MadSci Network: Zoology

Re: How do dogs hear a bigger range of frequencies than humans?

Date: Mon Dec 11 15:04:52 2006
Posted By: Sam Reyes, Otolaryngolgogy Resident
Area of science: Zoology
ID: 1165514115.Zo

Three anatomical areas are likely responsible for the differences between
human and canine hearing. In order of importance, they are the external
ear, middle ear, cochlea and auditory central nervous system.

External Ear
High frequency sounds are much more directional than those at low
frequency.  This is why many speaker systems in the past 10 years have gone
to using a single subwoofer for low frequencies.  It is also why animals
with mobile external ears, including many dogs, have better sound location
abilities than humans.  By changing the shape and direction of their ears
they are able to better transmit high frequency sounds to the ear drum. The
length of the ear canal may also have some effect on its resonant frequency
with shorter canals being better for high frequencies.

Middle Ear
The area of the skull between the ear drum (tympanic membrane technically)
and the inner ear (the cochlea is the hearing portion) is called the middle
ear.  This contains the bones of hearing. It also contains an air filled
portion of the skull.  The size of this air filled space (tympanum and
mastoid cavities) and the stiffness of the bones of hearing, have an effect
on the sounds that are transmitted through the middle ear. Animals such as
the kangaroo rat and chinchilla have a middle ear with similar volumes to
those found in humans. Their range of hearing is very similar to humans.
This makes these animals a favorite of hearing researchers. 

Inner ear
The canine cochlea has 3 1/4 turns compared to the human cochlea's 2 1/2
turns.  The additional length may provide some additional space for sensory

Auditory nervous system
The nerves directly attached to the cochlea are similar in dogs and humans.
 In the brain stem and thalamus, most of the same nuclei or processing
centers are present in humans and dogs, but they may vary in size.  There
are also connections with motor nuclei controlling ear movement, not
present in humans. The biggest difference is in the auditory cortex. 
Humans have much more cortical tissue in general.  Recent functional
imaging studies using PET and fMRI imaging have let us see some of areas
activated when performing some functions, but little beyond the primary
auditory cortex is likely frequency specific.


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