Toxic Toads Evolve Longer Legs, Study Says

John Roach
for National Geographic News
February 15, 2006
New generations of cane toads in northern Australia have longer legs
than those in older populations, according to a new study.

The longer legs are allowing the toxic toads to spread even faster to new territory.

Cane toads (Bufo marinus) are native to South America and can weigh up to 4.4 pounds (2 kilograms). They were introduced to Australia in 1935 to combat beetles that were devouring sugarcane crops.

But the toads began snapping up other bugs instead and quickly started competing with and beating out native insect-eaters.

The toads are also toxic, which means most predators die after eating the amphibians.

Thanks to these favorable conditions, the toads currently occupy more than 390,000 square miles (1 million square kilometers) of the continent.

When the toads were first introduced, they spread at a rate of about six miles (ten kilometers) per year. Today cane toads advance more than 31 miles (50 kilometers) annually.

This faster pace is happening, at least in part, because toads at the forefront have about 10 percent longer legs than toads of earlier generations, said Richard Shine, an ecologist at the University of Sydney in Australia.

Shine's team will report the find in tomorrow's issue of the science journal Nature.

Toxic Toads

No documented extinctions are attributed to the cane toads, Shine said. But the animals dramatically modify the abundance and diversity of plants and animals in the ecosystems they invade.

Shine said his finding is the latest example of how natural selection complicates the conservation challenge presented by the invasive species.

"Basically what has happened by introducing the toads is it has created really strong evolutionary pressure both on the toads themselves and on animals that interact on the toads," Shine said.

(Read a National Geographic magazine article about evolution.)

For example, Shine and his colleague Benjamin Phillips previously showed that two native Australian snake species have evolved smaller heads and are no longer able to eat the toads, which carry a lethal toxin.

Other studies have shown that some would-be toad predators have altered their diets to exclude toads, while others have evolved resistance to the cane toad toxin, Shine said.

"These studies tell us a lot about the evolutionary process," said Jonathan Losos, an evolutionary biologist at Washington University in St. Louis, Missouri.

"Invading species are a huge problem, and cane toads are a classic example of that," he added. "But they also represent an inadvertent evolutionary experiment, the sort of experiment you couldn't [normally] conduct."

Rules and regulations prohibit scientists from purposely confronting native species in the wild with a non-native competitor or predator to see how natural selection works, he explained.

Rapid Evolution

The evolutionary processes spawned by the cane toad invasion have occurred in a span of just 70 years. This adds to evidence from the past two decades that populations can adapt quickly when selection pressure is strong.

"We're taught evolution occurs over these very, very long time frames. But in systems like these, it's incredibly fast," Shine, the study co-author, said.

According to Losos, the unusual aspect of the toad leg length adaptation is the mechanism that drives it.

In most instances rapid evolution occurs when an organism enters a new environment and some variation that was previously irrelevant becomes favored. That variation is repeatedly selected until it becomes more common, he explained.

In the case of the cane toads, longer legs make the toads faster, and the fastest toads are always at the invasion front. The lead toads mate, passing their long legs to their offspring.

As long as there is no disadvantage to being the first into a new territory, this process should allow the toads to "evolve faster and faster rates of movement," Shine said.

Cane Toad Management

According to Shine, as scientists learn more about cane toad biology, they can devise strategies for eradicating local populations, such as changing the character of a breeding pond or staking out toad migration routes.

But the toads are likely to be permanent fixtures in Australia and will continue their spread, he said.

While Shine is optimistic that ecosystems will adapt, "there may be some parts of native systems that don't and, in time, will go extinct," he said.

"One message from the work," he added, "is to try to stop invasive species, you probably ought to start as soon as you get a chance. The longer you let it linger, the more formidable the adversary will be."

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