This is the first time I’m using the citations from researchblogging.org, and also the first time I’m trying to embed my own photos. We’ll see how it goes, and if there’s no success, I’ll be editing it until it’s right.
A friend of mine sent me this article about a week ago, and I just loved it.
Concealed Weapons: erectile claws in African frogs
(Blackburn et al, 2008)
So claws and hooves and nails in terrestrial vertebrates are very common (look at the hands sitting on your keyboard), and they’re especially common in amniotes. Amniotes are under the superclass tetrapoda and include mammals, reptiles, and birds. They are called amniotes because they have fetuses that develop with a series of membranes, including the amnion, chorion, and allantois that you probably learned about studying chicken eggs in middle school. Amniote claws are usually made as a keratinous layer (you’ll probably remember that keratin is the protein making up your hair and nails) covering the end of the phalanges, as in the case of horse hooves, chicken claws, or your fingernails.
However, amphibians are not classified as amniotes, and rarely have claws. The major amphibian that we can think of as having claws would be the African clawed frog, Xenopus Laevis, which is a very common laboratory model. They do grow keratinized claws, but the growth is very different, and many people believe that claw growth arose independently (Maddin 2007).
In frogs, claws are used as the last line of defense. When they are caught and can no longer swim or jump away, they wriggle like mad, using their claws to scratch up whatever it is that caught them. Apparently this is pretty effective, people who hunt the frogs used in this study for food use long spears to kill them, so that they don’t get scratched, and apparently the claws can cause “deep bleeding wounds” to people holding them. This study answers the question of why these frogs have claws and where they come from.
The scientists studied a genus of frog found in Cameroon, Trichobatrachus robustus, also known as the Hairy Frog. Apparently the frogs have bones inside the tip of their phalanges, and these bones are sharp, covered by another bony nodule, and suspended inside the skin of the frog’s toe with a suspendatory sheath, and padded on the bottom of the toe with lots of tissue. The bone is connected to an extensor muscle. When the frog is stressed or caught (they don’t know what triggers it yet), the extensor muscle contracts, and the sharp bone BREAKS away from the nodule covering it, and then tears open the tissue of the frog’s toe to come out. The bony claw remains anchored via strong collagen fibers, and might be able to retract once the extensor muscle relaxes. This is the only species found so far with claws that do not have a keratinous sheath covering them.
Lemme repeat that last bit: the frog BREAKS ITS OWN BONES AND SHOVES THEM THROUGH THE SKIN AS CLAWS. Not only that, this particular frog is known as the “hairy frog”, due to the growth of hair-like skin strands that the males grow during breeding season. It has sideburns! This is the freakin’ Wolverine of the frog world! I hereby declare that this frog be renamed “The Wolverine Frog”, or perhaps “the wolver-frog” for short, in honor of our favorite hirsute self-multilating X-Man.
Blackburn, D.C., Hanken, J., Jenkins, F.A. (2008). Concealed weapons: erectile claws in African frogs. Biology Letters, -1(-1), -1–1. DOI: 10.1098/rsbl.2008.0219
All frog photos courtesy of Newscientists.com, and the Wolverine is from semanticdrift.