What a Noise
It couldn’t have been their sense of hearing that has made human beings the dominant species on this planet. Numerous animal species are far superior to humans in this respect.
Especially in the high-frequency range, human hearing ability is rather paltry in comparison to other species, with its maximum audible frequency of just 20 kilohertz. Domestic animals like dogs, cats and mice can perceive sounds in a frequency range of up to 50 kilohertz. Dolphins and whales can hear in a range up to 80 kilohertz, while some species of bats can even manage to hear sounds at frequencies well above 100 kilohertz. The grasshopper, which doesn’t have ears at all, picks up extremely quiet sound vibrations using fine circular membranes called tympana on its legs, and even recognizes the frequency of the sounds and interprets them. In fact, it is similar to microphones and loudspeakers, which also pick up and generate sound using fine membranes. Their technology has been continuously refined over the decades, leading to the recording and reproduction of sounds that are far outside the human range of hearing.
Many animal species have the ability not only to hear but also to produce high-frequency sounds. They use these sounds for various purposes: whales and dolphins emit ultrasonic sounds for both communication and orientation, and their brains translate the reflected sound waves into an image of their surroundings. This is ideal for orientation under water, as vision is extremely limited at great depths. Sound waves, on the other hand, propagate more than four times faster in water than in air, with less energy loss.
Ultrasonic sound is also an excellent means of navigation in the air, especially at night. For example, bats find their way by emitting high-frequency screeches and then interpreting their echo. Some types of bats even produce sounds as loud as a pneumatic drill. In such cases, the human being’s limited audio range comes as a blessing, as noise at night would otherwise be unbearable. The animal kingdom as a whole is full of curious sounds. Herrings, for example, communicate by forcing air from their swim bladders into their anal duct, thus producing pulsating, high-frequency sounds that can cover three octaves. Every night, the male Chinese cascade frog performs songs that would honor any song bird, reaching frequencies that are well beyond the human hearing range. The complexity and frequency of the sounds determine the frog’s chances of attracting the opposite sex. Music is the food of love.
Elephants also have a talent for imitating voices. A female elephant from the Tsavo National Park in Kenya can imitate the engine noise of a truck two miles away on the highway to Nairobi. Elephants like to communicate with very low sounds, for example by using their feet.
The Colombian ground squirrel, a rodent found in North America, warns his fellow animals of danger by emitting sounds in the ultrasonic range that cannot be heard by his enemies. In the animal kingdom, an exceptional sense of hearing is often a means of survival in the wild.
But even the best sense of hearing doesn’t always help. The Egyptian cotton moth is equipped with very fine hearing that reliably warns it against approaching bats. It also has a very highly developed sense of smell. Unfortunately, this sense of smell can sometimes be a disadvantage: when the male moth picks up the scent of a female that is ready to mate, its sense of hearing is temporarily deactivated – and even the screech of an oncoming bat no longer triggers the flight instinct. It seems that love not only makes you blind, but it can sometimes make you deaf.
The Best of the Animal Kingdom
The loudest noise in the animal kingdom is produced by a tiny animal, the snapping shrimp. With an oversized claw resembling a boxing glove on its front leg, the shrimp creates an air bubble (cavitation bubble) that implodes with a loud bang at 150-200 decibels.
This is closely followed by the blue whales and fin whales, which “talk” to one another at up to 180 decibels.
The loudest land animal is the howler monkey. For its ear-splitting screech, it uses the frequency range around 200 Hertz, which is ideal for penetrating the dense rain forest.
The hearing ability of the lynx is proverbial. Its large ears with ear tufts up to 40 cm long that act almost like antennas allow it to precisely differentiate between different sounds. The lynx can hear a mouse rustling in the grass 65 meters away.
The gorilla has the largest eardrum, approximately 97 square millimeters in size. In second place is the domestic cow, with 86 mm2, followed by the horse, with 75 mm2 - and human beings, with 62 mm2.
Bats, whales and some types of rodents have been known to communicate via ultrasound for a long time. The cascade frog is a newcomer. It was only recently discovered by an American-Chinese group of explorers in Eastern China. The cascade frog (amolops tormotus) is a rare and rather small frog that lives in the Huangshan mountains with its many hot springs and waterfalls. In order to make itself heard against the constant roar of the water, the cascade frog has resorted to an acoustic trick: it has expanded its voice spectrum to include ultrasound. The male frog has funnel-shaped ear openings and is able to produce calls which are reminiscent of a bird’s song rather than the croaking of a frog. Its calls easily exceed 20 kilohertz – and thus the ultrasound threshold.