An elephant doesn’t need to hear another elephant to know it’s there. It can feel it. Yes, kind of like telepathy, but not exactly. You see, elephants can communicate with other elephants through sounds that can travel through the air up to five kilometres. But that’s not their only mode of communication. They have a second channel running alongside it: vibrations that move through the ground itself, climbing up through an elephant’s feet, legs and skull before reaching the inner ear. A new study published in the journal Frontiers in Audiology and Otology explains why this system works so well for elephants.
The second communication channel
Researchers at Harvard Medical School and Massachusetts Eye and Ear found that this second system, known as bone-conduction hearing, carries signals over 10 kilometres or more, roughly double the range of airborne calls.“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,” senior author Dr Sunil Puria, an associate professor in the Department of Otolaryngology at Harvard Medical School and Massachusetts Eye and Ear, said in a statement.“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.”
They have vibrating bones
To test bone conduction, the researchers used temporal bones, the part of the skull housing the middle and inner ear. They took samples from deceased elephants and human donors. They mounted the bones on a device that generates vibrations that mimic sound travelling through the body into the skull. Using a laser beam, they tracked how much tiny reflective markers on the middle ear bones moved. The ear canal was sealed with foam for each trial.Elephants’ middle ear bones vibrated most efficiently around 400 Hz. Human bones peaked closer to 1.2 kHz. Below those frequencies, the elephant stapes, a small bone that relays vibration to the inner ear, moved three to four times more than its human counterpart. More movement doesn’t automatically mean sharper hearing; however, it does mean more vibration reaching the cochlea, where it is converted into signals the brain can read.Previous research showed that elephants have better sensitivity to low-frequency hearing through air conduction, so it makes sense that elephants would also hear low-frequency vibrations through bone conduction better than humans.“Although we suspected as much based on their behaviour in the wild and responses to vibrational stimuli, it was very gratifying to show that elephants have excellent bone-conduction hearing,” first author Dr Caitlin O’Connell-Rodwell, formerly an instructor in the Department of Otolaryngology at Harvard Medical School, said.
The bigger ear
Nothing about an elephant’s middle ear is built differently from a human’s. It is simply larger. This could be the reason for elephants’ greater sensitivity to low-frequency sounds. The middle ear bones are nine times heavier and the eardrums seven times larger than those of humans. So, it means the elephant’s ear isn’t specialised in structure; it is just bigger.“Because of their ear size, elephants can better transmit lower-frequency sounds to the cochlea. The specialisation comes from the cochlea adapting to this greater input and generating neural responses that the brain can use and interpret for communication,” Puria explained.
A muscle that works like earplugs
Another fascinating fact is that elephants can voluntarily close their ear canals. Humans don’t have that ability. This could be another reason why elephants have excellent low-frequency hearing. According to the researchers, this happens through a muscle contraction, deployed when an elephant is listening for frequencies around 200 Hz or lower. This creates an effect similar to that of humans inserting earplugs or in-ear headphones.“Elephants produce infrasonic vocalisations in the frequency range of 10–20 Hz,” O’Connell-Rodwell explained.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 research also has limitations, as obtaining elephant tissue is difficult and the samples had gone through a long preservation process. The cochleae had been drained of fluid, which may have caused the study to understate the real effect.“There are few creatures more majestic than elephants. Their behavioural 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,” Puria concluded. Go to Source
