Rats are learning how to swim, dive and collect mussels
Cambridge University freshwater biologist David Aldridge says the rats are diving into a river near March, Cambridgeshire to eat mussels.
Local residents have discovered piles of discarded mussel shells in their gardens that he thinks have been left by rats.
"The rat population has already exploded in the area and will continue to explode," said Dr Aldridge, who works in the university's zoology department.
Plentiful supply
"They've tapped into a new food source and learned how to feed on freshwater mussels."
The rats swim into the middle of the River Nene, dive down and pull the mussels out. Some parts of the river near March are 1.5 metres deep.
Although Dr Aldridge has not seen the rats in action himself, local residents have reported piles of shells, in their gardens.
Some people in March have spotted rats dragging mussel shells from the river one at a time.
It is estimated that 30% of the mussels in a one-kilometre stretch of river at March have been eaten by rats during the past three years.
Dr Alridge said the policy of draining rivers to prevent flooding is possibly how the rats have learned about the new food source.
Clean water
"Levels can get very low and expose the mussels and that's probably how the rats have discovered them.
He also said that because freshwater mussels help keep the water clean, a marked decline in their numbers would lead to fewer plants, fish and insects in the river.
Mussels do have natural predators such as mink and ducks, but the rats could have a very heavy impact, said Dr Aldridge.
He said he would like to do further research into the diet and behaviour of rats in other areas of the country.