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Friday, January 23, 2009

Can You Hear Me Now?

And the questions keep coming! This one is from Steve...

I had a question regarding ear function in dogs. If pricked ears allow dogs to in general hear better, why would the majority of breeds not have ears of this type? For example, labs which are obviously used for hunting purposes, could benefit from the improved hearing while listening for game in the brush. My only speculation was that the floppy ears on sporting dogs may be beneficial to prevent poking the sensitive inner workings of the ear, but this theory does not hold as much weight after remembering that most wild canines have pricked ears. Unfortunately, I fear the answer may be simply "selective breeding" but was hoping you could offer a different insight.

I hate do disappoint you, but selective breeding is indeed the correct answer. Dogs in general have much more sensitive hearing than we do, and can hear more into the ultrasonic range than humans. Upright ears do help, but not necessarily how you would expect.

The actual hearing organs are in the inner ear. Let's trace a sound going to the brain. The sound wave comes from the source and travels to the ear. The outer portion of the ear, called the pinna, serves to capture and focus the sound into the ear canal. When you see a dog or cat swivel their ears, they are trying to isolate where the sound is loudest, which helps them determine in what direction the sound is originating (called "triangulation"). The sound travels down the ear canal and strikes the ear drum (tympanic membrane). This membrane vibrates at the amplitude and frequency of the sound. This vibration is transmitted through three connected bones, collectively called the auditory (or acoustic) ossicles. These connect to a structure in the inner ear, the cochlea. As the ossicles vibrate, they move a membrane on the cochlea back and forth (similar to the movement of the tympanic membrane). Inside the cochlea there is fluid, which will move in waves because of this vibration. Imagine pushing against the side of a plastic bucket filled with water and you have the idea. Inside the cochlea are tiny hair-like structures, that will move back and forth as the fluid moves over them. When these hairs move, they cause nerve impulses to travel in patterns along the vestibulocochlear nerve and into the brain. The brain takes these raw impulses and interprets them with our memories to help us recognize the sound. This basic process is true regardless of the species of higher animal. Different sensitivities and hearing abilities are determined by the number and type of structures within the cochlea. So really "hearing" takes place within the cochlea, and not in the part of the ear that we can actually see.

Since the pinna helps to focus sound, upright and mobile ears do help an animal hear better. However, it doesn't help too significantly. Wild canines have upright ears because it serves a benefit to them. We have selectively bred the different breeds of dogs in large part because of our own preference in appearance. Dogs with floppy ears came to be this way because people liked the way they look and bred dogs until they had this appearance. It doesn't make them hear better, but truthfully it doesn't hurt them either.

Labradors also don't hunt like you might think. The key is the "retriever" in their name. They were primarily bred to go into the water and retrieve game that had already been found and killed. They weren't designed to find and track prey. Other breeds have been bred for this trait, but not based on hearing. Beagles, bloodhounds, and similar breeds hunt and track based on their sense of smell. "Sight hounds", such as greyhounds and wolfhounds, hunt based on vision.

I hope that this explains it a little better!