Why some frogs have crazy heads

Most frogs look pretty benign, with round and friendly faces, but don’t be fooled. Below the surface, some sport faux fangs, helmet-like fortifications and even venom-delivering spikes.

Now US researchers have a better idea of how these evolved and why.

A team from Florida Museum of Natural History used 3D data to study skull shape in 158 species, representing all living frog families, and found that radically shaped skulls are often coverreced in intricate patterns of grooves, ridges and pits formed by extra layers of bone. 

This trait, known as hyperossification, has evolved more than 25 times in frogs, they discovered, and species with the same feeding habits or defences tended to develop similar skulls, even if they were separated by millions of years of evolution.

“Superficially, frogs may look similar, but when you look at their skulls, you see drastic differences; their skulls show how strange and diverse frogs can be,” says Daniel Paluh, lead author of a paper in the journal Proceedings of the National Academy of Sciences.

The work is long overdue. Paluh says the last comprehensive study of frog skulls was published in 1973, and since then scientists have doubled the number of described frog species and updated our understanding of their evolutionary relationships.

New analytical techniques are also a bonus. Paluh and colleagues were able to use 36 landmarks on frog skulls, scanned and digitised as part of the museum’s oVert project.

They found that frogs which eat other vertebrates, such as birds, reptiles mice and other frogs, often have giant, roomy skulls with a jaw joint near the back, which gives them a bigger gape with which to scoop up their prey. Their skulls are stippled with tiny pits, which Paluh suggests could provide extra strength and bite force.

Nearly all frogs lack teeth on their lower jaw, but some, such as Budgett’s frogs, have evolved lower fanglike structures that allow them to inflict puncture wounds on their prey. One species, Guenther’s marsupial frog, has true teeth on both jaws and can eat prey more than half its body length.

Other frogs use their heads to plug the entrance of their burrows as protection from predators. These species tend to have cavernous skulls overlaid with small spikes. A few, such as Bruno’s casque-headed frog, were recently discovered to be venomous.

The question is, which came first – hyperossification or fanciful skull shape? Did frogs start eating large prey and then evolve beefier skulls or vice versa?

The common ancestor of today’s 7000 frog species did not have an ornamented skull, says co-author David Blackburn, but heavily fortified skulls do appear in even more ancient frog ancestors.

“While the ancestor of all frogs did not have a hyperossified skull, that’s how the skulls of quite ancient amphibian ancestors were built,” he says. “These frogs might be using ancient developmental pathways to generate features that characterised their ancestors deep in the past.”

Previous studies have suggested frogs evolved hyperossification to prevent water loss in dry environments, but the new study found that habitat and hyperossification were not necessarily linked. 

Habitat does influence skull shape, however. Aquatic frogs tend to have long, flat skulls, for example, while digging species often have short skulls with pointed snouts and can use their mouths like chopsticks.