You’ve probably never had a reason to peer closely inside the mouth of a frog—but if you did, you might find small teeth on the roof of the mouth and along the upper jaw.
Researchers at the University of Florida spent the past year carefully examining the mouths of frogs, by looking at digital scans of frog specimens provided by museums. They found that, while all frogs once had teeth, over the course of evolutionary history, some species lost them. Their lockdown project, “Rampant Tooth Loss Across 200 Million Years of Frog Evolution,” was published Tuesday in the journal eLife.
Slate spoke to Daniel Paluh, who is the first author on the study and a Ph.D. student at the Florida Museum of Natural History, to learn more about the new research and what is going on with frogs and chewing in general. This conversation has been condensed and edited for clarity.
Sofia Andrade: OK, for starters, I was surprised to learn that frogs have teeth at all.
Daniel Paluh: I have talked to people who aren’t biologists about this project. And lot of them, at first, are very surprised that frogs have teeth at all. And then when I talk to other biologists, and especially those that study teeth across animals, they are very surprised to hear that many frogs don’t have teeth. I get the opposite response depending on who I’m talking to. But our study showed that there are about 100 genera of frogs that completely lack teeth, and they represent about 20 independent cases of tooth loss across species.
Your paper mentions that most frogs only have teeth on their upper jaw. How is that useful for chewing?
Generally, when frogs do have teeth, they may be using it to assist in capturing prey. But for the most part, frogs actually are catching prey with their projectile tongues. Their teeth likely don’t play a really important role in catching prey as compared to a mammal, which is going to be using their teeth a lot for catching and processing prey for the most part. No frogs or amphibians really chew their prey, and if they’re using their teeth at all, it’s just to grab on to and hold on to their food.
What do frog teeth look like?
In general, frog teeth are incredibly small, which is likely why they’ve been ignored for so long. The teeth of most frogs are actually less than a millimeter long and they’re actually often even smaller than half of a millimeter. But if you blow them up using some of the imaging analyses that we do at the Florida Museum of Natural History, you can actually see that a frog tooth, and other amphibian teeth are pretty complex. They’re usually bicuspid in shape, which means they have two little points at the end.
Your study looks at the evolution of frog teeth. When did animals first evolve teeth?
As far as we’re aware, teeth only evolved once in all vertebrates, and that happened over 400 million years ago. Since that one single origin of teeth, all animals that have teeth today have maintained teeth for the past 400 million years.
But some frogs no longer have teeth. Your paper explains that there have been more than 20 instances of tooth loss over millions of years among frog species, and that that’s uncommon. What is it about frogs?
Frogs have actually lost their teeth more times than all other vertebrate groups combined.
We don’t know exactly what has driven this repeated tooth loss. It’s likely related to the diet specialization that these toothless frog lineages have gained. We identified a strong correlation between tooth loss and species that specialize on eating, ants, termites, and mites, which are really small invertebrate prey items.
Most frogs that have teeth eat a broader diversity of invertebrates, and, really will eat anything that they can fit in their mouths. These species that only eat really small prey items, they can rely completely on their tongues to capture and consume these small prey items and so their teeth are no longer functionally required
You mentioned that the common thinking is teeth only evolved once. But your study suggest that some frogs might have actually lost teeth, and then evolved them again?
In our study, we did infer several cases of teeth reevolving in groups that have otherwise lost teeth. [If it pans out,] this would be something very unique to frogs. But it really requires further study.
We were using CT scanning to identify the presence or absence of teeth. Frogs will actually sometimes evolve “odontoid serrations,” which are basically just bony serrations on their bones that may superficially look like teeth but aren’t enamel or dentin, which are the tissue types that characterize true teeth. So, it’s not definitive that these species have truly reevolved teeth, but it’s something that we’re currently looking into.
How could your work investigating frog teeth be used?
Our next steps for further looking into the repeated tooth loss in frogs is to begin looking at the developmental and molecular mechanisms of tooth loss. All teeth are composed of enamel and dentin, and these tissue types are encoded by proteins, and we know what genes are responsible for those proteins. And so, we can begin looking at those genes in species that have lost their teeth to see if they have become nonfunctional through mutations, and begin testing whether each case of tooth loss is a result of a different mutation, or if it is the result of the same mutation that has occurred over and over again.