The Dog Ear – Hearing what’s out there

The Dog Ear – Hearing what’s out there
To understand how dogs hear, it might be good to look at the anatomy of the canine ear. The canine ear is similar to any mammalian ear with structural differences affected by the dog’s own evolutionary adaptation.
Structure – four parts
The canine ear has four parts that include the earflap, know as the pinna, the external ear canal, the middle ear and the inner ear. The pinna is breed dependent and has the ability of independent movement. The pinna contains “more than a dozen separate muscles” used to control the ear and many blood vessels and nerves. The pinna is mostly cartilage covered with skin. The outer portion is usually covered in hair and the inner part depends on the breed. More than 18 muscles found at the base control the movement of the ear enabling the best possible sound reception (think of a satellite dish). The pinna is also equipped with blood vessels and nerves that supply the area.
The outer ear or ear canal is a cartilage tube “…lined with both apocrine and sebaceous glands which…produce a protective coating of earwax (cerumen).” The dog’s ear canal is much longer making a horizontal turn joining the lower middle ear. The tympanic membrane or eardrum separates the outer and middle ear.
The middle ear is comprised of three small bones called the malleus, incus and stapes. The eustachian tube connects the mouth to the middle ear. The eustachian tube allows air to enter the middle ear helping to balance pressure against the eardrum.
The inner ear contains the cochlea, vestibule and semicircular canals. The cochlea is filled with fluid that flows back and forth with specialized hair cells used to detect sound vibrations that directly signal the auditory nerve to the brain. The tiny hair cells vibrate according to the exact sound frequency received. The semicircular canals are organs filled with fluids and used to maintain balance or equilibrium. Any shift in head position through fluid shifts is detected by the brain. This informs the dog its body position.
What they hear
It is not entirely true that dogs have better hearing ability than we humans do. The difference is dogs can hear certain sounds possibly “…hundreds of times better than ours,” says Coren and other sounds are heard the same. According to Coren, “[t]he ability to detect a sound depends upon two qualities of the sound signal…the intensity or volume…and its frequency.” Sound frequency is what we think of as high or low pitch. Frequency is determined by the number of sound wave cycles every second and measured as a unit called Hertz (Hz). The higher frequency the more sound waves per second and the higher pitched the sound (Coren, 2004).
Until recently, it was difficult to determine the dogs’ hearing ability. The “brainstem auditory evoked response” known as BAER “…has been used to measure hearing in dogs.” According to the test results the “…largest difference between the auditory ability of dogs and humans is in the high-frequency range” (Coren, 2004).
A decibel (dB) is the term used to measure sound intensity or volume. The “absolute sound threshold” is zero decibels and measured by sound barely detectable by a young human. Humans and dogs have about the same hearing sensitivity from 2,000 Hz down to 65 Hz, but from 3,000 to 12,000 Hz measured at -5 and -15 dB respectively are undetectable by the human ear (Coren, 2004).
Coren attributes the dogs’ ability to hear in such high frequencies to their evolutionary history. He says, “[w]olves, jackals, and foxes often prey on small animals…which make high-pitched squeaks” and their movement amongst leaves and grass “…produces high-frequency rustling and scraping sounds.” This was possibly the adaptation necessary for their survival (Coren, 2004).
Determining the direction
Dogs have a unique way of determining the direction of sound. They needed this ability to find prey and signal impending danger allowing them to determine the direction to flee. Dogs with pricked ears have enhanced ability rotating their ears to capture and determine the direction of sounds. Because floppy eared dogs are limited in rotation ability, they are disadvantaged.
The dogs’ ability to determine the direction of sound is usually because one ear is closer to the source. This is understood because “the sound reaching the nearer ear will be slightly louder…and then a fraction of a second later reaches the more distant ear.” The time delay between the two ears is increased for dogs with larger heads. The increased distance between the two ears provides an increased advantage in determining the distance the noise is originating.

Like the human ear, the dogs’ ability to hear diminishes over time. Other forms of hearing loss come from ear infections, trauma, loud noises, genetics and blockage caused by excess wax, foreign objects and infections and some drugs. Deafness in dogs can be a congenital disorder, stemming from pigmentation. Specific breeds such as the Australian Shepherd, Great Dane, Dalmatian, piebald colored dogs and still others have been linked to the “merle gene” and associated with an increased chance of deafness. The BAER test mentioned earlier can determine deafness, by measuring the “…electrical activity in the cochlea and other auditory nervous pathways in the brain” (Lindsay, 2000).