By reptile standards, alligators are
positively chatty. They are the most vocal of the non-avian reptiles and are
known to be able to pinpoint the source of sounds with accuracy. But it wasn’t
clear exactly how they did it because they lack the external ear structures, or
pinnae, that most mammals have.
new study by an international team of biologists shows that the alligator’s ear
is strongly directional because of large, air-filled channels connecting the
two middle ears. This configuration is similar in birds, which have an
interaural canal that increases directionality in the absence of external ears.
study was published online in the Journal of Experimental Biology on
March 26, 2014. The research was funded by the National Institutes of Health,
National Science Foundation, Danish National Science Foundation and Carlsberg
Tollin, PhD, associate professor of physiology and biophysics at the University
of Colorado School of Medicine—along with researchers from the University of
Maryland, the University of Massachusetts Lowell, and University of Southern
Denmark—collected anatomical, biophysical and electrophysiological measurements
of alligators to investigate the mechanisms alligators use to locate sounds.
measurements were performed in Tollin’s laboratory. Tollin and others from the
School of Medicine – postdoctoral fellow Jennifer Thornton, PhD, neuroscience
graduate student Heath Jones and postdoctoral fellow Kanthaiah Koka, PhD – were
involved in the design, performance, analysis
and figure production for acoustic measurements.
the team tested how sound travelled around an alligator’s head to investigate
whether the animal somehow channels sound to help it locate the origin,
listening for the minute time and volume differences between the sound arriving
at the two ears. But the team found no evidence that the animal’s body alters
sound transmission sufficiently for the animal to be able to detect the
difference. And when the team measured alligators’ brainstem responses to
sounds, they were too fast for the animals to be sensing the time difference
between a sound arriving at the two ears.
the team looked for internal structures in the alligators’ heads that might
propagate sound between the two eardrums. Sound reaches both sides of the
eardrum—travelling externally to reach the outer side and through head
structures to the internal side—to amplify the vibration at some frequencies
when the head is aligned with the sound. This maximizes the pressure
differences on the two sides of the eardrum, magnifying the time difference
between the sound arriving at the ear drum via two different paths to allow the
animal to pinpoint the source.
slices through the heads of young alligators, the team could clearly see two
channels linking the two middle ears that could transmit sound between the two
eardrums. They also measured differences in the alligator’s brainstem responses
to sounds depending on their location. And when the team looked at the
eardrum’s vibration, they could see that it was amplified at certain
frequencies, as they would expect if alligators use the pressure difference at
the eardrum for orientation.
all of the evidence together, the researchers suggest that the reptiles rely on
magnified time difference at the eardrum to locate noises. They also suspect
that this is the mechanism that the archosaur ancestors of modern crocodilians
and birds used to pinpoint sounds.
research was supported by the National Institute on Deafness and Other
Communication Disorders of the National Institutes of Health under Award Nos.
DC-00436, DC-04664 and DC-011555; the National Science Foundation under Award
No. 0817208; the Danish National Science Foundation under Award No.
DFF1323-00132; and the Carlsberg Foundation. The content of this article does
not necessarily reflect the views of the funding organizations.
The research paper, “Biophysics of
directional hearing in the American alligator
Hilary S. Bierman, Jennifer L. Thornton, Heath G. Jones, Kanthaiah Koka, Bruce
A. Young, Christian Brandt, Jakob Christensen-Dalsgaard, Catherine E. Carr and
Daniel J. Tollin, will be published online on March 26, 2014 in the Journal
of Experimental Biology.