Cane toads evolving to be bolder

Study finds possibly inherited behaviour traits in feral amphibians. Jeff Glorfeld reports.

Cane toads pioneers are much tougher than cane toad settlers.
Cane toads pioneers are much tougher than cane toad settlers.
Ian Waldie/Getty Images

The amphibian invaders that have spreading across ever-expanding stretches of northern Australia for more than 80 years are being led by an evolved cadre of the boldest, most daring individuals of their kind.

Cane toads (Rhinella marina), introduced to northern Queensland from Hawaii in June 1935 in an attempt to control two native sugarcane pests – grey-backed cane beetles (Dermolepida albohirtum) and Frenchi beetles (Lepidiota frenchi) – have formed into an “increasingly fast-moving invasion front” that “inevitably is dominated by fast-dispersing individuals, whose interbreeding produces even faster-dispersing offspring”, according to a new report by University of Sydney and Macquarie University scientists and published in the journal Royal Society Open Science.

Researchers looked at captive-raised and wild-caught toads, drawn from the invasion vanguard (known as the range edge) and populations long settled (the range core).

“A propensity to explore, take risks and engage with novel environments is likely to promote range expansion by stimulating dispersal,” says Jodie Gruber, the study’s author.

“These traits also enhance an individual’s ability to find water, food, shelter and mates in novel environments.”

Such behaviour has been linked to invasion success in several species, she notes, including the black rat (Rattus rattus) and various bird species.

The researchers noted that range-edge individuals are likely to be more aggressive than toads from the core. This is possibly because the conditions they experience foster such behaviours.

“Alternatively,” Gruber says, “the distinctive behaviour of invasion-front animals may reflect rapid evolutionary change. Behavioural traits are affected by genes, and traits such as exploration and boldness and aggression are heritable in some species.”

For example, front-line toads may benefit from reduced competition for resources, access to the best habitat and fewer predators, she adds.

In 2016 the researchers collected 68 adult cane toads (34 male, 34 female), half from around Cairns, Queensland, where toads have been established for almost 80 years, and half from a range-front population in Western Australia, where they have been present for fewer than three years. From these, they bred test subjects.

The study took into account factors such as the population source, and whether the animal was test-bred or wild-caught.

Each toad was tested in three different behavioural trials: willingness to explore a novel arena, preparedness to take risks (such as leaving a shelter to enter a test arena), and eagerness to check out a novel object.

They found that range-edge toads are more exploratory and willing to take risks than toads from long-colonised areas, and, importantly, that differences were evident even if the animals had been raised in captivity.

Gruber and her colleagues conclude that the differences in behaviour between toads in settled areas and those in the invasion vanguard was “a result of rapid evolutionary change”.

They add, however, that more work is required to measure how many of the observed traits are actually inherited, and to determine whether the changes in these traits during the course of the toads’ Australian invasion have been driven by adaptive or non-adaptive processes.

Jeff Glorfeld is a former senior editor of The Age newspaper in Australia, and is now a freelance journalist based in California, US.
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