Can You Hear Me Now?
A crying baby. A laugh of a loved one. A fire alarm. These are all sounds that have a distinct and evocative feeling associated with them and hearing each one calls for an entirely different response. Even by just looking at the photo above, you can probably generate an imaginary sound that the child is making.
Unlike our eyes, our ears cannot “blink” to close them and are constantly exposed to a cornucopia of sound. If you take a moment now to close your eyes, you are probably surrounded by all kinds of sounds that you can singularly pick out and identify. No one ever had to teach us how to hear, but over time we have heard sounds that we can now label and recognize.
Aural Tradition
Hearing is perceived through vibrations or pressure changes in a medium (gaseous, solid, or liquid matter) over time. For animals with hearing, they are able to detect phenomena without actually seeing it: a predator rustling in the grass, the quickening heartbeat of nearby prey.
As humans, we typically do most of our hearing through our ears by interpreting sound that is perpetuated through vibrations in the air. On the other hand, our marine based prehistoric ancestors developed a sense of hearing through water, but the underlying principle remains the same: a wavelength is sustained by a medium to be interpreted by an organ, translated into an electrical signal, and then sent to the brain for explication.
The marine animals that crawled onto land and consequently evolved into the first amphibians were the first organisms to develop hearing as we know it today. The prehistoric’s ears had to learn to comprehend signals through air, and these animals developed bone structures to mimic the vibrations that were captured.
For these early amphibians, these vibrations resonated in the back bones of their jaws, which could detect both vibrations along the ground and sound traversing through the air. Evolution has developed more siloed auditory systems since, but certain modern day animals still utilize these structures, e.g. snakes that are highly sensitive to vibrations in the ground and low frequency sounds. Since the millions of years that animals have been able to interpret sound, what has evolution been up to? Where are the best ears on the planet now?
The Rhythm of the Night
As the sun sets, the forest is cloaked in the shadow of night. The evergreen trees transform from nature’s mysterious towering cathedral to a dense maze of gnarled roots twisting in and out of the ground, fingers grasping thin air.
Mother Nature dims the lights over the landscape, but without intermission, the soundscape of the night changes acts and she invites new musicians to come to the stage. Millions of individual insects chirp and trill in a nocturnal chorus, each one secretly wishing he was singing a duet instead. Small rodents scamper across the forest floor while quietly rustling through foliage for bugs and seeds.
A deer mouse furiously digs into the ground and finally uncovers a tasty beetle. She hastily snatches it. As the mouse gnaws on her dinner (breakfast?), she enjoys the cover of night that keeps her safe from hawks and eagles. Little does she know that in her own single-minded pursuit, she has given away her location to another winged predator. A feathered whisper, a shrill gasp, she vanishes from the forest floor. A conciliatory hoot travels for miles through the forest night, warning other predators to keep their distance.
Who Gives a Hoot?
Owls have one of the best pairs of ears in the animal kingdom. The bird’s entire head is designed for listening, shaped like a satellite dish to capture and channel sound waves towards their ears. The tufts of feathers on top of an owl’s head are not actually ears, their real ears are actually hidden beneath the feathers in the side of their face.
For some species of owl, their ears are oriented one above the other and angled differently to be able to triangulate the height and direction of a sound. When a noise is heard, the slight difference between perceptions of the sounds helps the owl determine which direction to look for their target. For instance, if the owl hears the sound from their left ear even just milliseconds before their right, they know that their prey is on their left. The difference of height in the ears also improves the owl’s ability to determine if the source of the sound is above or below them.
Combining up, down, left and right signals as well as rotating their heads up to 270 degrees helps the brain generate a perfect mental model of their environment and their target. The owl calculates a flight path to their target and takes off, honing in on their prey and any changes that might indicate movement. Whether it’s a desert, a forest, a prairie, or the Arctic tundra, the owl’s pinpoint hearing provides a lethal method for tracking down prey. Even while kestrels might struggle to hunt for white rabbits hopping around snow banks, a snowy owl can hear them even under two feet of snow.
This ability to precisely map out a landscape as well as keep a mental store of sounds that indicate the presence of prey makes the owl a deadly flying arbiter over its domain by simply keeping open ears.
Under the Sea
The omniscient owl’s hearing ability makes it a threat anywhere where sound travels through the air, but what if we switch out air for water? It’s hard to imagine the threat imposed by an underwater owl, even if sound travels five times faster in water than in air. The owl can no longer pinpoint the direction of sounds with its radar shaped face because the sound waves vibrate through its entire skull rather than just the inner ear architecture. Trying to hear underwater would result in garbled and unintelligible audio for the owl.
The structures by which animals convert mechanical sound waves into chemical signals for our brains is specialized for the environment they are in. For an owl, its super-powered hearing is rendered useless underwater, but for animals that live underwater, they have adapted other means of auditory perception.
For cetaceans (whales, dolphins, and porpoises), their ears are remnants of their terrestrial past. Once they returned to the water, their auditory anatomy evolved to their environment by adjusting its sensitivity to the movement of molecules in the outside world. Unlike humans, who have a middle ear and inner ear attached to our skull, cetaceans have their entire middle ear encased in a dense bone structure called a bulla.
Instead of having incoming sound waves vibrate the entire skull, the bulla isolates the middle ear and creates a soundproof stage for an audio feed. A section of fat runs along the jawline to the bulla, and because water and fat are of similar densities (as opposed to bone and water) and the sound waves travel with high fidelity along the jaw straight back to the middle ear.
Cetaceans are able to localize the source of sound, detecting friend or foe, prey or predator, and navigate their way through the waters they call home.
Fin-stagram
Dolphins are highly social creatures and communicate using a series of signature whistles, echolocation and social communication that has evolved over the course of their five million years on this planet.
They will often have a unique signature whistle by the time they reach two years of age, which help dolphin calves and their mothers find each other, and allow lost dolphins to find their pods. Some dolphins are known to adjust their signature whistles upon forming alliances with another dolphin. Groups of dolphins that meet at sea will exchange signature whistles to identify themselves and facilitate their socialization.
For animals that hunt in packs, coordination is essential for a successful campaign. Using echolocation, dolphins can detect prey from as far as 200 meters away. As they approach a school of fish, they will vocalize more frequently, attracting other dolphins to the area to assist in rounding up the fish. Once the fish are herded, the dolphins take turns swimming through the assembly for a meal. This highly synchronized operation requires quick reflexes and telegraphic communication between the dolphins. The more expressive they can be in vocalizing their intent, the more fish and friends they end up with.
Dolphins aren’t the only intelligence on Earth that have realized the value of an underwater biosonar feedback system: the US Navy actually has a Marine Mammal Program in San Diego which as of 2019 employs 70 dolphins to help with harbor protection and mine detection. The dolphin is able to distinguish the density of the object with their echoes. Using echolocation, they are trained to detect and mark the location of mines on the sea floor to be disarmed. In 2003, the Navy flew nine of its dolphins to identify mines in the Persian Gulf and were deployed to hunt for mines. In a week, the dolphins helped the Navy identify and disable more than 100 anti-ship mines.
I’m All Ears
Hearing enables life on Earth to socialize and communicate. It connects life to the outside world and keeps us safe by warning us of potential danger. Regardless of where it comes from, a sound can immediately capture an animal’s attention. Whereas scientists have discovered many species of blind amphibians, reptiles, fish and mammals, no naturally deaf vertebrate species has been discovered.
While organisms might vary in the of range of frequencies that they can perceive, hearing’s inherent purpose remains the same: taking outside stimulus and ascribing meaning to the event. An dog bark causes a squirrel’s adrenaline to spike, while it simultaneously gives a child a rush of serotonin. The same external event is processed internally with two highly contrasting responses: one might start running away at full speed and the other towards.
The world around us is filled with all kinds of sounds with no inherent meaning, but that doesn’t stop us from enjoying the beauty of nature’s sound. Do animals that live in nature get the same feeling of serenity that we feel listening to the murmur of a creek? Or do their eyes get heavy to the lullaby of an ensemble of chirping crickets? The sound may not be received in the same intention it was produced, but yet we still choose to learn and engage with it, so long as we can so long as we are willing to keep our ears open and simply listen.
Human Ears
In human ears, the outer ear consists of three parts: the pinna, the auricle, and the ear canal. In several other species, the pinna is mobile and can be directed towards the source of sound, similar to how we are able to shift our eyes’ focus to a point of interest.
These sections work in conjunction to collect and focus sound waves onto the tympanum, more commonly known as the eardrum. The eardrum is a very thin membrane that oscillates in response to changes in air pressure and sound waves. These propagated waves transfer to the middle ear, which is constructed of miniscule bones known as the hammer, the anvil, and the stirrup. This begins a chain reaction in the middle ear, beginning with the hammer and ending with the stirrup to transfer the sound wave to the inner ear.
The cochlea, named after the Greek word for the spiral shell of a snail, kōhlias, is the unofficial functionary of the inner ear, and has fluid that reverberates with auditory stimulus and activates movement in the hair cells that cover the surface of the cochlea. This mechanical movement triggers a chemical response to hearing nerves, and the information is sent to the auditory cortex via electrical signals for interpretation.
This entire process detailed above seems lengthy and complex, and yet it happens automatically and almost instantaneously; we as humans can aurally recognize stimulus quicker than any other sense. A human brain can identify a sound within .05 seconds of its occurrence, nearly 10 times faster than the blink of an eye.
