The Mystery of the Earless Toads

Newswise — More than 200 species of “true toads” (Bufonidae) have fully functional inner ears, but cannot fully use them because they have lost their tympanic middle ears, the part of the ear which transmits sound air pressures from the outside world to the inner ear. These “earless” toads rely on sounds to communicate, so why would they lose a sense that is key to their survival and reproduction? “Losing an ear seems pretty maladaptive and we want to know why it’s happening so much,” explains Dr. Molly Womack, a researcher in Dr. Kim Hoke’s laboratory at Colorado State University.

The most obvious hypothesis for repeated ear loss is similar selection pressures: ear loss makes sense if you don’t need ears and can use the extra skull space for something more useful. Womack and post-doctoral researcher Dr. Jennifer Stynoski first looked for a common thread linking all earless toads. But earless toads are diverse—they are found in different habitats and reproduce in different ways—so there was no support for a common selective force driving ear loss.

Interestingly, many earless toads still call to attract their mates, so Womack and Stynoski wondered if earless toads could still hear sounds. To test this, Womack and Hoke headed to Peru and Ecuador to study hearing in closely related toads—some with ears and some that are earless.

Womack and Hoke were surprised to discover that many earless toads are not completely deaf. “Earless species can hear a lot more than you might expect, but they miss the high notes,” explains Womack. Toads without middle ears are thrifty and may use other parts of their body, not specialized for hearing, like their lungs or shoulder bones, to detect air vibrations. Womack and Stynoski suspected that development might be the key to understanding ear loss. Although the inner ear components are formed early in a tadpole’s life, the middle ear doesn’t start to form until much later, during metamorphosis. That makes sense because the aquatic tadpoles don’t need to relay air pressure differences until after they become terrestrial juveniles.

Returning to the laboratory, hundreds of tiny tadpoles of different species are lined up on shelves in their own individual vials. Stynoski carefully documents when the middle ear structures begin to form by slicing the delicate tissues and bones of tadpoles of different ages and reconstructing 3D models of their developing ears—a highly organized and intense operation that takes months to complete.

Stynoski and Womack hypothesized that the timing of ear development may explain why the middle ear structures stop growing in certain species. “When an ear develops relative to when metamorphosis happens is probably very important,” elaborates Stynoski.

Before the middle ear can be formed, a coordinated sequence of complex processes must take place, and if that chain of events is disrupted, the middle ear may not properly develop. Because the middle ear develops very late compared to other head structures, it may be more vulnerable to being lost. Ear loss may therefore be just an unexpected side effect of shifted developmental timing in this frog family. In some earless species, Stynoski finds vestigial structures where the middle ear should be—indicating the middle ear begins to form, but stops abruptly. It’s possible that in the ancestral toad, the genes that send signals to initiate ear growth, ear differentiation, and metamorphosis shifted out of sync, which interfered with proper formation of the middle ear.

This developmental mistiming, combined with the fact that many earless toads can still hear something, may finally solve the mystery of earless toads. More broadly, this research highlights how diverse factors can act on organisms to shape their features.

Womack and Stynoski’s findings will be presented at this year’s Society for Integrative and Comparative Biology meeting in New Orleans, Louisiana.