"Amphibian horror" isn't a movie genre, but on this
evidence perhaps it should be. Harvard biologists have described a
bizarre, hairy frog with cat-like extendable claws.
Trichobatrachus robustus
actively breaks its own bones to produce claws that puncture their way
out of the frog's toe pads, probably when it is threatened.
David Blackburn and colleagues at Harvard University's Museum of Comparative Zoology, think the gruesome behaviour is a defence mechanism.
The researchers say there are
salamanders that force their ribs through their skin to produce
protective barbs on demand, but nothing quite like this mechanism has
been seen before.
The feature is also found in nine of the 11 frogs belonging to the Astylosternus genus, most of which live in Cameroon.
Male Hairy Frogs grow threads of vascularised skin during mating season (Image: Gustavocarra / Creative Commons License) |
Instant weapon
"Some other frogs have bony spines
that project from their wrist, but in those species it appears that the
bones grow through the skin rather than pierce it when needed for
defence," says Blackburn.
At rest, the claws of T. robustus,
found on the hind feet only, are nestled inside a mass of connective
tissue. A chunk of collagen forms a bond between the claw's sharp point
and a small piece of bone at the tip of the frog's toe.
The other end of the claw is connected
to a muscle. Blackburn and his colleagues believe that when the animal
is attacked, it contracts this muscle, which pulls the claw downwards.
The sharp point then breaks away from the bony tip and cuts through the
toe pad, emerging on the underside.
The claw breaks away from a small piece of bone (see arrow) at the tip of the frog's toe (Image: Blackburn) |
Hirsute horror
The end result may look like a cat's
claw, but the breaking and cutting mechanism is very different and
unique among vertebrates. Also unique is the fact that the claw is just
bone and does not have an outer coating of keratin like other claws do.
Because Blackburn has only studied
dead specimens, he says he does not know what happens when the claw
retracts - or even how it retracts. It does not appear to have a muscle
to pull it back inside so the team think it may passively slide back
into the toe pad when its muscle relaxes.
"Being amphibians, it would not be surprising if some parts of the wound heal and the tissue is regenerated," says Blackburn.
Males of the species, which grows to
about 11 centimetres, also produce long hair-like strands of skin and
arteries when they breed (see image). It is thought that the "hairs"
allow them to take in more oxygen through their skin while they take
care of their brood.
The sharp bony claws look like small barbs (Image: Blackburn) |
Spiky snack
In Cameroon, they are roasted and
eaten. Hunters use long spears and machetes to kill the frogs,
apparently to avoid being hurt by their claws.
"This is an incredible story," says Ian Stephen, curator of herpetology at the Zoological Society of London, UK. "Some frogs grow spines on their thumbs during breeding season, but this is entirely different."
"For me, it highlights the need for a lot more research on amphibians especially in light of the threat of mass extinctions," he adds.
The existence of frogs with erectile claws like cats was first described by Belgian zoologist George Boulenger in 1900 in frogs found in the French Congo, now the Republic of Congo.