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Frogs legs have sprung a big surprise – contrary to textbook biology, they have primitive kneecaps.

The kneecaps are made of dense, fibrous cartilage rather than bone, and appear to be much better suited to soaking up the strains of leaping and jumping than the bony human patella.

They may have been missed until now because they are not clearly visible on frog leg bones, even with a microscope, says of Argentina’s Institute of Neotropical Biodiversity, who led the investigation. The researchers analysed full skeletons of 20 species, but they were only able to see kneecaps in the eight specimens from which they took tissue slices for analysis.

One implication of the discovery is that kneecaps like this began to evolve in the Devonian period 400 million years ago, when the first four-legged animals reached land, the researchers say.

“Until now it was thought that the evolution of kneecaps coincided with the arrival of tetrapods that lay eggs on land or retain fertilised eggs in the body,” says Abdala. This investigation shows that the process really started with fibrocartilage in frogs, she says.

Cushioning the blow

“It could be related to locomotion on land, which may have required reinforcement of precise limb points,” she says. The frogs’ fibrous kneecaps have a lot of elasticity, and so are suited to soaking up the large forces exerted during the act of jumping or leaping once tetrapods reached land.

Animals later evolved to walk rather than relying so heavily on jumping, and so developed bony kneecaps better suited to this. “The resting position in frogs is analogous to the jumping position in humans, so the knees of frogs are under constant stress, and the fibrocartilaginous kneecap might alleviate this,” says Abdala.

“It does matter what kneecaps are made out of,” says , also at the Royal Veterinary College. “Bone is a good lever, better at resisting compression than fibrocartilage, so animals using their kneecaps as levers rather than cushions would benefit from bony kneecaps.”

“Fibrocartilage is a good cushion, and might be the original state of kneecaps, possibly inherited by all legged land vertebrates.”

Abdala agrees: “The structures are probably protecting the knee from the huge mechanical effort necessary for the jumping of frogs”. In contrast, when humans jump, there’s not enough elasticity in the bony patella to soak up these forces, leading to injury and inflammation of the underlying tendons, as in “jumper’s knee”.

To knee or not to knee

“I think it’s very exciting to have potentially discovered a patella in frogs,” says   of the Royal Veterinary College in London. There are hints of non-bony kneecaps in frogs, crocodiles and turtles, and a non-bony kneecap or similar structure is well-documented in marsupials, she adds.

Not everyone believes the structures qualify as kneecaps, however. “The authors convincingly demonstrate that some frog species have one, sometimes two, fibrocartilagenous elements adjacent to the knee joint, but it’s premature to identify them as patellae,” says of Ontario Veterinary College in Canada.

He says that similar features – called patelloids – have been found in some mammals, in some cases alongside clearly genuine kneecaps, as in rabbits.

Vickaryous also points out that there are around 7000 species of frogs and toads worldwide, so it may not be justified to infer that they all have these features after analysing only eight species. “The data represent more of a starting point for future investigations aimed at resolving the evolutionary origins of these confusing elements,” he says.

of Southern Illinois University feels similarly. He recommends analysing tailed frogs and New Zealand frogs, because they branched off from other frogs 200 million years ago. If they too have fibrous kneecaps, it would reinforce the argument that the earliest frogs had them too, he says.