Sit and watch your horse in the pasture and notice how active his ears are - they flicker, swivel and turn with the slightest of sounds. Since horses first evolved from the Hyracotherium over 55 million years ago, they have been listening for the snap of a twig or a rustle in the underbrush. That noise often meant something hungry was nearby. Instinct has hard-wired into them the necessity of using all of their senses to make sure they are not next on some carnivores dinner menu.
Not only do our horses have big funnel-like exterior ears to capture and direct every sound, they also have a far greater range of hearing than we do.
Horses can hear moderately low to very high frequency sound, in the range of 14 Hz to 25 kHz, as compared to our measly range of 20 Hz to 20 kHz).
Horses ability to pick up ultrasonic sounds, while not quite as well as a bat, is infinitely better than ours. They will pick up the high-pitched squeal of a squeaky fan belt on your cousin's truck from miles away.
They are also able to capture the low frequency sounds, below our range. Research has shown that many sounds are transmitted to horses while they graze. They literally pick up vibrations via the bones of their jaw and the sounds travel up to their ears.
Now, let's talk about those swiveling ears. Just like old school satellite dishes, horses ears spin and twirl to capture sounds. They have an amazing 180 degree turn radius on those ears! Your horse will point his ears toward the sound he is focused on, even if his head is pointed another direction. The additional muscles that point and rotate equine ears allow this fine-tuning.
More than once I have tried to explain to my husband that women are hard-wired to be more aware of their surroundings and often more wary than men are. That we instinctively look under our vehicle as we enter the parking and hold our handbags close when walking down a dark street. He looks at me with puzzlement, as a burly big guy, he just doesn't get that.
Our horses have that same protective, awareness instinct times 1000. Of course, it is maddening when we are ambling along, enjoying a spring afternoon trail ride and your horse tenses, spins and bolts when he hears a twig snap. That is literally millions of years of keeping alive instincts kicking in.
Of course, some horses are wound a bit tighter than others (just like us). But if you learn your horse's reaction tolerance levels, it can make for a lot smoother trail ride!