New 3D Images Reveal the Evolutionary Wonders Hidden in Frog Skulls

As slick as a frog might seem, spikes and lumps erupt from their skulls in places we can’t always see. “A few of them, if you could poke them on the head, you could feel it,” says David Blackburn, a herpetologist with the Florida Museum of Natural History at the University of Florida. Most of the features are a surprise, visible only when peering beyond the skin and soft tissue.

So that’s what Blackburn and his student, Daniel Paluh, did. The team compiled digital 3D images of the skulls of 158 frog species to visualize all their lumps, bumps and enormous jaws. It turns out that frogs probably developed their strange head gear to help them hunt, protect their homes or even deliver poison, the team concluded in their research, published in the Proceedings of the National Academy of Sciences.

Historically, researchers haven’t paid as much attention to frog evolution as they have other vertebrates, Blackburn says. Connecting some of their cranial nooks and crannies to their abilities might open up more investigations into where these strange amphibians came from. “These things we think about as so froggy — when did they evolve, and why did they evolve?” Blackburn asks. “We just don’t know.”

Skull Scans

In the past 40 years, the number of known frog species has boomed, Blackburn says. Instead of studying museum specimens — which lose color over time, or don’t include tadpole phases that could mark two similar-looking species as distinct — researchers run genetic tests to differentiate the hopping amphibians. CT scans appeared as a kind of substitute, harmless surgery. These 3D X-rays map bone lying underneath the skin, showing everything researchers miss when studying the slick, bug-eyed heads from the outside.

When Blackburn and his team compared the CT scans with the genetic ancestry of these frogs, they found that some beliefs about these braincases don’t apply. Build-ups of bony growth don’t happen in mostly small frogs, like some research has hypothesized. Instead, those protuberances cropped up in all kinds of species, no matter how big the owners were.

The frog’s diet or behavior seemed a bigger determinant of head shape. For example, some had jaw joints that stretched so far behind their head, the hinge lined up with the middle of the frog’s back. These frogs all dined on other vertebrates and seemed to need massive mouths to conquer their meals. Others had jaw joints that were more proportional to their body size. “If you could imagine a diversity of people that had their jaw joints like that, it would look remarkably weird,” Blackburn says.

Spikes and Spurs

Some had aggressive spikes on top of their heads, something that might help deliver poison to predators. These species plug the entrance to burrows with their head as a blockade against pecking birds, and some have poisonous glands on top of their skull. The bony spikes lying just beneath the pocket of poison might help deliver the toxin to attacking birds, Blackburn says.

Because similar-looking skulls appeared amongst frogs that are not closely related, the team thinks these bony spikes and spurs evolved about 20 times along different lineages of frog. Ancient amphibian specimens show even more intense skull growths, Blackburn says, suggesting that today’s frogs are still tapping into ancestral growth patterns. “Somehow, these frogs are turning on some ancient developmental machinery in their DNA.”

More research will see how these theories hold. Researchers don’t know much about the biology and life cycle of many elusive frog species, and Blackburn would like to know more — for example, if many other frogs that barricade with their own heads also have poison glands and spikes on their skull.

As the discoveries continue, Blackburn can’t wait to see what else the CT scans reveal. “This is just the tip of the iceberg.”

When the study is made available online at PNAS.org, it will be posted here.

Check out their skulls, here:

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