This article is a detailed guide on Bat Echolocation to explore how bats navigate the dark.
Bats are among the most diverse mammals, having evolved unique and specialized methods for survival. Most bats possess specialized vocal capabilities that allow them to use sound to navigate in complete darkness. Before we delve into how bats accomplish this, let’s clarify the sound concept.
What Is Sound?
Sound is a wave of vibration that travels through the air. It arises when a sound source emits a wave of vibration that compresses the surrounding air molecules.
The frequency of these waves indicates how many times a wave of compression occurs per second. It is measured in Hertz (Hz).
The distance between each wave of compression is called the wavelength.
The distance from the center line of the waveform is known as the amplitude. Amplitude is a measure of the wave’s energy. A waveform’s amplitude (or energy) is quantified in decibels (dB).
When you look at a speaker cone, you can see this fascinating process in action. The cone moves back and forth, compressing the air around it to create sounds we can hear. As you increase the volume of the speaker, more energy is used to compress the air, leading to a higher amplitude of sound. This effect is similar to how your voice works; when you speak, your larynx compresses air to produce sound.
Bat Echolocation: Ultrasound In Bats
Humans can hear frequencies ranging from approximately 40 Hz to 20,000 Hz.
In contrast, bats can perceive even higher frequencies, ranging from 20,000 Hz to 120,000 Hz.
This is known as the ultrasonic range.
Wavelengths
The high frequencies of ultrasound generate sound waves with very short wavelengths. This allows bats to detect the positions of small objects more precisely than if they utilized low-frequency sounds. The capability is called echolocation. Whereby, bats emit sound pulses through their mouth and/or nose and listen for the returning echoes with their ears. They then process this information to create a mental map of their surroundings, determining the precise distances between themselves and nearby objects. This ability enables them to navigate through obstacles and locate prey.
Volume
Remarkably, while humans cannot hear most bat calls if we could, we would discover that bats are quite loud! Bats must produce high volumes of sound because high-frequency sounds quickly diminish in intensity as they travel. This is why humans transmit analog radio stations at relatively low frequencies, as it allows sound to travel longer distances whilst minimizing the energy consumed.
Bats can generate sounds at levels of up to 140 dB. For context, humans typically speak at about 55 to 65 dB. An average rock concert emits sounds at around 120 dB, which means bats are chattering at levels 20 dB higher than what humans experience at a concert. To clarify, the amplitude scale is logarithmic, meaning a 20 dB increase equals a 20-fold boost in sound energy. To put this in perspective, a fighter jet engine can emit sound levels of 140 dB and upwards!
Mechanics
While the exact mechanics of bats’ ultrasonic capabilities are not entirely understood, substantial theories have emerged based on scientific research.
Most echolocating bats transmit sounds from their larynx. The illustration on the left shows the anatomy of the larynx in these bats. A few bats, however, use their tongues to produce sound, as depicted in the illustration on the right.1
In both cases, bats, like humans, possess a bone called the stylohyal bone, which is part of the throat’s bone structure. This bone connects anatomically to the tympanic bone in the bat’s skull near the middle ear.
It is believed that this connection plays an integral role in how bats mentally store a representation of the signals they transmit and what they receive through their ears, allowing them to measure the differences between the sounds they produce and the echoes they receive.
Bat Echolocation: How Do Bats Hear?
Bats’ ears are similar to those of humans and other mammals. While their ears differ in size and shape, their function remains the same. One key difference is the large fleshy protrusion in the outer ear called the tragus, as illustrated here on the Natterer’s Bat. Although humans also have a tragus, it is much smaller.
It is thought that the tragus helps bats determine their vertical positioning, enabling them to create a fully three-dimensional picture of their environment.
Here is a diagrammatic representation of a bat’s ear.
Sound waves enter the ear and cause the eardrum to vibrate. These vibrations are then transmitted through three small bones in the ear: the malleus, incus, and stapes, before reaching the oval window. From there, the vibrations continue along the spiral canal of the cochlea. The auditory nerves pick up the electrical impulses generated by these vibrations and send the information to the brain.
Bats protect their hearing during echolocation using a reflex action in the middle ear that contracts the stapedius muscle and pulls the stapes away from the oval window just before they emit sound. This contraction dampens the loud echo that would otherwise reach their inner ear, allowing them to hear the returning echoes from their prey while safeguarding their sensitive hearing cells.
Furthermore, research indicates that bats may have evolved specific genetic adaptations in their cochlear hair cells to enhance their resistance to noise-induced damage caused by their echolocation calls.
Bat Echolocation: How Do Bats Process Sound?
Echolocating bats rely on their ability to process both the sounds they emit and the echoes they receive to navigate their environment and catch prey. They utilize two primary methods for this:
Time Separation: Used by over 80% of bat species, this method allows bats to distinguish between the outgoing pulse and its returning echo based on the timing difference.
Frequency Separation: Some bats, such as the Lesser Horseshoe Bat and the Greater Horseshoe Bat, differentiate between their calls and the returning echoes by using variations in frequency.
Let’s take a look at how this technique works in practice. We’ll use Frequency Separation for this example, If the bat’s prey is:
- Moving towards it when the bat echolocates, then the returning frequency increases, corresponding to the bat hearing a higher-pitched sound.
- Stationary when the bat echolocates, then the returning frequency is the same, corresponding to the bat hearing an unchanged reflected sound.
- Moving away from it when the bat echolocates, then the returning frequency decreases, corresponding to the bat hearing a lower-pitched sound.
Bat Echolocation: Advantages & Disadvantages
Bats rely on echolocation to navigate and hunt in the dark, but this comes with both advantages and disadvantages. One significant drawback is that bats must produce very loud sounds. Higher frequency sounds tend to dissipate quickly over distances, requiring bats to expend ten times more energy for communication compared to their resting metabolic rate.
Disadvantages
The main disadvantage, therefore, when using ultrasonics, is the amount of energy that is required to transmit signals over distances. This has been calculated that they need over 10 times more energy to communicate compared to their resting metabolism energy consumption. In comparison, the energy cost of flying is around 15 to 20 times their resting metabolic rate.
Interestingly, bats have developed an efficient method to echolocate with minimal extra energy. They synchronize their breathing with their wing flaps: exhaling on the upward stroke and inhaling on the downward stroke. Since the muscles that control their wing movements also manage their breathing, this dual function allows them to echolocate with virtually no additional energy expenditure, except when they’re feeding.
So, why do most bats use echolocation when many animals rely on eyesight, which requires significantly less energy?
Advantages
As mentioned previously, Echolocation allows bats to fly in complete darkness without colliding with obstacles. Some predators like the Barn Owl can also do this, but they have to facilitate their sight by using sound. Bats on the other hand excel in dark environments thanks to their unique echolocation abilities.
Approximately 70% of bat species are insectivores, and their ability to fly at night enables them to hunt nocturnal insects such as moths, flies, beetles, and midges. Many of these insects are quite small, and using high-frequency echolocation significantly enhances a bat’s ability to catch its prey.
Here’s how it works: If bats transmitted sound at low frequencies, there would be a high chance the sound waves would pass by their prey without detection.
In contrast, using high frequencies increases the likelihood that the sound waves will hit the insects, allowing for a reflected wave to bounce back.
Additionally, many bats fly at high speeds as they snatch their prey from the air. Therefore, it is crucial for them not only to detect their prey but also to track it accurately so that they can adjust their flight trajectory accordingly.
Some Insects Are Fighting Back!
As with all predator-prey relationships, evolution is constantly adapting to increase a species’ chance of survival. In the case of echolocation, some insects are fighting back!
Did You Know? Noctuid moth’s hearing is believed to have evolved to detect bat echolocation and enable the moths to take evasive action. It’s so effective, that bats that hunt Noctuid moths such as the Long-Eared Mouse Bat have to stop using echolocation and rely purely on their hearing!
CITATIONS
- Morphology images are based on Veselka et al. published in the Nature Journal in February 2010. [Accessed 16/02/2025] ↩︎
References
- Altringham, J.D. (2011) Bats : from evolution to conservation. 2nd ed. New York: Oxford University Press. [Accessed 16/02/2025]
