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.

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