For a few years, I’ve harbored a very cool (at least to me) natural history idea. But it’s a big technical challenge. The required fieldwork is never going to happen by me. So, I should write a blog post about it, right?
Bullet ants (Paraponera clavata) are one of the most charismatic creatures in Neotropical rainforests. My lab has done some work with them recently. These often-seen and well-known animals are still very mysterious.
Colonies of bullet ants nest at the base of canopy trees in rainforests. Aside from a stray ant here or there, these ants search for food in the canopy, going up and down trunks, vines, and lianas. This is where the mystery happens. Getting into the canopy isn’t easy, and if you do get into the canopy, you can’t exactly follow bullet ants around as they go from branch to vine to liana to branch while hunting and collecting nectar. There is a reason that most people who study the canopy look at plants. They won’t walk away from you. Or give you a sting that feels like bullet.
We can look at what these ants bring home to eat, and there are some papers on this (and another one, with a new twist, is in prep by us). But for the most part, when it comes to bullet ants, What Happens in the Canopy Stays in the Canopy.
Twenty-five years ago, it was observed that there is some smallish percent of bullet ant colonies that have actual nests in the canopy instead of the ground. This was determined from observations on the ground, though it does correspond to regular reports of climbers that there are entire nests in treeholes in the canopy. There clearly are substantial aggregations of bullet ant colonies up there. I recently heard of a recent treefall in which a colony was found in the cavity of a tree up in the crown, chock full of brood as well as adults. I’ve heard so much about arboreal nests of P. clavata from climbers, this leads me to suspect that there are far more nests in the canopy than is generally accepted. Maybe something else is going on.
My students and I have spent so much time watching what goes into and out of bullet ant nests, and we’ve seen some weird and uncommon stuff over the years. Once in a while, we see bullet ants carrying other workers down from the canopy into a nest on the ground. Social carrying is not uncommon in ants, but hasn’t been documented in bullet ants and it seems out of any understandable social context.
Another weird thing we see once in a while is that workers are carrying larvae up out of the nest. They are clearly bullet ant larvae, based on size and morphology. When I first saw it, I guessed they were hauling away a parasitized larva. But then others kept seeing it, too. It’s a regular thing.
I asked all of my students who watched bullet ant colonies how many times they observed the larval removal and social carrying. I went back to records to come up with a solid guesstimate about the number of person-hours each person spent watching colonies in the field. The number of larvae going up into the canopy was matched by an equivalent number of adults being carried down from the trees. The rate was about larval removal and one social carry per 12-24 hours of observation, including many hours at nighttime when there tends to be more traffic into and out of the nest.
Using some more Terry Erwin-style back-of-the-envelope calculation, based on estimates of colony sizes, growth rates and mortality rates, I realized that pretty much every larva produced by the colonies are going into the canopy. A colony that is steadily growing at a slow rate should make about 1-2 larvae per day.
The resulting inferred possibility: Bullet ant colonies keep nurseries in the canopy. Once larvae hit some developmental stage, then some or most of them get transported up to the canopy, in a nest protected and maintained by a bevy of workers. Once these workers emerge, they then are carried down to the nest on the ground by their nestmates, because they have not yet learned the route on their own.
Why would an arboreal nursery be advantageous? Larvae need to be fed frequently, and it would be energetically favorable to move larvae close to the source of prey and nectar in the canopy. The canopy is far warmer than the ground, and this would result in higher developmental rates, enhancing colony growth. This environment may be more favorable in some way to deter parasitism or predation. In particular, most army ant species don’t make it up into the canopy, and it may be more efficient to move brood to avoid army ants, which interact with bullet ants at least a few times per week. Why nest on the ground at all? Space in the canopy is probably limited. A whole mature colony of ants won’t easily fit. A more exposed nest in the canopy might result in more encounters with phorid fly parasitoids which that attack adults.
It makes sense. However, nature doesn’t always make overt sense. Sometimes, nature is more obvious to species other than our own. Is this story about arboreal nurseries in bullet ants actually true?
It would be easy to find out. You just watch some colonies, and mark workers with model paint. Do this while in a climbing harness with a rope secured over a good branch. When you see a larva being carried up, climb up and follow it. Then, you see where the larva goes, and if the arboreal nurseries indeed exist you’ll be led right to it. You gently explore the area to look inside whatever cavity or space the ants are inhabiting, perhaps with a telescoping flexible fiberoptic camera lens. Spending some time near this nursery, you can mark some of the workers and observe marked workers from the ground nest moving into this nursery on occasion. Then, on the ground, you can see nursery inhabitants also occasionally moving down into the nest, including those that are doing the social carrying of workers, who would be unmarked. You also would dig up multiple bullet ant colonies on the ground and check out the size distribution of larvae, expecting to find a consistent bias towards early instars.
Okay, that’s not so easy. At all. Even if I was an experienced climber, this is not a project I would be willing or able to take on. (If one of you wants to do this with me, or write a National Geographic grant to fund an expedition to discover the mysteries of the bullet ant, please drop me a line. I’ll leave the climbing to you and your crew, but otherwise, I would be thrilled to collaborate.)
But let’s just say, for argument’s sake, that I did this project. Let’s say that we showed conclusively that most bullet ant colonies have terrestrial nests, accompanied with satellite arboreal nests that are used as nurseries. So what? Where would I publish this? Ideally, it would be a natural history note in American Naturalist. (They did desk reject this natural history paper about bullet ants, though). Who would care other than super-duper ant enthusiasts? How would this change the world? Natural history truly matters, and I should do this project. But the ratio of [potential for academic gain]: [effort * expense * risk of massive physical pain] is miniscule. I’d love to do it, but it’s hard to justify and I have lots of other things about which I’m equally excited, so this is very low on the priority list. So I can sell the idea on a blog post and maybe it’ll get picked up.
If I’m really lucky, I might get to be involved. Watching from the ground with a little bottle of Testor’s Enamel and a video camera. And some antihistamine tablets for my collaborator poking into these arboreal nests.