Caecilians Evolved Unique Ears that Helps Sense Underground Vibrations

Caecilians - amphibians that resemble worms - have unique ears that can sense underground vibrations, scientists reveal.

Caecilians are tropical amphibians that resemble large worms or slick snakes. These limbless amphibians live underground, burrowing through loose soil and are known to ground litter with their extended streamlined bodies. There are nearly 200 species of caecilians; some are eyeless and the others have tiny eyes hidden under the skin.

In a new finding, scientists at the Carleton University in Canada discovered that these underground amphibians have developed an oversized organ in their ears in order to sense underground vibrations. This one organ in the ear is much larger than that of frogs and salamanders.

"Animals living in soil experience the same pressures as an animal flying or swimming, so we predicted that their ears should look the same," said Dr Hillary Maddin of Carleton University, who is also the lead author of the study. "But caecilians have a much larger ventral organ, which is probably used for transmitting vibrations from the ground to the ear."

The scientists explain that since these amphibians live underground, their ears are non-functioning for hearing airborne sounds. Being limbless, they have their heads on the ground all the time. In this case, the large organ helps the amphibians determine the vibrations that help to detect a prey or predator that is around them. This is unique to the group.  

In this study, the researchers focused on salamanders, frogs and caecilians. They used CT scanners that are used in hospitals. The CT scans produced a digital image of the animal's ears, and with the help of that image the team later built 3D digital replicas of each. They also measured the variation between inner canals and the shapes of the ears which help spot the unusual ventral organ in every caecilian specimen.

"This is the first time this organ has been quantified - people have noted it before but it's never been shown that it's truly distinct in all members of the group. Even primitive species all have a larger vibration sensing organ," said Maddin.

The larger ventral organ seen in caecilian was not detected in other subterranean animals. This indicates that caecilians have evolved a new technique of solving the problems of living underground.

"It was amazing to show that animals respond to the same environmental pressures but evolve different coping mechanisms. Caecilians are doing something different with their ears to other underground animals. So each group might be doing something different," Maddin explained.

The team further plans to investigate why only caecilians have evolved with enhanced vibration-sensing ability. They assume that the evolution of ears was based on the behavioral differences as opposed to the environment.

The finding was documented in the Journal of Anatomy.