echolocation -- 2/22/23
Today's selection -- from An Immense World by Ed Yong. Understanding the complexity of echolocation in bats:
"There are more than 1,400 species of bats. All of them fly. Most of them echo locate. Echolocation differs from the senses we have met so far, because it involves putting energy into the environment. Eyes scan, noses sniff, whiskers whisk, and fingers press, but these sense organs are always picking up stimuli that already exist in the wider world. By contrast, an echolocating bat creates the stimulus that it later detects. Without the call, there is no echo. As bat researcher James Simmons explained to me, echolocation is a way of tricking your surroundings into revealing themselves. A bat says, 'Marco,' and its surroundings can't help but say, 'Polo.' The bat speaks, and a silent world shouts back.
"The basic process seems straightforward. The bat's call is scattered and reflected by whatever's around it, and the animal detects and interprets the portion that rebounds. But to successfully do this, a bat must cope with many challenges. I count at least 10.
|Principle of bat echolocation: orange is the call and green is the echo.|
"First, distance is an issue. A bat's call must be strong enough to make the outward journey to a target and the return journey back to its ears. But sounds quickly lose energy as they travel through air, especially when they're high in frequency, so echolocation only works over short ranges. An average bat can only detect small moths from around 6 to 9 yards away, and larger ones from around 11 to 13 yards. Anything farther away is probably imperceptible, unless it's very large, like a building or a tree. Even within the detectable zone, objects on the periphery are fuzzy. That's because bats concentrate the energy of their calls into a cone, which extends from their heads like the beam of a flashlight; this helps the sounds to carry farther before petering out.
"Volume helps, too. Annemarie Surlykke showed that the sonar call of the big brown bat can leave its mouth at 138 decibels -- roughly as loud as a siren or jet engine. Even the so-called whispering bats, which are meant to be quiet, will emit no-decibel shrieks, comparable to chainsaws and leaf blowers. These are among the loudest sounds of any land animal, and it's a huge mercy that they're too high-pitched for us to hear. If our ears could detect ultrasound, I would have recoiled in pain while listening to Zipper, and Donald Griffin probably would have fled from the unbearable hubbub of his Ithaca pond.
"But bats can hear their own calls, which creates an obvious second challenge: They must avoid deafening themselves with every scream. They do so by contracting the muscles of their middle ears in time with their calls. This desensitizes their hearing while they shout and restores it in time for the echo. More subtly, bats can adjust the sensitivity of their ears as they approach a target so that they perceive the returning echoes at the same steady loudness, no matter how loud the echoes actually are. This is called acoustic gain control, and it likely stabilizes the bat's perception of its target.
"The third problem is one of speed. Every echo provides a snapshot. Bats fly so quickly that they must update those snapshots regularly to detect fast-approaching obstacles or fast-escaping prey. John Ratcliffe showed that they do so with vocal muscles that can contract up to 200 times a second-the fastest speeds of any mammalian muscle. Those muscles don't always contract so quickly. But in the final moments of a hunt, when bats are bearing down upon their targets and need to sense every dodge and dive, they produce as many pulses as their superfast muscles will allow. This is the so-called terminal buzz.... It is the sound of a bat sensing its prey as sharply as possible, and of an insect likely losing its life.
"Fast pulses address the third challenge while creating a fourth. For echolocation to work, a bat must match every outgoing call to its respective echo. If it's calling very quickly, it risks creating a jumbled stream of overlapping calls and echoes that can't be separated and thus can't be interpreted. Most bats avoid this problem by making their calls very short -- a few milliseconds long for the big brown. They also space their calls, so that each goes out only after the echo from the preceding one has returned. The air between a big brown bat and its target is only ever filled by a call or an echo, and never both. The bat's control is so fine that even during its rapid terminal buzz, there's no overlap.
"After receiving the echoes, the bat must now make sense of them. This fifth challenge is the hardest yet. Consider a simple scenario where a big brown bat is echolocating on a moth. It hears its own call on the way out. After a delay, it hears the echo. The length of that delay tells the bat about its distance to the insect. And as James Simmons and Cindy Moss have shown, the bat's nervous system is so sensitive that it can detect differences in echo delay of just one or two millionths of a second, which translates to a physical distance of less than a millimeter. Through sonar, it gauges the distance to a target with far more precision than any human can with our sharp eyes.
"But echolocation reveals more than just distance. A moth has a complex shape, so its head, body, and wings will all return echoes after slightly different delays. Complicating matters further, a hunting big brown bat produces a call that sweeps across a broad band of frequencies, falling over an octave or two. All of these frequencies bounce off the moth's body parts in subtly different ways, and provide the bat with disparate pieces of information. Lower frequencies tell it about large features; higher frequencies fill in finer detail. The bat's auditory system somehow analyzes all this information -- the time gaps between the call and the various echoes, at each of their constituent frequencies -- to build a sharper and richer acoustic portrait of the moth. It knows the insect's position, but maybe also its size, shape, texture, and orientation.
"All of this would be hard enough if the bat and the moth were staying still. Usually, both are in motion. Hence, the sixth challenge: A bat must constantly adjust its sonar. To even find a moth in the first place, it must scour wide expanses of open air. During this search phase, it makes calls that carry as far as possible -- loud, long, infrequent pulses whose energy is concentrated within a narrow frequency band. Once the bat hears a promising echo and approaches the possible target, its strategy changes. It broadens the frequencies of its call to capture more detail about the target and to more accurately estimate its distance. It calls more frequently to get faster updates about the target's position. And it shortens each call to avoid overlapping with the echoes. Finally, once the bat goes in for the kill, it produces the terminal buzz to claim as much information as possible as quickly as possible. Some bats will also broaden the beam of their sonar at this point, widening their sensory zone to better catch moths that try to bank to the side.
"The entire hunting sequence, from initial search to terminal buzz, might occur over a matter of seconds. Again and again, bats adjust the length, number, intensity, and frequencies of their calls to strategically control their perception. Handily, this means that a bat's voice reveals its intent. If its call is long and loud, it's focusing on something far away. If the call is soft and short, it's homing in on something close. If it produces faster pulses, it is paying more attention to a target. By measuring these calls in real time, researchers can almost read a bat's mind."