Achilles spine: The arrow points to an anomalous growth in the spine of a dissected cane toad.
Credit: Greg Brown, University of Sydney
SYDNEY: Australia's seemingly invincible and pesky cane toad may have a previously unrecognised weakness: spine arthritis from hopping long distances on unusually long legs.
The stress of travelling great distances in search of fresh habitat seems to have made the amphibians susceptible to infection from arthritis-causing soil bacteria according to a new study.
The discovery may point to a way of slowing down the toads' further encroachment.
Long-legged
Last year, Rick Shine, of the University of Sydney, and his team reported that cane toads (Bufo marinus) at the frontline of the invasion were bigger and had longer legs than their slower counterparts. This evolutionary advantage has helped the toad hop great distances across Queensland and into the Northern Territory – five times faster than the cane toads first introduced in 1935.
"The guys at the invasion front move every night," Shine said. "They really are invasion machines." Weighing up to two kilograms, the lead toads have been known to travel up to a kilometre per night in the wet season. In just 70 years they've covered about 1 million square kilometres of Australia and are "knocking on the doorstep of Western Australia" Shine said.
But a larger size and longer legs has its downsides too. Shine and colleagues report today, in the Proceedings of the National Academy of Science journal, that about 10 per cent of the large toads have arthritis in their spines.
"Bigger, longer legs increase their ability to seek out new territory, but also puts pressure on the body with every hop," said Shine. "And with much of their energy going towards movement, less is put into their immune system, which may predispose the toads towards infection with the soil bacteria that precipitate arthritis."
Big mistake
South American natives, Cane toads were introduced to North Queensland in 1935 to wipe out the cane beetles that were then devastating sugarcane crops. The robust toads instead ate just about everything except the beetles they were supposed to feed on. They decimate frog populations and compete with other animals for food. From egg to adult, the toads are toxic to any animals unlucky enough to try and eat them. They've been labelled one of the world's worst invasive species.
While the toads' newly discovered weakness won't stop their progression, it may slow them down. "The good news is that they're not going to accelerate anymore, because they've probably achieved the limits of what their bodies can handle," Shine said.
"It is particularly important that the sick toads are just being found near the invasion front," commented ecologist David Skelly of Yale University in Connecticut, USA. "This links illness with rapid evolution of traits like leg length and body size that help [toads] move farther and faster."
By "asking an uncommon question," the authors will have a big influence on how the invasion of species is studied in the future, he said.
While cane toads are native
While cane toads are native to South America they were actually introduced from Hawaii in the 1930's.