Jan Freedman, curator of natural history at The Box — a museum in Plymouth — was walking with his family at the Venford Reservoir in Dartmoor when his 8-year-old son spotted the gory corpse, he told Live Science in an email.
In a photo of the remains, which Freedman shared in a tweet on March 24, the toad's glasslike intestines spill onto the ground, and the peeled skin of its underside — still attached below the jaw — stretches over its back. [Beastly Feasts: Amazing Photos of Animals and Their Prey]
"The head was the only part that wasn't turned inside out, so we could see it was a common toad (Bufo bufo)," Freedman said. "We have seen plenty of toads in our garden and at other spots on Dartmoor, but nothing like this." Freedman wrote in the tweet that he suspected this was the work of a predator — but what kind of predator was it?
Freedman hoped that biologists on Twitter could explain the toad's intriguing (and gory) state, and they did not disappoint. One of the responders, Jack Ashby, a museum manager at the University Museum of Zoology in Cambridge, initially thought that the frog had been pulled apart by a crow, "as this is something that Australian crows do to invasive toxic cane toads."
In Australia, crows have learned to sidestep toxic glands in the toads' heads and backs by flipping the toads over and slicing into the skin of their bellies to devour their insides, a behavior that was documented in 2018 (opens in new tab) by photographer Steve Wilson.
"European toads also have toxin glands in their skin, so it's not surprising that a predator would remove it," Ashby told Live Science in an email. In the photo of the inside-out toad, its flesh appeared to have been delicately removed, "which one might expect to be more easily done with a nimble beak," he added.
The jaws that bite, the claws that catch
However, there's another predator known for adroitly stripping toxic toads of their skin — the European otter (Lutra lutra). Though these otters are primarily fish eaters, they are also known for eating mammals, birds and amphibians, Amy Schwartz, a researcher and doctoral candidate with the School of Biosciences at Cardiff University in the U.K., told Live Science in an email.
Toads are often on the menu for otters in spring, when large numbers of the amphibians gather in ponds to breed, Schwartz explained. She suspected that an otter had skinned the toad in the photo because she had previously seen similar evidence: at a pond in Pembrokeshire — "in an area full of otters" — where multiple toad skins floated on the surface of the water.
After closely examining the image, Ashby also concluded that the predator was probably an otter. Many of the muscles and bones of the leg and spine were missing, which hinted that the toad had been attacked by a mammal — "something large enough to chew up whole toad legs," he said.
The detail that cinched it for Ashby was the empty skin that once held the toad's foot, which must have been yanked from the skin by a predator strong enough to remove the entire leg — "muscle, bone, tendons and ligaments" — in one piece.
"This requires a serious amount of force, presumably by an animal holding the carcass in its paws and pulling the leg out of its skin by its teeth," Ashby said. "It’s a lot easier to imagine an otter doing this" than a bird.
And as it happens, toad skin is surprisingly easy to remove, as it is only very loosely attached to the animal's body, Ashby said.
"The only places it is attached firmly is around the hands and feet, and the skull," he said. "If you're not worried about keeping the skin in one piece, it can be pulled off like a pair of leggings and a skin-tight shirt after snipping around the 'waist.'"
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Originally published on Live Science.