Elephants can communicate with other elephants across distances of up to five kilometers by producing sounds that travel by air. However, they have a second way of sending signals: seismic waves traveling through the ground. These vibrations are transmitted from elephants' feet through their legs and ultimately through the bones of their skull directly into the inner ear. They can be perceived across distances of 10 kilometers or more. This is called bone-conduction hearing.
Now, a Frontiers in Audiology and Otology study has found why bone-conduction hearing works so well for elephants: it may all come down to size and a special muscle.
"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," said senior author Dr Sunil Puria, an associate professor at the Department of Otolaryngology at Harvard Medical School and at the Massachusetts Eye and Ear. "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."
Vibrating bones
For the study, published in the journal's Auditory Science section, the scientists used temporal bones, a part of the skull that houses the middle and inner ear. Samples came from deceased elephants and human donors. The 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 these 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.
Previous studies showed that elephants have better sensitivity to low-frequency hearing for air conduction, so it stands to reason that they would also hear low-frequency bone-conducted sounds better than humans, the team 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," said first author Dr Caitlin O'Connell-Rodwell, formally an instructor in the Department of Otolaryngology at Harvard Medical School.
Sharp ears
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. This means an elephant's middle ear isn't specialized in its structure compared to a human one – it's just bigger.
"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," explained Puria.
Elephants' ability to voluntarily close their ear canals – a capacity humans lack – could further improve their excellent low-frequency hearing. The authors 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.
"Elephants produce infrasonic vocalizations in the frequency range of 10 to 20 Hz," O'Connell-Rodwell explained.
"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," Puria elaborated. "However, the exact improvement in sensitivity would depend on the extent to which the ear canal volume is blocked by the muscle."
This study opens avenues for further research in this intriguing field. 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, the team pointed out. Elephant tissue is scarce to obtain, which is why the number of samples available was limited.
"There are few creatures more majestic than elephants," concluded Puria. "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."
