Cane toads in the Northern Territory and Western Australia have evolved “very very rapidly” to hop in a straight line and cover up to six times more ground than distant cousins back in Queensland, scientists say.
Research published in the Proceedings of the Royal Society of London journal shows “a new kind of evolution” and a phenomenon dubbed “spatial sorting”, according to the article’s co-author and University of Sydney professor Rick Shine.
“The front has accelerated from about 10-15 kilometres per year to about 60 kilometres per year over the time toads have been in Australia,” Professor Shine said.
“To move at that rate, toads have to behave in very strange ways – ways that no other frog has before.
“All you get at the front are the offspring of the fastest toads who were themselves the offspring of the fastest toads who themselves were the offspring of the fastest toads.
“Genes for fast dispersal end up concentrated at the invasion front.
“This is evolution through space rather than time.
“It’s quite different to the sorts of things (Charles) Darwin talked about.
“Cane toads are the best example in the world of this new evolutionary process.”
‘Giant Brazilian frog’ on the march: Toad Hall
The headquarters of cane toad research is located an hour out of Darwin among mango farms and goose hunting reserves, and past a notorious bikie hangout as well as a military radar facility.
Officially it is called the University of Sydney Tropical Ecology Research Facility, but its handful of scientists call it Toad Hall.
Here, tadpoles breed in vats of water under flywire. They have been collected from all over northern Australia to document the widening cane toad genetic spectrum.
Professor Shine has been studying toads since the invasion reached Darwin in 2005.
“We expected they would behave as they had in Queensland,” he said.
“We put transmitters on them and lost all our toads because they went hurtling over the horizon.
“The early days were pretty much strap yourself in for wild ride. We began to understand the 80-year footrace across Australia has created an animal very different to the one found on the Queensland coast.”
All the cane toads in Australia – there are now estimated to be as many as 200 million – are descended from 102 toads released in June 1935 in northern Queensland.
The Bureau of Sugar Experiment Stations intended them to control a native cane beetle. There is no evidence the toads had any impact on the cane beetles.
“You’ve got a giant frog from the Brazilian rainforest that has triumphantly marched across the driest continent on the planet,” Professor Shine said.
“They’re incredibly flexible. They’re surmounting a range of challenges we never thought they could deal with. There’s a very real sophistication to a lot of the things that they do.”
The introduction of cane toads to Australia, which has caused massive declines in a number of native species, has also been recognised as a rare opportunity to document adaptation through evolution.
“When cats and foxes were released it was a long time ago,” Professor Shine said.
“Even for more recent invasions we don’t know how quickly they spread.
“But people care about cane toads and recognise them. We have very good documentation of how the toads have spread across the continent.
“As an evolutionary biologist, we are usually stuck with looking at the slowly changing results of thousands of millions of years. Here we’re looking at evolution in action.”
Boffins weaponise toad research
The direct practical outcome of the toad research has been a more effective toad trap.
Researchers have learned the alien invaders communicate with each other through a sophisticated exchange of pheromones.
“Native tadpoles don’t seem to eavesdrop on that,” Professor Shine said. “They don’t even seem to know the chemicals are there.
“For the last few million years toads in the native ranges have been competing with each other.
“It turns out they have worked out some pretty clever ways of doing that.
“When the cane toad female lays her eggs in the pond, if there is any existing in that pond they detect the eggs and race across and kill them and eat them.
“We can take the poison from adult toads from the shoulder glands and put that in a funnel trap.
“It catches thousands and thousands of cane toad tadpoles very quickly and doesn’t catch anything else.
“Cane toads 20 million years ago worked out quite complicated ways of talking with each other and competing with each other. We’re just starting to get glimmerings of how that all works.
“We can take the toads’ weapons and turn them against the toads.”