Dozens of scientists recently glued fake green caterpillars onto plants around the world in an unusual study to see how the caterpillars' risk of getting eaten varied from pole to pole.
Any ant, slug, lizard, bird or beetle that attacked the soft clay caterpillars left telltale bite marks that were later analyzed by a lab in Finland.
It turns out the risk of being eaten was eight times higher at the equator than close to the poles, according to a newly published report in the journal Science.
"I mean, it sounds kind of like child's play, I realize," says researcher Liz Nichols of Swarthmore College. "But this kind of massive, simple, standardized technique is really powerful when you can implement it at a global scale in a really well-replicated way."
The idea for the study came when researcher Tomas Roslin at the Swedish University of Agricultural Sciences was trying to use fake caterpillars to study predation risk in Greenland. He says it's an old technique that's been around for a long time. But he wasn't having any luck — predators just weren't attacking the dummy caterpillars.
When he talked about his problem with his colleague Eleanor Slade at the University of Oxford, she told him she'd had a lot of success using them in rain forests in Borneo. The pair realized that a caterpillar's risk of being eaten might vary around the globe. And they figured they could find out by asking scientific friends, and friends of friends, to help with a little bit of fieldwork.
"We've known for a really long time that there are more species in the tropics than there are in polar regions," says Will Petry, a biologist with the Swiss Federal Institute of Technology who was on the research team. "But we don't have as good of an idea of the geography of interactions between species."
That's why Petry was eager to be a part of the study. He was doing research in California when the fake caterpillars arrived in the mail. He glued them onto plants, including cacti, then waited a few days. Then he carefully placed them into individual tubes, to preserve any bite marks in the plasticine clay, and mailed them back to the lab in Helsinki, Finland.
Nichols also got a box full of the fake critters, and colleagues helped her glue them on leaves at her research sites in Brazil.
All in all, nearly 3,000 dummy caterpillars were deployed in 31 sites from the Arctic Circle to southern Australia.
It turned out that not only was the risk of predation less at the poles, but also the risk decreased at higher elevations. "If you go up a mountain slope at the equator, you would find the same type of gradient — a decrease in the risk of getting eaten," Roslin says.
The pattern was driven by small predators like ants rather than by birds or mammals. "It's actually reinforced the idea that insects are far more important," says Yves Basset, at the Smithsonian Tropical Research Institute in Panama.
He notes that in addition to predators that might eat them, caterpillars are plagued by parasitizing wasps that wouldn't be detected by this study, and he thought that might muddy the predation pattern. "I was expecting this kind of result. I wasn't thinking it would be so clear," Basset says. "So this was a welcome surprise."
Phyllis Coley, a biologist at the University of Utah who was not part of the research team, told NPR that she'd seen the study: "I think it is great."
She says this global and controlled study strongly supports higher predation on caterpillars at lower latitudes and elevation.
Petry notes that "there are some migratory butterfly species, like the monarch. And one reason that they might migrate could be to avoid predators."
What's more, scientists can now look to see whether caterpillars tend to focus their defenses on insects, rather than on mammals or birds.
AUDIE CORNISH, HOST:
Sometimes good research can get done with a tool so simple a child could do it. For instance, scientists around the world recently set out thousands of fake caterpillars made from clay. NPR's Nell Greenfieldboyce explains why.
NELL GREENFIELDBOYCE, BYLINE: If you've got a preschooler at home who makes little worms out of Play-Doh, you should easily be able to imagine the fake caterpillars that scientist Tomas Roslin was putting out in Greenland. He wanted to know a little critter's risk of getting eaten. And this is an old technique.
TOMAS ROSLIN: Somebody came up with it, like, decades ago.
GREENFIELDBOYCE: He says of a bird or lizard or whatever tries to bite the caterpillar, telltale beak or tooth marks get left in the soft clay. Trouble is, at his research sites up north, it wasn't working.
ROSLIN: We tried it one summer, and very few of them ever got eaten.
GREENFIELDBOYCE: He's a researcher at the Swedish University of Agricultural Sciences, and he talked about his problem with another scientist, Eleanor Slade at the University of Oxford.
ROSLIN: My friend Eleanor told me that, oh, she has been using the same technique in Borneo, and everybody gets eaten.
GREENFIELDBOYCE: The pair realized that maybe the risk of getting eaten, at least if you're a caterpillar, is actually higher in the tropics than near the poles. To test this idea, they enlisted 40 researchers in 21 countries from the Arctic Circle to southern Australia, including Liz Nichols, a biologist at Swarthmore College. She was doing research in Brazil.
LIZ NICHOLS: And then a box arrived with bright green gummy-looking caterpillars.
GREENFIELDBOYCE: Her team glued them onto leaves using glue and instructions that came in the box. A few days later, they shipped them back to a central lab.
NICHOLS: I mean it sounds kind of like child's play, I realize. But this kind of massive, simple, standardized technique is really powerful when you can implement it at a global scale in a really well-replicated way.
GREENFIELDBOYCE: Biologists Will Petry with the Swiss Federal Institute of Technology set out his dummy caterpillars at field sites in California. He was eager to be part of this study.
WILL PETRY: We've known for a really long time that there are more species in the tropics than there are in polar regions, but we don't have as good an idea of the geography of interactions between species.
GREENFIELDBOYCE: What the scientists found was a striking pattern. If you're a caterpillar at the equator, your chance of getting attacked by a predator is much higher than it would be if you were closer to the poles - about eight times higher. Petry says all of this could help explain why certain creatures do what they do.
PETRY: There are some migratory butterfly species like the monarch, and one reason that they might migrate could be to avoid predators.
GREENFIELDBOYCE: A detailed analysis of these results, a far cry from child's play, appears in the journal Science. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.