How elephants feel each other’s messages through the ground

By Stephen Beech

Elephants can communicate with each other through ground vibrations thanks to their massive ears, reveals new research.

Their large and heavy lugholes - along with ear canals that can be sealed off at will - allow the iconic species to stay in touch with other herd members through "vibes" over six miles away, say scientists.

Elephants are able to communicate with each other across distances of up to five kilometers (3.1 miles) by producing trumpeting sounds with their trunks.

But they have a second way of sending signals - seismic waves traveling through the ground.

Scientists explained that the vibrations are transmitted from elephants' feet through their legs and ultimately through the bones of their skull directly into the inner ear.

The method - called "bone-conduction hearing" - can be perceived across distances of 10 kilometers (6.2 miles) or more.

Now a new study has found why bone-conduction hearing works so well for elephants: it may all come down to size and a special muscle.

Senior author Sunil Puria said: "Ear canal listening devices such as AirPods can be annoying because we hear body-generated sounds louder than normal, for example, when we walk or chew.

"Elephants, however, may use the ability to close their ear canals to their advantage in long distance communication.

"We found that elephants' bone-conduction hearing is significantly improved through their larger middle ear structures and possibly further enhanced by voluntarily closing the ear canal."

For the study, published in the journal Frontiers in Audiology and Otology, researchers used temporal bones, a part of the skull that houses the middle and inner ear.

Samples came from deceased elephants and human donors.

The research team attached the temporal bones to a device that creates vibrations mimicking sound traveling through the body into the skull.

By shining a laser beam, it measured how much tiny reflective markers placed on the middle ear bones moved in response to vibrations generated by low- and high-frequency stimulation.

The ear canal was closed with a soft foam plug for the experiment.

Elephant middle ear bones vibrated most effectively at a frequency of about 400 Hz, whereas human bones did so at around 1.2 kHz.

Below those frequencies, elephants' stapes - a small middle-ear bone that transmits vibrations to the inner ear - moved three to four times more than humans' stapes.

Greater movement doesn't equal better hearing ability, but it means more vibrations are transmitted to the cochlea – the part of the ear where vibrations from sound waves are translated into neural signals.

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Previous studies showed that elephants have better sensitivity to low-frequency hearing for air conduction, so the researchers say it stands to reason that they would also hear low-frequency bone-conducted sounds better than humans.

Study first author Caitlin O'Connell-Rodwell, of Harvard Medical School in Boston, said: "Although we suspected as much based on their behavior in the wild and responses to vibrational stimuli, it was very gratifying to show that elephants have excellent bone conduction hearing."

The researchers say the reason for elephants' greater sensitivity to low-frequency sounds is likely anatomical.

Their middle ear bones are nine times heavier and their eardrums seven times larger than those of humans.

In most mammals, organ size scales with body size - meaning an elephant's middle ear isn't specialized in its structure compared to a human one, it's just bigger.

Puria, from the Department of Otolaryngology at Harvard Medical School, said: "Because of their ear size, elephants can better transmit lower frequency sounds to the cochlea.

"The specialization comes from the cochlea adapting to this greater input and generating neural responses that the brain can use and interpret for communication."

The researchers say elephants' ability to voluntarily close their ear canals – a capacity humans lack – could further improve their excellent low-frequency hearing.

They hypothesized that when listening to low frequencies of around 200 Hz or lower, elephants may contract a muscle that closes the ear canal, achieving an effect similar to that of humans inserting earplugs or in-ear headphones.

O'Connell-Rodwell said: "Elephants produce infrasonic vocalizations in the frequency range of 10 to 20 Hz."

Puria added: "Based on our estimates, elephants' ability to close their ear canals could enhance their bone-conduction hearing by up to 30 times when listening to these infrasonic frequencies.

"However, the exact improvement in sensitivity would depend on the extent to which the ear canal volume is blocked by the muscle."

The team says the study opens avenues for further research.

They explained that, for their experiments, the cochlea had been drained of fluids due to a lengthy procurement and preparation process, which may have led to an underestimation of the reported results.

Elephant tissue is scarce to obtain, which is why the number of samples available was limited.

Puria added: "There are few creatures more majestic than elephants.

"Their behavioral characteristics might be better understood through their hearing capabilities.

"We need better data about their absolute hearing sensitivity across frequencies with air and bone-conduction stimulation.

"We have tried this and found that it is easier said than done."