Sam McCann wasn’t always into toads.
By her own admission, the 24-year-old University of Sydney biologist started late. “In herpetology—[the study of reptiles and amphibians]—you have all these boys who start early with pet lizards and frogs.” Growing up in a big city, she says she never imagined a career in biology until she took the class in high school.
“My teacher was this little Indian woman. She told us to draw what a biologist looks like. Everyone in the class drew a bearded old, white man. Then my teacher said, ‘Look at me. Nobody drew me and I’m a biologist.’ That’s what made me believe that I—as a girl—could become one.”
Inspired, Sam went on to study biology at the University of Sydney, where a Tropical Ecology course took her to Mary River National Park in Australia’s Northern Territory, 90 miles east of Darwin. A recent cyclone had flooded the landscape.
“I just remember being out in the middle of the night during the rainy season. I was holding my toad, looking for the spot to release it, all while swatting mosquitoes and trying not to be eaten by crocodiles. And I just remember thinking, 'This is so cool! I want to do this for the rest of my life!'”
This is what Barrington calls “selling the finish line.” Mentors showed Sam what was possible.
Now she is three months into her Ph.D. program at the University of Sydney’s Tropical Research Facility in Darwin. We drove to this tiny outpost in the Australian bush to see Sam’s research firsthand. Together with her teammates on “Team Bufo”, Sam is using biology—and a good deal of ingenuity—to stop one of the world’s most invasive species: the cane toad.
What is an Invasive Species?
When the Flying Classroom landed in Australia, customs officers fumigated the cabin with disinfectant and inspected all of our belongings for evidence of termites and other wood-boring insects. They weren’t messing around. Brian had bought two hand-carved picture frames in Bali. In one, they found tiny little pinholes. They confiscated it and told Brian he could pick it up again when he left the country. I have a feeling he will never see that frame again.
“Australia has some of the strictest customs in the world,” Sam explained. “It’s an island. When things get transplanted here, it’s a big deal.”
Cane toads were brought to Australia 80 years ago from Central and South America in order to eat cane beetles, which we’re eating sugar cane. The plan didn’t work. Not only did the toads not eat the beetles, with no natural predators, they began spreading like wildfire across the continent. The toads were introduced on the eastern coast in Queensland. Since then, they have crossed half the continent. Now they’re spreading southward into New South Wales and Southern Australia. As Sam noted, “it’s a very good example of why you shouldn’t mess with an ecosystem.”
We were standing out back behind the Tropical Research Facility. It was 95°F in the shade. Brittle leaves littered the red, dusty ground. Sam reached into a large white tub and pulled out the fattest toad I’ve ever seen. It was easily the size of a grapefruit. She held the toad with her thumb on its back and her palm wrapped around its hind legs. Its floppy body squished around in her hand like a water balloon.
“As we say in the ‘hood,” said Barrington, “that is a swoll toad.”
Sam explained that this was a female and that she was puffing herself up to look more threatening. “When you tell people that you’re a toad biologist,” she said, “they start telling you how much they hate them.”
“Well, what is a cane toad good for?” Barrington asked.
Sam was silent.
Eventually, she said, “In their natural environment in Central or South America, they’re part of the ecosystem’s food chain. Snakes evolved to eat them. Here in Australia, that hasn't happened yet. When cane toads enter an area, the populations of crocs, snakes, and lizards go straight down. They’re definitely one of the world’s worst invasive species.”
But why? What makes the cane toad so destructive? For one, there are just so many of them. A female can lay 20,000-40,000 eggs in one clutch and can breed multiple times a year. Multiply that by thousands of toads and you’ve got more than an invasion; it’s an infestation.
Their population remains unchecked because anything that tries to eat them dies. The puffy glands behind the eyes of a cane toad contain a toxin strong enough to kill a crocodile. The toxin goes straight to the heart and stops it. In fact, it works so fast that Team Bufo has found dead snakes and crocodiles with toads still in their mouths.
“Do birds eat cane toads?” Barrington asked.
“Kites will,” she said. “They’ll swoop down and catch them and flip them over and only eat the guts, not the poison parts.”
Other local animals are being trained not to eat them. For example, northern quolls—an Australian wild cat species—were getting wiped out. So researchers started feeding sausages with traces of cane toad in them to quolls in areas where the toads hadn’t yet migrated. The quolls would eat them, get sick and learn to never go near that scent ever again.
“How long will it take before animals in Australia will be immune to the toxin?” Barrington asked.
“Not any time soon,” said Sam. “That kind of evolution takes generations. But we are seeing another type of evolution here. To eat a cane toad, you need a pretty big mouth. Snakes with big mouths are eating the toads and dying from the poison. Through time, what we end up with is a lot of snakes with smaller mouths because they can’t eat the toads.”
“Wow! Who thinks of that?” said Barrington.
The great scientist never made it to northern Australia, but his theory of evolution certainly did. “Usually we talk about evolution over time,” Sam said. “This is also evolution through space.” Toads with longer legs can hop further and faster. As they cross Australia, all of the toads with longer legs gradually move to the front. Toads with shorter legs are left in the dust. “There’s always variation in nature,” said Sam, “but when challenged by an environment like the Australian outback, toads with longer legs perform better.” This is what Charles Darwin called “natural selection.”
In Australia, survival is not about being the biggest, fastest, or strongest; it’s about who can best adapt to the environment. In that respect, the cane toad is king.
Halting the Invasion
In ideal conditions—when it’s warm, with lots of food and little stress—a cane toad can grow from egg to tadpole to a tiny toad called a metamorph in two weeks. In poor conditions, the process can take up to two months.
As Sam’s colleague Michael Crossland explained to me, “taking a long time is not a good strategy. The longer you are a tadpole, the greater risk you’ll be eaten or your pond might dry up.”
In their lab, Team Bufo is raising hundreds of toads in various stages of development. Cane toad tadpoles are very competitive. If they’re in a pond with other cane toad eggs, they’ll start eating the eggs. In fact, the eggs actually release a chemical that lures tadpoles to come feed on them. “It’s a really dumb strategy," said Michael. “We don’t yet understand why they release a chemical to attract tadpoles to come eat them.”
Yet this ‘dumb strategy’ gave Team Bufo an idea. What if they could trick the tadpoles with their own chemical communication system? The toxin in adult toads is like a fruit punch of chemicals. At least one of those chemicals—let’s call it banana—is very similar to the attraction chemical exuded by the eggs. So much so that Team Bufo could interchange the two.
Sam led us into the workshop where she had set up a series of long, shallow plastic tubs. At one end of each of the tubs was another plastic container with a red funnel opening (above). “These are our traps,” she said. “First we’ll add water to the tub. Then we’ll put about twenty tadpoles in the water.”
Next she had Barrington pour a bit of banana—the Bufo toxin—into the water in the trap area. Then we waited. It didn’t take long. One by one, the tadpoles changed course and started swimming toward the red funnel. Fifteen minutes later, every single tadpole was in the trap.
“We are manipulating their own desire to eat toad eggs,” said Michael.
In their experiments in local ponds, Team Bufo has caught and removed thousands of tadpoles, 90 to 100% in some cases. “If we trap continuously each day for a week,” Sam said, “we can capture every one.” No tadpoles means no new toads. Not bad for the first three months of her program. A few days later, she would fly back to Sydney to present her findings. The next step for Team Bufo is to identify the precise makeup of the attraction chemical so they can make a synthetic version for use around the country.
“It’s what we love about collecting data,” said Sam. “You collect it, study it, and it tells you a story.”
Back in the lab, the February 2009 issue of National Geographic lay face up on a table. The cover headline read, “What Darwin Didn’t Know: Evolution Now.”
The cover image was a frog.
NEXT EXPEDITION: Sydney, Australia