Quick-moving toads take the straight and narrow path

Since 101 cane toads were introduced to Australia in 1935 — in a misguided attempt at pest control in sugarcane fields on the northeastern coast of Queensland — the invasive amphibians have quickly spread west and wreaked havoc. The toads are toxic, and many native animals have died after eating them.

One curious aspect of the toad invasion has been its speed, which has been increasing. Soon after their arrival, cane toads spread at a rate of about 10 kilometers per year. Now that rate is  55 kilometers annually. Scientists have documented some interesting features that have allowed that increase in invasion speed: Toads at the front line of the expansion have longer legs and are more active at night. The toads also move in a straighter path, according to a new study, published October 8 in the Proceedings of the Royal Society B.

“It’s not just an issue of toads evolving longer legs, and running faster — they have also changed the way they move. Somehow or other, evolution has furnished the invader with a compass,” University of Sydney herpetologist Rick Shine said by e-mail.

There are lots of different ways that an animal can move faster, but often those ways come at a cost. A toad might need to eat more to grow and power its longer legs, for example. Moving in a straighter path, though, may be more effective because there wouldn’t be such a big trade-off, Shine’s group proposes.

The team worked with cane toads at a farm near Darwin, Australia. When the toad invasion arrived there in 2005, the researchers began radio-tracking adult cane toads for five days at a time over the next 10 years. This let the scientists track the toads’ movements. The first cane toads to arrive tended to move in straighter paths than those living there years later, the scientists found.

“Invasion-front toads appear to treat the landscape into which they are moving as one that needs to be traversed quickly and efficiently,” the researchers write, “whereas later-arriving toads may see the landscape and its resources as something to be explored and exploited at leisure.”

In another experiment, the researchers collected frogs in 2006 from four sites across northern Australia, spanning a distance of 1,600 kilometers. They brought those toads back to Darwin and bred them. Generations of toads later, in 2008, the researchers attached radio trackers to the animals and released them, again measuring their movements over five-day periods.

The farther away that the cane toads had been collected from the invasion front, the less straight they moved. Similar results were found for their descendants, which showed that this directionality is probably rooted in the toads’ genes.

“Thus, the invasion process has evoked a significant evolutionary response in a parameter critical to dispersal rate: the degree to which an individual consistently moves in some specific direction,” Shine and colleagues write.

By moving in straighter and straighter paths, the toads can invade new territory faster and faster as they spread.

“Conservation managers need to predict when invasions will arrive in the areas they are protecting — and to do that, they need to understand rates of invasion-front acceleration,” Shine says. “Our paper shows that these kinds of behavioral changes can happen rapidly, and that we should expect most invasions to accelerate.”