I remember vividly how, more than fifty years ago, a ranger at the Wankie National Park in Rhodesia (now the Hwange Park in Zimbabwe) noticed how, as a herd of elephants was moving towards a water hole in the evening, other herds, often as far as ten miles away, would abruptly change their direction and head towards the same watering place. Similarly, it intrigued me as I occasionally watched from a blind that, as a herd left a watering hole, there was invariably another one on the perimeter waiting to take their place. In both cases there seemed to some form of communication involved, but if so, what and how?
The mystery was solved in 1984 by Katy Payne, an audiobiologist at the Washington Zoo in Portland, Oregon, who, while observing Asian elephants at the zoo, noticed an unusual vibration in the air when she was in their presence – infrasound. From the Latin (infra meaning below,) infrasound is defined as sound waves with frequencies below 20Hz, and which are thus inaudible to humans. And indeed, when humans are exposed to sufficiently powerful infrasound, the stimulus may be felt in one’s body rather than heard through one’s ears.
As air flows through the dense folds of the elephant larynx , the relationship between the length, mass, and elasticity of elephant vocal folds allows them to produce sounds lower in frequency than any other terrestrial animal. In addition, the anatomy of the outer and inner ear of elephants is comprised of large, specialized components which allow them to detect low-frequency sound at long distances.
Infrasound offers a number off advantages when communicating over long distances – coordination of herd movements to improve resource availability (or, as in the first paragraph above, to regulate arrivals at the water source so as to avoid over-crowding,) the avoidance of rival herds and predators, and the facilitation of mate-finding for bulls in musth.
It makes sense in evolutionary terms. In the dense forests in which elephants evolved, limited vision necessitated other forms of communication. This likely led to enhanced senses of smell and hearing as early elephants directed their herds, located each other, and warned others of predators. And why low frequency sound? Because unlike high frequencies, they are are not blocked or muffled by thick vegetation.
Nor is it limited to communication. It is reported that during the 2004 Indian Ocean earthquake and tsunami, elephants in Thailand, Sri Lanka, and India displayed unsettled behavior and moved to higher ground before the tsunami reached the coasts of those countries. The supposition is that they were able to react to infrasonic sounds from the massive disturbances long before humans were aware of it. Similarly, in both India and Kenya, traditional lore has long associated the beginning of a period of rain with the sudden appearance of elephants. Scientifically, both Asian and African elephants have been shown to predict thunderstorms at distances of up to 90 miles. The movements within cumulonimbus clouds produce strong infrasonic signals, and it is thought that elephants may be able to hear these and locate the storm.
The use of sound outside of our ability to hear is not unique to elephants – bats, alligators, giraffes, hippopotamuses, peacocks, rhinoceroses, and whales are all known to utilize infrasonic communication. Incredibly, infrasonic calls produced by blue whales have been recorded crossing the entire Atlantic Ocean, from South America to Africa. And infrasound has natural uses beyond just communication – homing pigeons and migratory bird species may use naturally occurring infrasound as a navigational tool.
Incidentally, An Immense World by the Pulitzer-winning science journalist, Ed Yong, is a spectacular and fascinating account of the latest research as to how animals perceive the world around them. It ties in neatly with David Papke’s observation elsewhere in this newsletter that “Anthropomorphic … errors in interpreting bee behavior might not be an over-estimation of their intelligence but something entirely different – overlooking completely another form of intelligence that differs radically from our own.”
What is the connection between elephants and honey bees? Earlier this year a team led by Yossi Yovel at the University of Tel Aviv has shown that plants ‘scream’ when under stress from drought or physical damage, emitting high frequency noises undetectable to the human ear but audible to many species of insects, bats and rodents. In particular, they found that female moths avoided laying eggs on tomato plants that were emitting ultrasonic noises associated with stress. The researchers conjecture that the moths interpreted the sounds as a sign that the plants were unhealthy. “This is speculation at this stage,” says Yovel, “but it could be that all sorts of animals make decisions based on the sounds they hear from plants, including about feeding, pollination and shelter.”
Seventy years ago, when Karl von Frishch interpreted the bee dance, he believed that it would lead a forager bee directly to a flowering plant. Using von Frisch’s own data, we know now that it is not sufficiently specific to do that; it gets the forager out of the hive and flying in the right direction, but there are two further stages before she arrives at the flower, which Jurgen Tautz explores in Communication Between Honey Bees; More than Just a Dance in the Dark. The first he labeled ‘the search zone’ in which returning foragers fly with their Nasanov glands open, thus laying a chemical trail to the food source. Tautz concedes that this needs more research. The final stage is when the forager uses the scent released by the original dancer in the hive to confirm the flower source.
In the light of Yossi Yovel’s research, is it only scent that finally attracts the bees? Could it be that plants with available pollen and nectar also emit a sound that is inaudible to us but is a trigger to the sensitive antennae of a honey bee? After all, we know that plants that have had their supplies of nectar exhausted emit a signal to let foragers know they have closed up shop. It seems reasonable to posit that plants also signal audibly to foragers that they are open for business.
Our Green Cathedral 