Secrets of how and why frogs developed extreme heads

Frog skull bony spines

Anotheca spinosa, a tree frog from Central America, is likely to use the bony spines on its skull as a defense against predators. Photo credit: Edward Stanley / Florida Museum

Many frogs look like a water balloon with legs, but cannot be fooled. Some types of spines, spines and other skeletal secrets bear under smooth skin.

While most frogs have a simple skull shape with a smooth surface, others have developed more chic features, such as artificial fangs, ornate crests, helmet-like attachments, and gift-giving spikes. A new study is the first to deal in depth with the development and function of this armored frog skull.

Researchers at the Florida Museum of Natural History used 3D data to study the skull shape of 158 species that represent all living frog families. Radically shaped skulls were often covered with intricate patterns of grooves, ridges, and pits formed by additional layers of bone. The research team found that this characteristic, known as hyperossification, has developed more than 25 times in frogs. Species with the same eating habits or defense mechanisms tended to develop similarly shaped and patterned skulls, even if they were separated by millions of years of evolution.

Giant gape and fake fangs allow frogs to eat large prey

Hemiphractus scutatus, a horned frog from South America, can hunt other vertebrates thanks to its wide skull with a large gap. This species also has muzzle-like structures on the lower jaw. Photo credit: Edward Stanley / Florida Museum

“On the surface, frogs may look similar, but if you look at their skulls, there are drastic differences,” said Daniel Paluh, lead author of the study and a PhD student at the University of Florida. “Some of the strangest skulls are found in frogs that eat birds and mammals, use their heads as a shield, or in some rare cases are poisonous. Their skulls show how strange and diverse frogs can be. “

The last comprehensive study of frog skulls was published in 1973. Since then, scientists have doubled the number of frog species described, updated our understanding of their evolutionary relationships, and developed new analytical techniques using CT scans.

In this way, Paluh was able to use 36 landmarks on frog skulls, which were scanned and digitized as part of the National Science Foundation-funded oVert project to analyze and compare shapes in the entire frog tree of life.

“Before we had methods to digitize samples, the only way to quantify the shape was to measure each skull linearly,” he said.

Paluh found that hyperossification and bizarre skull shapes not only occur together, but are also often associated with frogs that either eat very large prey or use their heads for defense.

Frogs skull evolution

Most frogs have a simple skull shape with a smooth surface. However, some species developed artfully armored skulls that were suitable for catching large prey or for defense. Photo credit: Daniel Paluh

Frogs that eat other vertebrates – birds, reptiles, other frogs and mice – often have huge, spacious skulls with a jaw joint near the back. This gives them a larger gap with which to collect their prey, Paluh said, citing Pacman frogs as an example. His analysis found that the skulls of this species are littered with tiny pits that could offer additional strength and bite power.

Almost all frogs lack teeth on the lower jaw, but some, like Budgett’s frogs, have developed lower tuck-like structures that allow them to stab their prey. One species, Günther’s marsupial frog, has real teeth on both jaws and can eat more than half its body length.

Other frogs clog their caves with their heads as protection from predators. These species tend to have cavernous skulls covered with small spines. Some, like Bruno’s cascaded frog, have recently been discovered to be poisonous. When a predator rams the head of one of these frogs, specialized spikes prick right under the skin to defend against poisonous glands.

While the study showed a continuing overlap between hyperossification and imaginative skull shape, the researchers aren’t sure which one came first. Did frogs start eating big prey and then develop stronger skulls or vice versa?

“It’s kind of a” chicken or egg “question,” said Paluh.

The common ancestor of today’s 7,000 species of frogs had no decorated skull. But heavily fortified skulls appear in even older frog ancestors, said David Blackburn, curator of herpetology at the Florida Museum and co-author of the study.

“While the ancestor of all frogs had no hyperossified skull, the skulls of fairly old amphibian ancestors were built this way,” he said. “These frogs could use old development paths to create traits that characterized their ancestors deep in the past.”

Previous studies suggested that frogs developed hyperossification to prevent water loss in dry environments. However, Paluh’s research showed that habitat and hyperossification were not necessarily related. The characteristic is shown in frogs that live underground, in trees, in water and on land.

However, the habitat influences the shape of the skull: water frogs usually have long, flat skulls, while excavation types often have short skulls with pointed snouts. This shape also allows them to catch small, hasty prey like ants and catch termites with their mouths like chopsticks, Paluh said. These include the Mexican turtle and the Australian turtle frog – distant relatives that live in different parts of the world.

As the study sheds new light on the shape of the frog’s skull, Blackburn says that we still don’t know much about the basic biology of frogs.

“Oddly enough, it’s easier for us to make beautiful skull pictures than to know what these frogs eat,” said Blackburn. “Natural history remains pretty difficult. Just because we know that things exist does not mean that we know anything about them. “

The study will be published this week in National Academy of Sciences procedure.

Reference: National Academy of Sciences procedure.
DOI: 10.1073 / pnas.2000872117

Florida Museum’s Edward Stanley was also a co-author of the paper.

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