Hearing a Moth in the Dark

Calls No One Heard

In 1793 Lazzaro Spallanzani found that blinded bats still flew around a room without hitting anything. His contemporary Louis Jurine, in Geneva, found that bats with plugged ears crashed. Bats found their way by ear, yet no one could hear them make a sound.

In 1938 Donald Griffin, a student at Harvard, took bats to the physicist G. W. Pierce, who had built a detector for ultrasound. Their calls were recorded for the first time. In 1944 Griffin named the skill: echolocation.

Model: a common pipistrelle and a noctuid moth, about 16 times life size; the distance between them is not to scale

One Call

A common pipistrelle searching in the open makes calls about 6 milliseconds long: the frequency sweeps down from fifty or sixty kilohertz and levels out near 45 kHz. Human hearing stops at about 20 kHz.

Bats that hunt insects in the open air call loudly: 120 to more than 130 decibels, measured 10 cm in front of the mouth. The call leaves through the open mouth and goes mostly forward.

Sound travels 343 metres a second. A call 6 milliseconds long is stretched out in the air into a train of waves about 2 metres long.

Sound slowed about 300 times; the crests drawn at the real wavelength, enlarged like the model

The Late Echo

When the call meets a moth, a small part of it bounces back. The echo makes a round trip: from a moth 1 metre away it arrives 5.8 milliseconds later; from 2 metres, 11.7 milliseconds.

The bat reads the distance from this delay. In experiments in 1973, bats told apart distances 1 to 3 centimetres different, a difference of less than 0.2 milliseconds in the echo’s timing.

The echo is far weaker than the call. Sound spreads out as it travels, so it weakens once on the way out and again on the way back, and the air absorbs some of it.

Red: the moth’s echo. The readout is a spectrogram: time across, frequency up

Why Ultrasound

When a wave meets something much smaller than its wavelength, the echo is disproportionately weak. At 45 kHz the wavelength is about 7.6 mm, smaller than a moth; at 20 kHz it is about 17 mm, as big as a small moth. The higher the frequency, the clearer the echo from a small insect.

The price is range. Air absorbs high frequencies more strongly: at 20 °C and 50% humidity, about 1.5 dB per metre at 45 kHz and about 3.3 dB per metre at 100 kHz. The higher the call, the shorter the distance at which prey stands out.

Crest spacing is the real wavelength, at the moth’s scale; cycling through 20, 46 and 100 kHz

Two Ears

The bat also needs to know which way the moth is. An echo from one side reaches the nearer ear first. But a bat’s head is small: a big brown bat’s ears are about 12 mm apart, so the difference is at most about 35 microseconds.

Loudness is the more useful cue. The outer ear works like a forward-facing horn and is very directional at high frequencies: the ear on the side of the echo hears it louder.

The small flap in front of the ear opening, the tragus, helps with up and down. In a 1982 experiment, big brown bats told apart heights about 3 degrees apart; with the tragus glued aside, it took 12 to 14 degrees.

A Wing Made of a Hand

A bat’s wing is a hand. The upper arm is short and the forearm long, just over 3 cm in a common pipistrelle. The thumb stays free on the leading edge with a small claw; the other four fingers are very long and spread a thin skin. The wings span 20 to 23 cm, and the bat weighs a few grams.

The skin runs from the fifth finger to the body and the hind leg; between the legs is a tail membrane, held out by a spur of cartilage from the ankle called the calcar. Pipistrelles often scoop insects up with the tail membrane or a wingtip.

While searching it calls once with every wingbeat. Calls are timed with the wingbeat and the outward breath, so they cost almost nothing extra.

Search, Approach, the Final Buzz

Searching, a pipistrelle calls about 10 times a second. It detects an insect 1 to 2 metres away and starts the approach: calls come faster and each one gets shorter, from 4–7 ms down to just over 2 ms. Short calls keep each call clear of its own echo, which comes back sooner and sooner.

About half a metre from the insect the calls run together into a buzz, the feeding buzz. In the last 20 cm or so the bat calls nearly 200 times a second, each call only half a millisecond long and lower in pitch. The whole buzz lasts little more than a tenth of a second. A few centimetres from the insect the bat falls silent and scoops it up.

Flight slowed about 7 times; in this chapter the speed of sound is not to scale

Horseshoe Bats Retune

Horseshoe bats call differently: each call lasts tens of milliseconds at an almost constant frequency, about 83 kHz in the greater horseshoe bat.

Flying forward, the echoes from ahead come back higher because of the Doppler effect. The horseshoe bat lowers its own call just enough to cancel this, so the echo lands on one narrow frequency where its hearing is sharpest, to within a tenth or two of a percent. Hans-Ulrich Schnitzler found this Doppler compensation in 1968.

An insect beating its wings makes the echo flicker in pitch and loudness. Listening at that fixed frequency, the horseshoe bat picks out these flickers and finds a flying insect among the echoes of leaves.

The bat’s speed is exaggerated so the squeeze of the wavefronts shows; the readout assumes a flight speed of 4 m/s

The Moth Can Hear

Many noctuid moths have an ear on each side of the hind thorax: a thin eardrum with only two auditory cells beneath it, one sensitive and one much less so. It cannot tell pitch, but it is very sensitive to bat ultrasound.

In the 1960s Kenneth Roeder found that noctuids hear a bat long before the bat can detect them. At a faint, distant call a moth turns and flies away; when the call suddenly gets loud, the bat is within a few metres, and the moth folds its wings and spirals down.

Some tiger moths answer back. In 2009 an experiment showed one species clicking ultrasound rapidly once a big brown bat had locked on to it, jamming the bat’s ranging so that it missed.

The moth’s ears are on the sides of the hind thorax, in front of the waist; it folds its wings as it drops