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Leggy cane toads lead the march

Cane toad

The cane toad front is moving across Australia five times faster than it was in the 1940s (Image: Reuters/David Gray)

The invasion of the dreaded cane toad across Australia has taken a new turn with research showing the toxic toads are evolving longer legs that allow them to move faster.

This means they can encroach even faster on new territory, biologists say in today's issue of the journal Nature.

Cane toads (Bufo marinus) were introduced to Australia more than 70 years ago to control insect pests in cane fields. And their toxic nature and invasiveness have made them a major ecological menace.

The toads have expanded to cover more than a million square kilometres of the continent and are now progressing at a rate five times higher than in the 1940s to 1960s.

Professor Richard Shine from the University of Sydney and team stationed themselves at the invasion front 60 kilometres east of Darwin.

They then waited for the toads, which can travel almost two kilometres a night, to arrive.

Measurements revealed that the 'super athletes' at the front of the pack had longer legs than those at the back.

Why are their legs longer?

Shine says there are two "possible" and "probable" explanations as to why toads at the front have longer legs.

The first is the conventional evolutionary reason, that there are advantages to being at the front.

Toads with longer legs have the first access to virgin territory full of juicy grubs, so the evolving species is favouring this characteristic.

The second explanation is more immediate and practical.

"These guys sort themselves out spatially because the only ones that can be at the front are the fast ones," says Shine.

Put simply, if the faster ones with the longest legs were at the back, they would trample the shorter slower ones ahead.

"So even if there is no evolutionary advantage to being at the front you are still going to end up with the front guys being the fast movers," he says.

Understanding the enemy

Shine says understanding the enemy is the first step in combating the toads in the future.

"People tend to take a very straightforward view that cane toads are bad and we just have to work out ways to kill lots of them. But history is telling us that that doesn't work that well," he says.

"What we need to do is find what kind of animal toads really are, what bits of the landscape they use, how they use it and what they depend on."

Tags: biology, zoology