By Rebecca Morelle
BBC News science reporter
The toxic cane toad in Australia is evolving into an "eco-nightmare" capable of covering huge distances, a study in the journal Nature reports.
Cane toads can move up to 1.8km in one night
Scientists say the species Bufo marinus is developing a leggier, faster-moving form that is now hopping out rapidly across the continent.
The toads were introduced 70 years ago to control pests, but have since wrought havoc on indigenous animals.
They kill snakes, lizards, water birds - even crocodiles and dingos.
When harassed they secrete poison carried in two sacs behind the head which is lethal to a potential predator within minutes of being ingested.
The amphibians, which can weigh up to 2kg, are now found in an area covering over a million sq km.
They were first introduced to Queensland from South America in 1935, in an attempt to wipe out cane beetles, a pest that was destroying sugar crops in north-east Australia at the time.
Since then, the toads have been sweeping through Australia leaving a trail of dead creatures in their wake.
To investigate their worrying spread, scientists looked at cane toads invading the Northern Territory of Australia, at a site about 60km east of Darwin.
They caught the toads, measured them, and also attached a radio-transmitter, weighing about 5-6g, around their waist to track their movements.
"During an invasion process the individuals at the front are there because they have moved the furthest," explained Dr Ben Phillips, an author on the paper and an evolutionary biologist at the University of Sydney, Australia.
"We showed that the toads that are the first to arrive at the front are the ones with the longest legs, and the ones last to arrive have shorter legs.
"The front toads also have much longer legs than the older populations in Queensland."
They discovered that the toads were moving incredibly quickly, covering distances about five times faster than when they arrived 70 years ago.
"They are moving around 55km a year on average, which is a long way to hop if you are a toad," said Dr Phillips.
The researchers believe their findings indicate evolution is favouring longer-legged toads which can travel further, quicker, meaning they can encroach on new territories faster than ever before.
The scientists say the toads are causing an "ecological nightmare".
So far, researchers have been unable to find a successful way of controlling the ever-spreading invaders, which are now on the cusp of invading Darwin.
"How to control them is the $64m question," said Dr Phillips.
"A lot of time and money has been spent researching how to control toads, but it is an ongoing problem."