Water voles eat their food at the water's edge
Ecologists have discovered that normally vegetarian water voles living in Wiltshire have started eating frogs' legs and discarding the bodies.
The mammals usually live on grass and plants, and experts from British Waterways said it was "incredibly unusual behaviour".
Ecologist Robert Randall said pregnant water voles may have been feeding on frogs because their diet lacks protein.
He said the Kennet & Avon Canal was littered with dead frogs without limbs.
Water voles often bring their food to the water's edge and leave the chewed up remains in neat piles which ecologists use to monitor their eating patterns.
Mr Randall, who carried out the survey with fellow ecologist Oda Dijksterhuis, said: "We found a number of typical water vole feeding areas that were littered with dead frogs, minus their legs.
"As a water vole's diet is normally vegetarian, this rather gruesome scene isn't what we'd expect to find at all.
"We're not really sure why it's happening, but as the evidence coincides with the water voles breeding season we think it may be that pregnant mothers are snacking on frogs' leg as they lack protein in their diet.
"This is incredibly unusual behaviour and as far as we know this is the first recorded evidence we have of them eating frogs' legs, so it's a really exciting discovery.
"We'll be keeping an eye on what happens next over the coming months."
Mr Randall added: "There are reports of animals in some low calcium environments like northern Scotland, the Shetland Islands, where they've had reports of sheep eating wading birds to get calcium out of their bones, but we think here it could be protein."