Horses hear sounds over a wider range of frequencies than we do, although the decibel levels they respond to are about the same.
As Prof Bright says: “Humans with good hearing perceive sound in the frequency range of 20 Hertz to as high as 20,000 Hertz, while the range of frequencies for horse hearing is reported as 55 to 33,500 Hertz with their best sensitivity between 1,000 and 16,000 Hertz.
“As anyone who works with horses knows, that means they can often be aware of something outside our range of hearing and react to it.
“Dogs, on the other hand, can hear frequencies as high as 45,000 Hertz and possibly higher, while elephants hear much lower frequencies and only to about 10,000 Hertz on the high-frequency end of the spectrum.”
2. Their vision is better than their hearing
Hearing serves the same role in all mammals. It allows us to detect other animals - since it is animals that make most of the sounds that other animals hear. Once an animal is detected, ears tell us where it came from so we can scrutinise it visually to decide whether it is a danger or not.
“Horses are not good at this,” says Prof Heffner. “This is probably because their vision is so good and takes in virtually the entire horizon.
“They don’t seem to need much accuracy from their ears regarding location, just enough to give their eyes an approximate location to direct their gaze to examine what made the sound.”
3. Body language is key to communication
As prey animals, horses prefer to stay in herds, and communication is accomplished by body language rather than vocalisation and sound. They rely on body position and subtle body and head cues, even the twitch of an ear or the widening of an eye, to communicate within the herd.
Prof Bright adds: “That said, they do use their exquisite sense of hearing to pick up on changes in their environment, also a characteristic of prey animals.
“I suspect that stabled animals who may not be able to see each other communicate via sounds more than animals in a herd. Imagine the comforting sound of rustling hay versus the sound of snorting in a neighbouring stall.”
4. Their ears can rotate independently up to 180 degrees
Like all mammals, a horse’s ears are shaped to locate, funnel, and amplify sounds.
Not only are equine ears shaped to hear, but because of the musculature around the ears, they can rotate each ear independently as much as 180 degrees to pay attention to a sound without turning the head.
Prof Heffner says the rotation is because they are not good at locating the source of sounds.
She adds: “That is probably why they raise their head and rotate their ears when they hear a brief sudden sound—they are trying to locate it. Other hoofed animals (but not pigs) have this same feature.”
5. Their ears tell a story
Horses’ ears are primarily for hearing, but they are also utilised to express and communicate.
When a horse puts its ears back this usually means they are angry, and they may be threatening or warning another horse.
If the ears are held forward and the horse is attentive, it means the horse is listening. If the ears are tipped forward and stiff and the nostrils flare, it means the horse is scared or really interested in something.
6. Hearing issues can affect behaviour
Horses with hearing loss may exhibit behavioural changes - not responding to voice cues, for example - or may be more anxious than usual. And in many cases, hearing loss can be undetected or overlooked.
“My experience is that hearing is not something a horse owner or trainer normally thinks about when a horse has a behaviour change,” Prof Bright says.
“Horses are good at communicating in other ways (for example, interpreting visual and olfactory cues) so that they may be able to hide any changes in hearing from the humans who work with them.”
7. Horses aren’t as affected by hearing loss as humans
Hearing loss related to ageing, also known as presbycusis, is a form of sensorineural hearing loss that presents as increased hearing thresholds and changes in auditory processing.
Loss of hearing with age does not appear to be as significant for horses as it is for humans, as the FETCHLAB data in the above chart illustrates.
Prof Bright adds: “One of our doctoral students, Brenna Melvin, recently completed a study comparing the brainstem auditory evoked responses (BAERS) of young and old horses. She found no differences in responses for a group of horses aged 20 to 31 years compared to a group of young horses (<7 years).”
However, it’s possible environment rather than physiology is the cause of the difference between the deterioration of human and equine hearing.
She adds: “We concluded from this study that what we consider age-related hearing loss in humans may actually be the cumulative effect of noise exposure, toxic substances, and other environmental factors occurring over a lifespan.
“It is possible that purely age-related effects cannot be studied in humans because of all the confounding factors inherent in modern living. For animals not exposed to the same environmental conditions, such as horses, age-related changes in hearing may not be as pronounced as those we see for humans.
“We do see significant hearing loss related to ageing in dogs, and it may be that dogs are more likely than horses to be exposed to the same types of environmental conditions such as noise and environmental toxins as their human companions, resulting in a similar pattern of age-related hearing loss.”
8. Hearing loss can be affected by numerous factors
In addition to ageing, the disorders that affect equine hearing are temporohyoid osteoarthropathy (THO), congenital deafness (often related to coat colour), brain diseases, head trauma, and ear infections.
Prof Bright says: “I don’t believe we know as much about the prevalence of hearing loss in horses as we do in other species.”