Relentless invasion of the cane toad continues

When people say they love all creatures great and small, they’re surely not thinking of the warty, poisonous cane toad that has wreaked havoc across tropical Australia since its ill-fated introduction.

It gets worse. The ugly toads (Rhinella marina) are evolving in leaps and bounds according to scientists who’ve had a closer look at their bodies and movements.

A hallmark of successful invasive species – one of the greatest threats to the world’s ecosystems and biodiversity – is their ability to adapt and spread rapidly without the checks and balances of their native environment.

Cane toads are no exception. They were brought to Queensland from Puerto Rico and Hawaii in the mid-1930s, despite protests by scientists and naturalists, to eat beetles that damaged sugar cane crops, and have since spread like wildfire.

Australian researchers have now found that those on the frontline evolved to move differently and thus bolster their invasion, as reported in the journal Proceedings of the Royal Society B.

The study shows “there is a definite link between the enhanced locomotor performance and different body shapes that we see at the invasion front of cane toads,” says co-author Marta Vidal García from the Australian National University.

“While this might seem obvious, it has never actually been proven before.”

It explains why cane toads are so successful as an invasive species, she adds, and have dispersed substantially faster in less than 100 years than would be expected in a typical evolutionary scenario.

Lead author Cameron Hudson, from the University of Sydney, and colleagues used high-speed videos to measure the distance, height, speed and angles of take-off and landing of toads from populations that stay home or are on the invasion-front.

They tested 195 wild-caught adult toads from four invasion fronts and three populations of 137 captive individuals.

In jumping and raceway trials, the critters were prodded with a blunt pole to encourage them to hop. Those that obliged had their time and number of hops recorded and morphological characteristics measured.

Results showed toads that had dispersed more had longer limb bones, larger hindfeet and smaller front feet relative to their body length, and these changes were correlated with their movements. Their directions were also more consistent, with less meandering. Those that hadn’t travelled were more like their South American ancestors.

“In sum, dispersal-relevant traits have shifted profoundly during the toads’ Australian invasion,” write Hudson and colleagues.

Their study adds to previous work showing changes in cane toads’ physiology and purposeful direction, greater boldness and risk-taking behaviours.

Both studies have bad news – their babies inherit these new adaptations

Enhanced movements give species several advantages. By dispersing more quickly, they can more efficiently catch prey and escape predators, access new resources and escape competition.

The new study lends weight to theories that invasive species evolve features to improve their locomotion.

Voles are thought to have evolved longer feet to swim more efficiently and reach remote Swedish islands, for instance, while invasive bush crickets on Britain’s front line have larger wings, which is believed to have improved their flight ability. Even invasive pine trees have lighter seeds which might help them travel further on the wind.

As far as cane toads, the study confirms they “have evolved rapidly during their Australian invasion – they now move in ways that make them better at covering long distances,” says senior author Rick Shine.

“As a result, toads are able to invade new areas at ever-increasing speed.”