In August of 2004, in Tucson, Arizona, our backyard Devil’s Claw plant was in bloom. When I was looking at the plant’s pretty purple/violet blossoms, I saw that there were Dorymyrmex bicolor ants the plant’s leaves and stems.
Seeing Dorymyrmex bicolor on a plant wasn’t unusual – the ants love the sugars that plants make, and so I’d often see Dorymyrmex crawling on plants.
The strange thing was that these ants were dead. They were glued in place on the Devil’s Claw.
Devil’s Claw is covered in sticky hairs, and the ants had become trapped in the glue of the hairs.
The sticky hairs are present on the leaves, but are especially abundant on the stem of the plant. The hairs are long, tapered and flexible, and they have a ball of gummy substance at their tip.
Some of the ants were stuck on the stem of the plant, and other stuck on the leaves.
There were not a lot of ants dead on the plant, perhaps four or five. But there were a couple of strange things about this situation.
One strange thing: I’d not noticed any dead ants stuck to this plant in the past. Maybe I just hadn’t noticed them before, but saw them on this day because I was attracted to the plant’s flower? That’s possible, I suppose.
Another strange thing: the ants had been stuck to the plant not on the plant’s periphery, but well into the plant, spatially. This implies that the ants had been able to navigate at least partway through the thicket of sticky hairs. It further implies that they had been motivated to try to get through the sticky hairs.
Was it possible that the ants had fallen off of a nearby wall and landed on the Devil’s Claw? Maybe, but some of them were stuck in positions that would have been unlikely had they fallen. The ants were all belly-down, as well, as though they had become stuck while in the process of walking. I’d expect some of them to be on their sides, or on their backs, had they not crawled onto the plant themselves.
So…I’ve got no evidence for this at all…but I wonder if the ants were attracted by the odor of the flowers. Perhaps they were looking to raid the flower nectaries in the same manner that they had raided our Squash’s flower nectaries.
The ants don’t seem to be very good pollinators for some plants – the ones on the Squash plant, for example, had very, very few pollen grains on them. So from the plant’s point of view, the ants would be nectar thieves – taking nectar, but not performing the pollination service.
The tubular shape of the Devil’s Claw flower seems like it’s for something like a bee pollinator. The bee is forced to crawl through the tube of the flower, the tight confines of the flower force contact of the bee with the pollen. The hairy body of the bee facilitates the pollen sticking to it, etc. The ants would probably bypass the Devil’s Claw flower’s pollination mechanisms.
So I wonder sticky leaves and stems help the plant by preventing access to its flowers by ant nectar thieves.
The sticky hairs would also, of course, prevent at least some other herbivores from eating the leaves of the plant. I could see that some insect had been eating the Devil’s Claw leaves – there were leaf miner tunnels on some of them, and others had damage that looked like it may have come from caterpillars. So they weren’t 100% effective in the role of preventing insect herbivores. Perhaps the sticky hairs guard the plant from large herbivores? I could imagine that being true.
Here’s another strange thing. Why were the ants trying to travel through the Devil’s Claw gum in the first place? What was motivating them? If it were just their normal exploring, I’d expect to find the ants all the time, throughout the year.
Again – I have no evidence for this at all, but I wonder if the ants were responding to the odor of the flowers. They could smell that the flowers were in bloom, and were trying to get to the source.
I’ve not heard of ants homing in on a scent, though certainly their world is one of odors and pheromones. Maybe some ants also use scent to locate food sources?
Addendum – 2022-04-27: I’ve recently been reading Donald Schnell’s book, Carnivorous plants of the United States and Canada. He’s got a chapter titled “Other Possible Carnivorous Seed Plants”, and that chapter contains a section on Devil’s Claw.
He discusses work done by a number of people over the years on Devil’s Claw, starting with W.J. Beal in 1875. Beal noticed lots of small insects trapped on his Devil’s Claw plant. He tried putting fragments of beef on the plant, and found that they “disappeared”. E. Mameli, in 1916, saw dissolution and apparent absorption of hard-boiled egg white on the plant. Both of them concluded that the plant was carnivorous.
More recently, other researchers determined that the glands on Devil’s Claw don’t produce proteases, suggesting that the plants are not carnivorous.
My suspicion is that carnivory in Devil’s Claw is a side-benefit to the plant, and that the sticky stems evolved to protect the flowers from nectar thieves. If that’s correct, then you’d expect a lot of variability in “how carnivorous” the plants are, at least between populations.
I don’t know the answer for-sure, but I do so love this kind of thing – a situation where you observe something about the world, wonder how it works, and then find out that other people have had similar observations more than a hundred years ago.
Sources:
Schnell, Donald E. 2002. Carnivorous plants of the United States and Canada. 2nd Edition. Timber Press, Inc, Portland Oregon. ISBN 0-88192-540-3.
