How does it benefit the plant? I mean, why did it evolve to be so mean?
I agree that there might not be a main reason for the skin itching since you might not associate the two. However, it is also apparently an irritant if eaten, and many animals are very good at associating what they eat with later bad reactions.
It keeps many grazing animals from eating it.
Several herbivores can browse poison ivy / oak, deer and rabbits among them. It’s also claimed that horses and goats can browse it. A lot of other animals seem not very sensitive to the urushiol, and don’t usually get the skin rash. Dogs and cats can, but usually don’t, although your dog can roll around in the stuff and give YOU a rash. For some reason, people seem to have a greater sensitivity than most other species.
BTW, the plant could have still originally evolved the urushiol as protection from herbivores, but the herbivores in turn evolved immunity. There are a few claims out there that rabbits and deer eat poison ivy, but not preferentially, so maybe it at least tastes bad to them.
I believe it’s just by chance that the poison ivy oil binds to a protein in humans. It’s not a typical defense mechanism that would affect a wide variety of attackers. Likely if our protein was slightly different, we wouldn’t be bothered by poison ivy.
Although, it works out to be a good defense mechanism in the current time. It easily proliferates in yards and green belts because we’re reluctant to touch it and there aren’t a lot of other animals around the neighborhood which will eat it.
It’s evolutionary beneficial if humans think twice about stomping through a patch of it where they can get its nasty little essence on their skin. On the other hand, it is not a sound evolutionary strategy when the sight of it spurs me to douse it with herbicide. Your classic two-edged sword; it doesn’t want to be messed with, but neither do we.
You’re gonna need an ocean
Of calamine lotion
You’ll be scratching like a hound
The minute you start to mess around.
I can confirm that goats can eat it. We used goats on our property to remove its West Coast cousin. Worked like a charm.
I think the OP got it right, plants are just mean spirited … poison ivy is just more mild than the homicidal tendencies of Douglas fir or hemlock … also why roses smell nice and are pretty, they’re trying to stab you with thorns …
Well, at least in this study poison ivy fell into the “moderate” preference range for White-tailed deer and I’ve seen our local variety of Mule deer happily munching away on poison oak. Also urushiol seems less of a mechanical/chemical irritant than an allergen. Which is to say even some human beings are completely immune to the stuff.
Give that I’m just a little dubious that it arose primarily as an anti-grazing defense. Seems like most native major grazers are relatively unaffected by the stuff. Just why many humans seem to have a problem with it is unclear to me, but perhaps it is just a coincidental quirk of evolution. Human kryptonite, as it were.
This may be helpful medicinally.
Most people (about 85%) are allergic to poison ivy to some degree but it certainly isn’t everyone. I can pull it up with bare hands all day long and nothing will happen (the same is true with its cousins poison oak and poison sumac). Countless other people are also not allergic to it. That leads me to believe it is just a really widespread allergy on the human side and not a defense mechanism those plants specifically evolved against humans.
Run hot water over it, hotter and hotter til you can’t stand any more. The itch will just scream til you quit the water. Then, just gone, no itch for about 8 hours.
The hot water uses up the histamines in your skin and it takes about 8-10 hours to regenerate.
Which part is the reason you don’t believe it evolved as a defense mechanism? Is it because the number of people allergic to it isn’t higher than 85%? That’s a pretty high number. I would bet lots of adaptations have evolved successfully not because they affect 100% of predators negatively, but much smaller numbers.
Something I always come back to with notions like this, is that evolution doesn’t make decisions the way lots of folks were accidentally taught to think.
Basically, some stuff is the way it is, just because it came with the package. Yeah, MAYBE the itchy stuff has a “purpose,” but maybe it doesn’t. After all, millions of other plants survived the eons just fine, without being annoying like that.
Evolution is a lot like single generation success stories about people who you see. All kinds of examples you could find, but a silly one…a hot woman can make it to the top of the modelling world, and then go on to a profitable acting career, all because she’s born to be good looking. But she might have six toes on one foot, or be near sighted or something too. She doesn’t make it to the top BECAUSE she’s near sighted. That factor just goes along for the ride.
You can also develop the allergic reaction. I never got it until I was in my 40s, in spite of having total disregard for it - then I had to learn to be careful after a few nasty cases of it. One just this year after a few years of managing to avoid it. CA Bay Area hiking trails are loaded with poison oak, and this year it’s supposedly especially prolific because of the wet winter.
Where is “CA Bay Area”? Hopefully not San Fran because my relatives want me to visit there in the next year or so.
Sadly, SF does indeed fall into the CA Bay Area ;). And yes, poison oak is just about everywhere outdoorsy around here - it’s positively weedy. You just have to learn how to identify and avoid it.
I am not an evolutionary botanist but many plants that have defense mechanisms are much more obvious like thorns, strongly adverse tastes or fast acting toxins to make the cause and effect clear. Poison ivy doesn’t have that and immunity to it is fairly common but seemingly scattershot. Even it it did, the defense mechanism should be more targeted towards grazing animals than people because even the common allergic response doesn’t prevent people from trying to get rid of it whenever possible and a usually mild threat would put negative pressure on that.
It is perfectly possible (and even likely in my opinion) that the common allergic response is just an accidental crossover effect when it comes to allergic reactions in people. The allergic reaction may have nothing to do with defense at all. It already grows extremely well over huge areas and the allergic response in humans due to its compounds may just be coincidental.
You can look at it from the other side as well? Why are so many people allergic to poison ivy when it isn’t a true threat?