Australia, but only two of them are the tall woody types - the other four are lowish shrubs. They live along the East coast of Australia, from Cape York in the north, to Victoria in the south. They grow only if they get both strong sunlight, and protection from the wind. So you'll see them along tracks, the banks of creeks, and where the rainforest canopy has been broken by a falling branch or tree. They also pop up after a storm has ripped through a forest, or an area has been cleared for forestry or land development.

Stinging trees play an important part in the ecology of a rainforest. Many native Australian animals, birds and insects are not bothered by the sting, and happily devour the leaves and fruit.

We know a lot more about stinging trees, and what lives off them, thanks to Marina Hurley. She's an entomologist and ecologist, and got her doctorate at James Cook University in Cairns, for her research. She looked at two shrubs, the Gympie-Gympie (Dendrocnide moroides) which is the most painful of all the stinging trees, and another closely related but slightly less harmful shrub, D. cordifolia.

Even though they don't hunt in packs, these stinging trees are pretty vicious. The sting is delivered through tiny silicon hairs that cover the leaves and the fruit of the plant. You can think of the silicon hairs as tiny fibres of non-transparent glass. Dr. Hurley found that the only way she could handle the leaves safely without getting stung, was to wear incredibly thick and bulky welding gloves. These silicon hairs penetrate your skin, and then break off. They're so tiny, that often the skin will close over the hairs. So sometimes, once you've been stung, you can't remove the stinging hairs.

The silicon hairs cause pain, because they carry a neurotoxin. One scientist, Oelrichs, purified the poison and injected himself with it and suffered intense pain. He proved that the toxin, not the silicon hairs, caused the pain. If you have stabbed yourself with the hairs, you can release the neurotoxin from the hairs by heating or cooling your skin, or just touching it. This neurotoxin is very stable. Experiments have been done with hairs that were collected nearly a century ago, and they can still cause pain.

The reaction depends on what species of animal gets stung, and how many hairs get stuck in the skin. But we humans feel something between mild irritation and intense pain and death. The pain comes immediately after touching the plant, and it gradually increases to a peak after about 20-30 minutes. The Dutch Botanist H. J. Winkler made the only official recording of Death By Stinging Tree, for a human. It was in New Guinea, back in the early 1920s. There have been other anecdotal stories from soldiers in WW II suffering intense pain, and of an officer shooting himself because of the unrelenting pain - but these are just word-of-mouth.

But you can suffer even if you don't touch the plant. The plants continuously shed their stinging hairs. Stay close to the stinging trees for more than an hour, and you can get an allergic reaction - intensely painful and continuous bouts of sneezing. You can even get nose bleeds from these silicon hairs floating in the air. But Dr. Hurley found that if she wore filter masks, which she replaced regularly, she could work near the plants for a few hours at a time.

Now there are two weird things about these stinging trees.

First, these stinging trees are harmless to many native Australian species, but very nasty to introduced species such as humans, horses and dogs.

The second thing is even weirder. The pain is real and intense, but your body does not suffer any damage. Fire and snake bites cause pain, AND they damage you as well. But it seems that the pain from this tree could be the only pain that is not related to any damage. If we look at the neurotoxin involved, we might learn a lot about the mechanism of pain.

So what's the best way to get the hairs out of you, once you've accidentally got stuck on a stinging tree. Don't even think about rubbing the affected area with the sap of nearby trees, or the ground-up roots of the tree that stung you. No, it was a student from James Cook University in Cairns who discovered the best way - you can remove these hairs with a hair-removal wax strip. In fact this is now the official recommendation in a Queensland ambulance journal.

It might sound like a good way to get a free leg wax, but the pain is not worth it.

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