When the first rains arrive in the forests of northwest Costa Rica, yellow toads gather in the hundreds for an intense but short-lived breeding season. This year, however, they will be joined by a handful of intruders called “RoboToads.” These motorized amphibians—3D printed in a lab—could help uncover the secrets behind the real toads’ unusual mating ritual. And that’s just one of many mysteries that 3D-printed animals are starting to reveal.
“3D printing is really advancing the questions that we’re able to ask as field biologists,” says Daniel Mennill, who has been researching the toads for a decade alongside Stéphanie Doucet, both of whom are behavioral ecologists at the University of Windsor in Canada.
Several years ago, the husband-and-wife duo discovered that during the breeding season—which lasts little more than a day—male toads change color from their normal dull brown to a striking lemon-yellow. Using handmade clay models, they figured out why: The brief switch to yellow helps the males identify females, which remain brown. But now the team wants to figure out how the females choose among similarly colored males, a much harder task to do with crude clay models.
Enter the RoboToad. Created by Mennill and Doucet’s Ph.D. student Lincoln Savi, the toads were painstakingly sculpted into exactly the right shape and texture with computer software, after being scanned in from a handful of photos. Savi 3D printed several copies and painted them to look like males, some bright yellow and some a dull chartreuse. Motors make the models move around randomly, giving the illusion of a living creature. With the RoboToad, the team no longer needs to sculpt individual clay models—or struggle to get them painted the precise shades they need for their research.
When the rains begin, the team will set up arenas containing two 3D-printed male toads of different shades, and see which one a wild female chooses. The brief breeding window means that time is of the essence, and Mennill says the team is already out in Costa Rica, “sitting there staring up at the sky [and] waiting for the first rains to arrive.”
Back in Canada, biologist Grégory Bulté, of Carleton University in Ottawa, is using 3D printing to answer a question that’s been bugging him for more than a decade. Bulté studies northern map turtles, whose females can grow to twice the length of males. He wondered whether males might be attracted to larger females, but the turtles’ skittish nature—and the fact that they mate on the floor of the lake—made observation tricky.
Bulté’s team printed two 3D models of female turtles, identical in every aspect except size, and placed them a meter apart on the lakebed, with cameras rigged up to record how wild males reacted. As predicted, the males attempted to mate with the large model more often than the smaller one, the researchers report this month in Animal Behaviour. Using live animals in this sort of study would introduce a host of other variables that would be difficult to control, Bulté says. For example, one of the animals could be related to the male, which could affect his choice. In contrast, 3D printing is “almost an ideal system.” And multiple copies can be printed relatively cheaply. That’s a far cry from past eras, when an artist or taxidermist needed to create each individual model.
3D printing is also allowing scientists to create models at a much finer scale, says ornithologist Mark Hauber at the University of Illinois in Urbana. Hauber studies a behavior known as brood parasitism, in which birds lay eggs in the nests of other species, leaving the unwitting foster parents to rear their chicks. Previously, researchers studied host birds’ reactions by placing plaster and wood models of parasitic eggs in their nests. But using 3D printing, Hauber’s team created far more realistic looking cowbird eggs, which allowed his team to examine whether variations of just a few millimeters in size influenced robins’ decisions to throw the parasitic eggs out of their nests.
Using these new models also means people can easily replicate experiments, Hauber says. He has made the digital models of his eggs freely available online, so anyone can print off their own copy and reproduce the research.
But the number of groups using 3D printing is still small. That may be because beginners are intimidated, says Jocelyn Behm, an ecologist at Temple University in Philadelphia, Pennsylvania, who studies how predators respond to invasive lizards. Rather than bringing a live exotic species into an ecosystem, she is using 3D-printed models. To help other researchers, Behm posted last month about her own experiences in a preprint on bioRxiv that also reviews recent papers using the technique. “I really thought 3D printing was difficult … but then once I got into it, I realized it’s not that hard,” she says. The key to using the technique, says Bulté, is collaboration. “If people don’t talk to each other, then [they] might think it’s less accessible than it really is.”