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Source: Ohio State University
(<a href="http://www.acs.ohio-state.edu/units/research/";
rel="nofollow">http://www.acs.ohio-state.edu/units/research/</A>)
Date: Posted 3/7/2002
Lizards And Salamanders May Use Lungs To Hear, Study Says
COLUMBUS, Ohio - Certain species of salamanders and lizards can
actually hear through their lungs, according to a new study at Ohio
State University.
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."
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