Beagles and bassets have huge ears, but it's the terriers that actually use their ears for hunting.
Underground, a dog can see nothing -- zero, zilch, nada. Past the first turn of a sette, all is black. All the dog knows for sure is that somewhere in the blackness are white teeth waiting to bite and slash if the dog gets too close.
It is the ears that tell the dog where "too close" is -- the breathing of the animal, the sound of claw and paw against dirt, even the heartbeat of the animal at the opposite end of the pipe.
Though some people will tell you terrier's ears flop over to keep out dirt, that is pure nonsense -- every wild animal that burrows has an ear that stands upright, from rats to fox, from badger to wart hog. Terrier ears flop over, not out of necessity, but out of convention. In all of the wild animal kingdom, only the elephant has ears that flop over, and these are hinged for an entirely different reason -- ventilation.
Deafness in terriers is a very serious problem, especially in Bull Terriers and Jack Russell Terriers. For reasons that are not well understood, deafness in dogs is strongly correlated with the gene for merle coats, and shows up in a higher-than-random incidence in dogs that are mostly white and have blue or "glass" eyes.
Dog that have solid-colored coats or which have large amounts of color in them (such as Border terriers, Fell terriers, Patterdale terriers, and the beagle-colored Jack Russells) are less likely to have heriditary deafness.
All Jack Russell terriers should be BAER tested before breeding. BAER stands for "brainstem auditory evoked response" and involves placing two very small wires into the skin on the dog's scalp just above the eyes. These small wire electrodes register a response if the neural pathways work in response to sound (i.e. they dog is hearing).
Dogs that are unliateral or bilateral deaf should not be bred, nor should dogs that produce deaf offspring.
BAER testing needs to be done only once in the life of a dog, can be done on very young puppies (any time after 5 weeks of age), and costs only $40 a dog or so.
Remember that a dog is a long-term invesment of time as well as money, and a deaf dog is useless in the field, more trouble at home, and a drag on the gene pool.
If you are in the market for a puppy, especially a Jack Russell Terrier or any other breed at high-risk for deafness, demand that the puppy be BAER tested.
For more information, and a list of BAER testing sites, see >> deafness in dogs and cats.
This is what a dog can see underground when it is working, and it is why hearing is so vital to a working terrier.