In 2004, when an 810-mile swath of the Sunda megathrust fault ruptured beneath the Indian Ocean, it triggered a 9.1 magnitude undersea earthquake off the coast of Sumatra, Indonesia, the third largest tremor ever recorded. The abrupt rise of the seabed displaced a staggering volume of water, generating a wave of tsunamis from the quake’s epicenter in every direction.
Some 230,000 people across fourteen coastal countries died, but, in the aftermath, locals and rescuers in certain areas noted a conspicuous absence of animal casualties. In the following weeks and months, stories emerged of some animals acting oddly just before the tsunami hit: Eyewitnesses in Sri Lanka and Thailand told of elephants that trumpeted before seeking higher ground, dogs that refused to go outside, and flamingos that suddenly abandoned low-lying nesting areas. For centuries, anecdotal stories have circulated about animals possessing some primal sixth sense that alerts them to an imminent natural disaster, but does science back it up?
While it’s clear that animals have different or heightened sensory capabilities compared to humans, very few scientists will go on record to support the idea that animals and insects possess a biologically determined sixth sense that allows them to portend coming trouble. In the case of the elephants that reportedly made for higher ground before the tsunami hit, one theory is that they picked up on infrasound waves generated by the tremor. These waves have a fundamental frequency of 20 Hz or lower, and fall outside the limits of normal human hearing (the bottom note on a piano, A0, has a frequency of about 27.5 Hz, and is generally the lowest tone humans can differentiate).
Infrasonic sound waves can be spawned by intensely energetic occurrences like earthquakes, volcanic eruptions, avalanches, lightning, meteors, and iceberg calving. Elephants, rhinos, hippos, whales, felines, dogs, and many birds rely on infrasonic sounds for both communication and navigation. When the Sri Lankan elephants detected the initial low frequency rumblings coming from the Indian Ocean, it wasn’t as if they sensed the coming tsunami, they just instinctively moved away from the source of the sound, which, in this case, happened to be the right decision.
Many animals, insects, and birds are also particularly sensitive to Rayleigh waves, a type of surface wave that travels along solid ground. After the initial rupture, the waves would have traveled through the earth’s crust from the epicenter, causing minute vibrations. The waves are inaudible and travel at ten times the speed of sound, and could therefore have been noticed by those animals sensitive to them well before the slow-moving tsunami crashed ashore. Humans actually have mechanoreceptors in our skin called Pacinian corpuscles which act to detect changes in vibration and pressure, but, as their optimal sensitivity is 250 Hz, and Rayleigh waves generated by earthquakes are typically below 20 Hz, they do little for us in these situations.
Ants have a fascinating ability to seemingly anticipate both earthquakes and coming rainstorms. A recent study in Germany documented red wood ants and their propensity to build nests along active fault lines. The three year study showed that the ants, in the hours leading up to a quake, would go about their daily routine, but would stay awake and outside their mounds at night, even though this made them vulnerable to predators. The day after the quake hit, the ants would revert to their normal behavior.
Though researchers are still trying to figure out the mechanism that causes the change in behavior, it’s proposed that ants have receptors that can pick up on barely detectable changes in atmospheric gases and electromagnetic fields that are the byproduct of tremors and storms. Ants will often build up mounds around their ground holes for extra protection before heavy rains. They’ll also seek out higher nesting spots, such as the tops of tree stumps and potted plants, in an effort to avoid being washed away. For years, farmers have been tipped off to coming rain by noticing a dramatic uptick in ant activity before a downpour.