Lizards And Salamanders May Use Lungs To Hear, Study Says

The research extends previous studies showing that some types of earless frogs and toads use their lungs to pick up sound vibrations, said Thomas Hetherington, an associate professor of Evolution, Ecology and Organismal Biology at Ohio State.

The results of the current study suggest lung-based hearing may exist in a variety of land-based animals.

"This primitive system of hearing may have been the auditory system for the first animals that lived on land," Hetherington said. "And it appears that it may still be important for some species today, particularly ones that lack middle ears."

Hetherington examined four species of salamanders and three species of lizards to determine if the lungs might play a role in their hearing. Although salamanders lack middle and external ears, both groups of animals have inner ears that can process sound.

In his studies, Hetherington found that sound causes the animal's chest to vibrate, and the vibrations are carried by air from the lungs to the animal's inner ear where it is processed as sound.

The experiments make clear the importance of the lungs for hearing - one species of salamander that lacked lungs did not show the chest vibrations that the others did. And when the lungs of the other species were filled with oxygenated saline instead of air, the animals' chests no longer showed vibrations.

The study was published in a recent issue of the Journal of Comparative Physiology A: Sensory, Neural and Behavioral Physiology.

Hetherington put the animals on a table in a soundproof chamber. He bounced a beam of laser light off of each animal's skin to measure the skin's movement when exposed to various sound frequencies emitted from a speaker inside the chamber.

Low frequencies caused the greatest vibrations: peak motion ranged from 1,600 to 2,500 hertz in small newts (newts are a type of salamander); from 1,250 to 1,600 Hz in larger salamanders; and from 1,000 to 2,000 Hz in lizards. Lizards have middle ears, which is where the eardrum is located, and the skin covering this area of the animal's head vibrated at slightly higher frequencies of about 2,000 to 3,000 Hz. The lungless salamanders didn't respond at any frequency.

To determine how dependent the animals were on their lungs for hearing, Hetherington filled the lungs of three red-spotted newts and three green anoles (an anole is a tropical lizard that can change color) with oxygenated saline solution - the oxygen in the solution allowed the animals to keep breathing. Sure enough, the response to sound - the vibrations - dropped.

"It practically disappeared," he said. "While sound may get in through other routes, the lungs are clearly the most sensitive to sound waves," Hetherington said.

After filling the lungs with the saline solution, the vibrations noticeably decreased by about 90 percent in all of the animals. The animals' sensitivity to sound was restored when the lungs were emptied and filled again with air.

While Hetherington knew from his previous research that certain frog species depended on their lungs to conduct sound, he wasn't sure before these studies that the same process held true in other amphibians and small reptiles, whose lungs are covered with ribs and muscle.

"Using the lungs to detect sound seems to be especially useful for small animals with really small lungs," Hetherington said. "Thinner body walls respond more readily to sound, so it may be that the lungs can capture a wide range of frequencies only in small animals."