Imagine yourself wandering around the woods during the summer. You're hungry. The sunlight shines through the trees and highlights a plant with three leaves tinged with red. You bend down and take the leaves in your mouth, tearing them with your teeth, and grinding them with your molars to let the juices flow, before swallowing.
You've just eaten poison ivy, also known as Toxicodendron radicans. If you are a human, here's what would happen to you:
Resin released from inside the leaves and stems would bind to the membrane surfaces everywhere it touches — your lips, gums, tongue, throat, esophagus, and stomach. After several hours, these membranes would begin to disintegrate, oozing blood and fluid from your cells. Blisters would form. Nerve cells would be damaged, causing itching and burning Nausea, fever, chills Possibly shock, hypertension, renal failure, or development of ulcers
If you are a deer, goat, horse, rabbit, or bird that just ate poison ivy, here's what would happen if you:
In fact, studies have shown that white-tailed deer prefer eating poison ivy over other plants. To most animals, poison ivy is delicious and nutritious. Poison ivy guru Jon Sachs (www.poison-ivy.org) says that "Everything I have ever read or heard says that no animals other than humans are affected by poison ivy - even chimps, which are 98% the same DNA as people."
So what's going on here? Why is it that humans react to poison ivy in ways that other animals do not?
One explanation is that poison ivy produces urushiol. Urushiol is a mixture of phenolic compounds and is the substance that is responsible for our reaction to the plant. The production of urushiol by the plant though is thought to provide a protective benefit to the plant: they are less likely to get eaten and are therefore more likely to survive to reproduce than their competitors.
But since grazing animals like deer have no ill effects from feasting on poison ivy, this explanation doesn't make much sense. Maybe the urushiol gives some other benefit to the plant itself or maybe just has no benefit to the plant at all. The evolution of the poison in poison ivy remains an open question.
The interesting thing though is that technically, urushiol is not a poison at all! It does no damage of its own to tissues. In fact, the red, painful, oozing rash that can result from exposure to urushiol is merely the result of an allergic reaction. It is our OWN bodies that are doing that to us, not the plant.
Here's how the allergic reaction happens: The phenolic compounds in urushiol are absorbed by the skin where they chemically react with our cell membranes, and bind to them. These compounds are then recognized as a foreign substance by our body by special immune cells called T- cells which try to destroy it. This army goes a little overboard in its protective reaction though, releasing enzymes and protein toxins that destroy everything in the area. The result, in the worst-case scenario, is a red, oozing, painful mess.
Because the reaction is an immune response, it varies between people... and animals! Other animals don't react the same way because they do not share the complexity of our immune system.
The good news is you don't have to feel sorry for all those animals out there feeding in poison ivy-ridden fields and woodlands. And since humans are unlikely to accidentally eat poison ivy during a walk in the woods, there's no reason for concern there either.
That said, it's still worth taking extra special caution when you're trekking around plants that you think look like poison ivy to avoid getting an annoying rash. You can also certainly feel sympathy for all those forest firefighters out there battling the blazes. They've got it real bad — as urushiol is highly stable, and the smoke of a forest fire can be laden with it. But that's another story...
- Professional Education Service Group. Toxicodendron Dermatitis: Identification, Immunologic Mechanisms, Diagnosis, and Treatment. Thomas W. McGovern, MD. Volunteer Clinical Assistant Professor of Dermatology, Indiana University School of Medicine. Fort Wayne, IN http://www.pesgce.com/toxicodendrondermatitis/derm.asp
- Stratford Landing Elementary School, Fairfax County Public Schools. Study of Northern Virginia Ecology. Mark Moran ttp://www.fcps.k12.va.us/StratfordLandingES/Ecology/mpages/poison_ivy.htm
- US Department of Agriculture Forest Service (2006) Species Information, Toxicodendron radicans.
- Wayne's Word: An On-Line Textbook of Natural History. Poison Oak, More than Just Scratching the Surface, Volume 8 (Number 2) Summer 1999. http://waynesword.palomar.edu/ww0802.htm
CurioCity would like to thank Judith Wearing for her expertise on this topic and help with this question. Judith has degrees in evolutionary biology and education. She has worked developing science education programs and products and is now the Manager of Product Development with an education publishing company. She is currently writing a book of her own entitled Planetary Stew and other science recipes.