Shooting down a widely held scientific myth

Bullet ant image by Benoit Guenard

Science is full of ideas that people somehow accept to be true, just because people say it’s true. We’ve all heard wonderful just-so stories that are waiting to be dispelled by data.

Let me tell you about three myths.

The first myth was that gastric ulcers are caused by stress. All kinds of medical treatments were predicated on this notion. When a researcher figured out that gastric ulcers were caused by bacterial infection, it was considered so outlandish that he had to infect himself to convince the medical research community. (In 2005, the Nobel Prize was awarded for this finding.)

For the second myth, consider the three-toed sloth. For about a century, it’s been said they specialize on Cecropia leaves. One twist on the story is that that the trees are tastier to sloths because they have weaker chemical defenses, because the plants are defended by ants. Then, in the 1970s, two biologists radio-tracked sloths for a couple years in Panama and found that yes, they eat Cecropia, along with many other plant species. If you track them with radio collars, then you get to see that they are not Cecropia specialists.

The people who radio-tracked the sloths did not receive a Nobel Prize.

In fact, their results haven’t permeated widely, and the story is still out there that sloths are Cecropia specialists. Why is that? I guess that’s because when you look for sloths, you find them in Cecropia trees a lot. The open architecture is great for sloth spotting. But the radio-tracking data show they don’t spend an inordinate amount of time in Cecropia, compared to other options. Unsupported myths die hard.

More than 40 years after the radio-tracking study came out, a brand new paper just claimed that three-toed sloths ingest almost Cecropia “almost exclusively.” Did they have support for this? They cited three papers. One was a 1978 chapter on habitat selection (not diet), the second didn’t mention Cecropia at all, and the third reported that three-toed sloths had a highly variable diet and fed on fifteen different species (including a bit of Cecropia)in a plantation. Is this a problem? You bet it is, when that paper is about the gut microbiome of three-toed sloths!

Author Erica Parra in the field. (Doing a project not related to bullet ants. She’s multi-faceted.)

The third myth is featured in the most recent paper from my lab, a project done with Erica Parra. We dispel the notion that ants in rainforest canopies are dominant because they have a high carbohydrate diet.

It’s a common idea I hear all the time — not from ant specialists, but it’s a thing that people say. Ants in the canopy eat a lot of nectar. This sugar gives them an excess of energy, and they use this energy to kick the asses of other ants and be dominant. Canopy ants are on a sugar high. It’s a nice story, but is it true?

If you look in the trees of rainforests, you’ll see them covered with aggressive and dominant ants. Several decades ago, it was imagined that these canopy-foraging ants were fearsome predators, stripping the forest of herbivorous insects. Then, about 20 years ago, some compelling work showed that the aggressive and dominant ants in rainforest canopy are almost exclusively sugar eaters. Sure, they take an insect when they can, but the bulk of their diet is sugar produced by the trees. This is known.

Canopy ants on a sugar high makes sense. The animals are clearly Nitrogen-limited with high carbohydrate diets. The ants haven’t converted this sugar into fat (apparently?) so they must be burning it by running around a lot and kicking butt. That explains why they’re so aggressive and ecologically successful. Right?

So we did an experiment evaluating some mechanisms by which canopy-foraging ants could use sugar to become dominant. They could use this sugar to aggressively defend their nests. They could use the extra sugar to go out and actually catch more prey. Or they could use the extra sugar to send more individuals out of the nest to forage and patrol.

Or, maybe, they just are collecting the sugar when it is there, and don’t collect the sugar when they can’t collect it.

This isn’t as easy as it sounds. Canopy ants tend to be tiny and quick-moving, and nest in cavities up at the top of the rainforest. Manipulating sugar, counting them, and measuring stuff involving these little critters is a total bear. So instead, we chose to study a dominant canopy species that is huge in body size and happens to nest at the base of the canopy trees. These are bullet ants. Our lab’s been working with them for a little while now. Here’s a study we’ve done on the field ecology of the Nitrogen-cycling microbes in their guts, and here’s another paper about how bullet ants try to handle protein-filled liquids as if they are solids. Their brains are apparently hard-wired that way. And here’s an idea about their nesting biology that I’m not prepared to test.

It turns out that when bullet ant colonies are taking in more sugar, they’re not using it to become behaviorally dominant. They’re just collecting the sugar because it’s there. These findings suggest that ants rule the canopy because they’re simply accessing a resource that other animals aren’t accessing. It doesn’t make them kick butt, it just allows them to exist in a place where they otherwise wouldn’t exist.

So, the next time someone implies that sugar makes canopy ants dominant, ask them for some evidence.

Are you not yet 100% convinced that this suggestion is applicable to all species in all environments? Great! Neither are we. It would be wonderful to see more work on the topic. There’s still a lot of mystery out there.

How this project happened in a teaching-focused university

This project was conducted by Erica over the course of a summer at La Selva Biological Station, in Costa Rica. She’s a student in my lab, who at the moment is shopping around for Ph.D. programs. My lab is running a long-term demography study of bullet ant colonies, piggybacking on prior work from another student in my lab, so we have a lot of marked colonies to work with. Because bullet ants tend to be far more active at night than in the daytime, especially when it’s been rather dry, she did this fieldwork at night. (Some people even call them nocturnal. That’s just silly though.) Working out in the forest with bullet ants all night isn’t the easiest or most fun to thing to do.

Erica’s talk about this project at the 2015 Entomological Society of America meeting won the Audience Choice prize in the Three Minute Presentation Competition. I think it’s rockin’ that she got this honor as an undergraduate competing mostly against grad students.

Here’s the paper:

McGlynn, T.P. and E.L. Parra. 2016. Mechanisms of carbohydrate-fuelled ecological dominance in a tropical rainforest canopy-foraging ant. Ecological Entomology 41. DOI: 10.1111/een.12294

You can also find it via my page on Researchgate.

The project was part of the NSF-supported Tropical Ecology Mentorship Program of CSU Dominguez Hills.