Seeing with their ears
The tiniest bat and mightiest toothed whale navigate and hunt in darkness using echolocation. Even some humans have this remarkable skill.
How bats and whales navigate in darkness using echolocation.
DEEP IN THE ABANDONED mine shaft, bats were on the move. It was dusk – time for insect hunting. At the mine entrance, Kyle Armstrong knew they were coming because his bat detector device was picking up their ultrasonic echolocation calls, normally inaudible to the human ear. It was the late 1990s. Kyle was working on his zoology doctorate at the University of Western Australia and wanted to catch some bats to collect non-lethal biopsy samples for a genetic study. The species he was focusing on was the orange leaf-nosed bat; the mine was at Bamboo Creek, in the East Pilbara, in northern WA. He’d set up a f ine-mesh mist net across the entrance and was confident he’d catch a few.
The bats arrived from the depths – but refused to f ly into the net. Detecting the f ilmy material in front of them, they spun around, tried again and spun around again. “They did this for some minutes, backwards and forwards, and then they discovered a pre-existing tear in my net, and in the blink of an eye they all zipped out through the hole and were gone,” Kyle tells me.
The experience left a lasting impression. “It brought home to me how clever bats are at processing their ultrasonic echolocation signals at high speed and f inding out about their environment, even something as f ine as a mist net,” he says.
For Kyle, it led to a career-long fascination with bats, their echolocation abilities and calls, particularly as a tool for identifying species. It’s all part of a branch of science known as bioacoustics. “I’m interested in bat echolocation because it underpins so much of their ecology and evolution,” he says. “You can see bats as similar to rodents, with two extra layers of complexity: they f ly and they echolocate.”
Kyle’s bat research has taken him to many parts of Australia, to Japan’s islands and Papua New Guinea’s rainforests, as an independent zoological consultant or on behalf of institutions such as the University of Adelaide and the South Australian Museum. When I met him in early 2018, he was running a citizen science project for the museum to gather information on microbats in the Murray-Darling Basin.
ECHOLOCATION IS A biological sonar capability that enables some animals to ‘see’ in murky to pitch-dark conditions. An echolocating animal emits specialised calls and detects echoes of those calls ref lecting back from surrounding objects. From those echoes the animal builds a comprehensive, three-dimensional mental image of the world around it.
Nature’s finest echolocators are microbats and toothed whales, which include sperm whales, orcas and dolphins. Two kinds of cave-dwelling birds – swiftlets and oilbirds – also echolocate. So do a few small mammals, such as shrews and shrew-like creatures called tenrecs, although their techniques are more basic. Increasingly, blind people are also learning to use echolocation with remarkable success (see page 82).
There are two main bat types. Megabats include large fruit-eaters and nectar-feeders, such as f lying foxes, and smaller blossom bats. Microbats mostly hunt insects, although some feed on small animals, blood and nectar. Megabats usually don’t echolocate; microbats mostly do, each species in its own way.
The expression ‘blind as a bat’ is a timeworn slur. Although eyesight in many microbat species isn’t great, visual deficiency is more than offset by echolocation and hearing skills, to the point where a bat on the wing on a pitch-black night can detect and catch a mosquito and perceive something as fine as a human hair.
PRODUCING SIGNALS IS ONE half of echolocation; the other is detecting and analysing returning echoes. These are so faint that not even an electronic bat detector can pick them up. So it’s hardly surprising bats are gifted with one of nature’s most sensitive hearing systems. And tiny though its brain may be, a bat is capable of analysing vast amounts of information streaming in from its ears at mind-boggling speed.
Delays are important to this process. The nanosecond delay between the call and the echo tells how far away an object is. And, like other animals, if the bat detects an echo arriving in one ear before the other, it can fix the object’s horizontal position.
Internally, the bat’s hearing equipment is much like that of other mammals, except that in the inner ear of some species a section of the cochlea called the basilar membrane is super-sensitive to the frequencies of returning echoes.
Signals from the cochlea pass via auditory neurons to an organ called the inferior colliculus, in the midbrain, where ‘delay-tuned’ neurons precisely measure the gap between outgoing calls and their echoes. From there, the results go to the auditory cortex, which combines the information with a mass of other data, including memorised material about the environment garnered during past f lights. Not surprisingly, the bat auditory cortex is relatively large.
With this comprehensive auditory and neural toolkit, a bat can very precisely gauge an object’s position, size, shape, texture, density, movement, speed and direction. Zeroing in on a moth f lying across its path, a bat works out exactly where the insect will be in a moment’s time and aims for that spot so as to capture it with minimal use of energy. It makes these calculations in microseconds.
The image of the outside world that the bat creates in its head through echolocation may be as comprehensive and precise as that created by the visual cortex of a visually guided creature. It’s a remarkable feat for such a small brain. One scientif ic study concluded: “Bats face some of the most diff icult neurobiological challenges of any mammal…and solve them with apparent ease and grace.”
IN SUNLESS OCEAN DEPTHS or in murky, silt-laden river shallows, toothed whales can’t rely on their eyes to navigate or catch food. Instead, these marine mammals – including
A bat can very precisely gauge an object’s position, size, shape, texture, density, movement, speed and direction.
sperm whales, belugas, narwhals, orcas, porpoises and river dolphins – echolocate. And they’re every bit as proficient as bats.
However, although there are many similarities in the way they do it, the underwater realm demands some very different sonar tools. Most signif icantly, water is more dense than air. Sound travels 4–5 times faster in water than in air, as well as much further. Dr Brian Miller, a marine mammal acoustician with the Australian Antarctic Division, says this allows toothed whales to detect prey hundreds of metres away and, in the case of the sperm whale, possibly more than 1km. Contrast this with the bat’s maximum range of 30m.
Water’s density also means echoes from echolocation calls are more stable. Prey such as fish and squid fleeing a predator move very differently from fast-fluttering insects and create their own echo patterns.
Brian says sperm whales and harbour porpoises use mainly clicks to perceive their environment and communicate. “However, dolphins and killer whales, which are taxonomically dolphins, create sounds beyond echolocation clicks,” he says. “They produce whistling and singing sounds, burst-pulse sounds halfway between a whistle and a click, and quite complex communication sounds beyond that.”
The frequencies of echolocation signals of toothed whales range from three to 150 kilohertz (kHz), so humans can hear a few of them. Like bats, they modify their sonar signals as they detect and approach prey. They adjust the sound’s intensity, auditory sensitivity and pulse rate according to the distance to the target, ending a chase with a high-repetition, low-intensity buzz. And, again like bats, they can widen the echolocation beam at close range to get a wide-angle ‘view’.
Toothed whales generate their echolocation calls in their heads, and the sound streams out through a large blob of fat called the melon that sits on the upper jawbone. This melon accounts for the bulbous shape of all toothed whales’ heads. It acts as a kind of acoustic lens; massive surrounding muscles squeeze and change its shape, organising and focusing the sound vibrations.
The vibrations are generated in the air passage directly beneath the blowhole and behind the melon. Under water, the animal keeps the blowhole closed and can circulate pressurised air through the passage, associated air sacs and what are called phonic lips, setting up sound vibrations that reverberate through surrounding tissue and bounce forward off the front of the skull. In sperm whales, the blowhole is further forward and the anatomy is different, but the mechanism works similarly.
To detect returning echoes, toothed whales have hearing that’s as sensitive as any bat’s. But, as with sound production, the mechanism is different.
A toothed whale’s external ears are just tiny holes and play no hearing role. Instead, incoming sound is picked up by fatty deposits in the lower jaw and carried by fat-f illed channels to the internalised middle and inner ears, housed in two bony capsules, called the auditory bullae, under the skull. There, they are shielded from the intense outgoing calls.
The toothed whale’s cochlea is as specialised and attuned to higher frequencies as the bat’s. The specialisations continue into the auditory cortex, ensuring that some of the biggest creatures on Earth can hear the smallest of sounds.
“Generally, on land, bigger animals hear lower frequencies. That’s a function of bigger mechanical structures being better able to pick up those frequencies,” Brian says. “But in dolphins and larger whales you can still have large hearing structures that are tuned to very high frequencies.”
That bats and toothed whales, in a beautiful case of convergent evolution, separately honed echolocation to the same heights of perfection may have something to do with their shared mammal ancestry. Some birds also echolocate, but are their skills as highly developed? Brian is doubtful. “It does seem there might be something in the mammalian auditory system that makes it better suited to echolocation than the avian auditory system,” he says.
BIRDS WEREN’T ON DERMOT SMYTH’S mind when he visited Chillagoe Caves, west of Cairns, while doing his zoology PhD on fruit bats at James Cook University in 1979. “A friend of mine invited me on a f ield trip to the caves to do a bat inventory,” he said. “We set some nets 30m underground in total darkness and to our surprise birds f lew into them, instead of bats. That really got me intrigued.”
The birds were Australian swiftlets. “Echolocation had not been conf irmed in those swiftlets at that time,” Dermot says. “It had been discussed as a possibility but this idea of them f lying in total darkness had not been proven. It was known about in other swiftlet species in Asia and the Pacif ic but not in the Australian species. So I dumped the fruit bats and switched to echolocation in swiftlets.”
Swiftlets are small, fast-f lying relatives of swifts that hunt insects in f light by day, especially at dawn and dusk, and roost during the night in caves or rock shelters. There are 30 species, mostly spread around southern Asia and the Pacific. Only the Australian swiftlet breeds on this continent, in north-eastern Queensland. It’s among 16 swiftlet species where individuals use echolocation to navigate in caves to f ind their nests among hundreds, even thousands, of others.
Dermot and two colleagues conf irmed in a 1976 research paper that the Australian swiftlet uses double clicks to echolocate. Because the clicks are of low frequency, ranging from one to 10kHz, humans can hear them. The clicks speed up when the bird approaches an obstacle, enters its cave or is close to its nest.
The swiftlet produces the clicks in its syrinx, which corresponds to the larynx of mammals. This organ doesn’t seem to be specialised for producing echolocation-specif ic calls, nor are the ears or brain adapted for receiving or analysing them.
In a 1983 study, Dermot and a colleague found Australian swiftlets weren’t able to detect objects smaller than 1–2cm wide. “Echolocation in swiftlets is less sophisticated than in bats, but then it’s not doing the same job. It’s not used for hunting insects; it’s an orientation system,” Dermot says.
Dr Mike Tarburton, a Melbourne-based ornithologist who has been researching and writing about swiftlets for more than 30 years, says swiftlet echolocation may not be on a par with bat echolocation but is very good nonetheless.
A double click gives more information than a single click, Mike says. “It can show whether an object is stationary or moving, whether it’s a rock wall or another swiftlet, or a predator like an owl. It’s good enough to enable the birds to avoid each other in the air or in a cave.”
As well, the Australian swiftlet’s eyesight is exceptionally good. “Their eyes occupy 50 per cent of the skull, more than normal for birds. That shows the eyes are very important. In fact, a lot of the birds don’t echolocate in the entrances to caves they’re familiar with because they can see and also they have a good memory,” Mike says.
Only one other kind of bird echolocates – the oilbird of northern South America, a relative of nightjars. Like them, it’s nocturnal but, unlike nightjars, the oilbird feeds on fruit rather than insects. It roosts in caves by day and emerges at dusk to f ly to its feeding grounds, which may be up to 120km away. It uses bursts of clicks generated in its syrinx, usually in the frequency range of 1–15kHz, to navigate in the darkness of caves, but relies on its sharp eyesight and sense of smell when f lying in the open and searching for food. Like the swiftlet, the oilbird has no echolocation specialisations.
The oilbird has a wingspan of nearly 1m and is skilled at hovering and twisting in f light. Its common name derives from the fact that young birds carry a lot of fat and Indigenous people would boil them down for their oil, which they used in food and lamps. The f irst European to see oilbirds was the Prussian geographer and naturalist Alexander von Humboldt. While exploring what is now Venezuela in 1799, he visited a cave that was home to as many as 18,000 of them.
Some 172 years later, I visited that same cave with my father, a dedicated birder. My journal entry for 15 August 1971 tells how a guide carrying a gas lamp led us deep into the cave, to an enormous chamber where thousands of birds f lew around us, “cackling, twittering, screeching and clicking… More and more came into view. Some were f lying, some sat on the rocks above us… I felt things dropping on my head”.
My dad was spellbound, but I couldn’t share his passion: at that moment, my mind was concentrating not on all that raucous life in the gloom around us but on the precise nature of what was falling on my head.
Swiftlet echolocation may not be on a par with bat echolocation but is very good nonetheless.
Zoologist Kyle Armstrong holds a bat detector, which helps him identify bat species in northern Australia and PNG from their echolocation calls.
Sound waves travel further and faster under water than through the air. This allows a sperm whale’s echolocation signals to detect prey as far away as a kilometre.
Marine mammal acoustician Dr Brian Miller launches a sonobuoy, which will monitor whale calls in the Southern Ocean, where he’s studying blue whale population numbers.
An Australian swiftlet negotiates a tight passage in northern Queensland’s Chillagoe Caves, using echolocating skills. The swiftlet is one of only two kinds of bird known to echolocate.