How elephants use their footsteps to send messages across the ground and listen via skull vibrations
An elephant does not need to hear another elephant to know it is there. It can feel it. Yes, it’s a bit like telepathy, but not quite. You see, elephants can communicate with other elephants through sound, which can travel up to five kilometers through the air. But that’s not their only way of communicating. They have a second pathway running parallel to it: the vibrations travel through the ground itself, crawling up through the elephant’s feet, legs and skull, and then to the inner ear. A new study is published in the journal cutting edge audiology and otology Explains why this system works so well for elephants.
Second communication channel
Researchers at Harvard Medical School and the Massachusetts Eye and Ear Institute have found that a second system called bone conduction hearing can transmit signals up to 10 kilometers or more, roughly twice the range of an airborne call.“Ear canal listening devices like AirPods can be annoying because we hear sounds from our bodies louder than usual, for example, when we walk or chew,” senior author Dr. Sunil Puria, associate professor of otolaryngology at Harvard Medical School and Massachusetts Eye and Ear, said in a statement.“However, elephants may exploit the ability to close their ear canals to communicate over long distances. We found that elephants’ bone conduction hearing is significantly improved by their larger middle ear structures and may be further enhanced by voluntary ear canal closure.”
they have vibrating bones
To test bone conduction, the researchers used the temporal bone, the part of the skull that houses the middle and inner ears. They collected samples from dead elephants and human donors. They mounted the bones on a device that produced vibrations that simulated sound traveling through the body and into the skull. They used a laser beam to track the amount of movement of tiny reflective markers on the middle ear bones. The ear canal was sealed with foam during each trial.The elephant’s middle ear bones vibrate most effectively around 400 Hz. The peak value of human bones is close to 1.2 kHz. Below these frequencies, the elephant stirrup, a small bone that transmits vibrations to the inner ear, moves three to four times as much as the human stirrup. More movement does not mean sharper hearing; however, it does mean more vibrations reach the cochlea, where it is converted into signals that the brain can read.Previous research has shown that elephants have better sensitivity to low-frequency hearing through air conduction, so it makes sense that elephants also hear low-frequency vibrations better than humans through bone conduction.“Although we had doubts based on their behavior in the wild and responses to vibratory stimuli, it is very exciting to learn that elephants have excellent bone conduction hearing,” said first author Caitlin O’Connell-Rodwell, PhD, a former lecturer in the Department of Otolaryngology at Harvard Medical School.
The ears are bigger
An elephant’s middle ear is no different than a human’s. It’s just bigger. This may be why elephants are more sensitive to low-frequency sounds. The middle ear bones are nine times heavier than humans, and the eardrum is seven times larger. So this means that an elephant’s ear is not structurally special; it’s just larger.“Due to the size of their ears, elephants can better transmit low-frequency sounds to their cochlea. This specialization comes from the cochlea adapting to this larger input and producing neural responses that the brain can use and interpret for communication,” Priya explained.
Muscles that work like earplugs
Another interesting fact is that elephants can voluntarily close their ear canals. Humans don’t have this ability. This may be another reason why elephants have excellent low-frequency hearing. According to the researchers, this is accomplished through muscle contractions that unfold when elephants listen to frequencies of 200 Hz or lower. This creates an effect similar to a human inserting earbuds or in-ear headphones.“Elephants emit infrasonic vocalizations in the frequency range 10-20 Hz,” O’Connell-Rodwell explains.Puria added, “We estimate that an elephant’s ability to close its ear canal when hearing these infrasonic frequencies could improve bone conduction hearing by up to 30 times. However, the specific increase in sensitivity will depend on the extent to which the ear canal volume is blocked by muscles.”The study also had limitations because of the difficulty of obtaining elephant tissue and the lengthy preservation process the samples underwent. Fluid in the cochlea was drained, which may have led to studies underestimating the true effect.“Few creatures are more majestic than elephants. Their behavioral characteristics can be better understood through their hearing abilities. We need better data to understand their absolute hearing sensitivity across frequencies under air-conducted and bone-conducted stimuli. We have tried this and found that it is easier said than done,” Priya concluded.
