THE SECRETS OF BAT ECHOLOCATION

ABAT PRODUCES CALLS in the same way most mammals make sounds – by forcing air through vocal cords in the glottis, part of the larynx.

“A vibrating signal is produced at a particular audio frequency. It has overtones or harmonics, so it’s rich in various harmonic characteristics,” explains zoologist Kyle Armstrong.

The calls are essentially squeaks or chirps sent out in bursts or pulses at up to 200 per second.

The audio frequencies of microbat echolocation calls generally range from 12kHz to above 200kHz, although Kyle says the highest known in Australia is 167kHz. Because humans can’t hear sounds above 20kHz, nearly all of these calls are inaudible to us and termed ultrasound.

Bats vary their calls to suit their environment and prey. A call can sweep through a broad band of frequencies or stay at one frequency. Generally, low frequencies travel further and are good over longer ranges. High frequencies are good at close range and broadband sweeps are good for discerning detail.

Echolocation calls vary in intensity (from 60 to 140 decibels), duration and pulse interval. Bats alter the characteristics of their calls according to whether they’re navigating in open sky or dense forest, searching for prey, closing in on prey or making the final approach.

On final approach, the call pulses can speed up to what’s called a terminal buzz, allowing the bat to accurately track its target’s movements at a fast-diminishing range.

“Search-phase signals are quite stereotypical and are useful for telling species apart,” Kyle says. “When a bat approaches something interesting or gets too close to a wall of vegetation, it changes the shape of the signal and that gives different

information on things like texture and distance. The bat will also change the rapidity of its calls; when it’s about to catch an insect, the call’s repetition rate might get up to 200 pulses per second.”

During the search phase, calls can be coupled to breathing and this, in turn, is coupled to wing beats, saving energy.

At the same time, because search-phase calls are so loud – they’re among the loudest sounds in nature – bats momentarily desensitise their hearing so that they don’t deafen themselves and miss the very soft return echoes.

Microbats beam their echolocation calls into the outside world either through their mouths or nostrils. “Some species of mouth-emitting bats have beautiful parabolic lips that open in the shape of a horn to project the call,” Kyle says.

“Nose-emitting bats may have interesting nose specialisations, complex skin structures, called a nose leaf, around the nostrils to project the signal. Every species has a different nose leaf.”

Not all bats generate echolocation calls in their larynxes; some fruit bats use low-frequency tongue clicks for navigating dark caves.

Bursts of echolocation calls produced in the larynx are projected from the nose. The nose leaf directs and shapes the signals. Bat ears come in different shapes and sizes, but all are geared to picking up the often faint echoes of echolocation calls. Outer ears funnel incoming echoes to super-sensitive inner ears, from where they move to the enlarged and highly specialised auditory cortex. A pitcher plant in Borneo (left) lures bats with a specially shaped reflector to enhance and return their echoes. The bat will roost in the pitcher during the day and leave nutrient-rich droppings for the plant. The nose leaf of echo-locating bats (below) comes in all shapes and sizes.