Tag Archives: Bats

Do bats know the speed of sound from birth or do they have to learn it?

Kuhl’s pipistrelle bat

Most bats use echolocation to orient themselves in space. This means that they produce high-frequency sounds and can judge the distance of obstacles that reflected the sound based on the return time of the echo. However, to judge the distance based on the time the echo returns, one needs to know the speed of sound. The problem is that the speed of sound is not constant: it can vary in nature by more than 5% depending on temperature, humidity or altitude*.

What would be your guess: are bats born with the innate knowledge of the speed of sound and therefore distance judgment, or do they need to learn this?

Since the speed of sound is not constant, scientists predicted that knowledge about it is not innate in bats, but rather gained through experience. However, research on Kuhl’s pipistrelle bats (Pipistrellus kuhlii) has shown that the opposite is true.

These bats occur mainly in the Mediterranean region and in the West Asia. In their natural environment, they experience a wide range of temperatures and thus conditions with different speeds of sound.

To assess the effect of the speed of sound on bats’ echolocation and behavior, scientists conducted experiments in normal air and helium-enriched atmospheres – with a higher speed of sound and therefore a faster echo return time**. Young bats that were reared from birth in different conditions were compared to each other as well as to adult bats that had grown up in normal air.

It turned out that in the atmosphere with helium, bats, regardless of their age or rearing environment, assessed the distance to a target (feeding platform) as being closer than it actually was. Their echolocation pattern was the same as for closer objects in normal air. Additionally, in helium atmosphere, bats often slowed down and prepared for landing too early, thus landing ahead of their intended target. Even after several days in the helium atmosphere, the bats did not change their behavior and missed their target as often as at the beginning of the experiment.

This indicates that the speed of sound is encoded in the bats’ brains from birth, and the world seems to be perceived in terms of time rather than distance.

But how do bats deal with changes in the speed of sound in the wild? Firstly, these changes are usually smaller than those encountered in experiments. Secondly, as the bat approaches the prey or obstacle, the absolute error in distance judgement decreases. The error is a fraction of the distance – for example, if the speed of sound is increased by 10%, the object that is 100cm away seems to be 90cm away – error of 10 cm. But if the object is 10cm away the error is only 1cm. Lastly, bats usually don’t catch prey with their mouths, but entrap it with their wings and tail membrane, so they don’t have to be very precise. Although, even in the wild bats may miss their target.

While animals have amazing abilities to learn and adapt to their environment, in this case they seem to rely on innate skills even if they are not perfect in all conditions.

* For example, in standard air at 0°C, sound travels at about 332 m/s and at 30°C it travels at 350 m/s. So, the echo returns faster at higher temperatures.

** The experimental conditions increased the speed of sound by 10%, 15% or 27% – depending on the amount of helium.

Photo: Leonardoancillotto86 – Italy, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=15410420

Did you know that some bats have masks?

Wrinkle-faced bat with (a) and without (b) the mask on.

People wearing masks is a common sight in most of the world nowadays. However, a wild animal covering its face is a rarity. As far as I know only a couple of species of bats from Central and South America wear masks.

I have a face mask in my jacket pocket, handbag and backpack, just to make sure that I won’t forget it when I go shopping. But these bats don’t have to worry about that – they always have their mask with them. It is a skin fold under their chin that they can raise with their thumbs (the only free fingers on their wings) to cover the lower part of their face.

Why do they do that? That’s not exactly clear, as not much is known about them (yet). Maybe they want to protect their face during sleep, as some of the bats raise their mask when roosting. But it seems that for at least one bat species – the wrinkle-faced bat (Centurio senex) – the mask has a special role during mating. As the name says, this bat has deep hairless folds of skin on their faces. In males the wrinkles are more pronounced, and only they have masks. Any difference between the sexes (such as the big colourful “tail” of peacocks) already suggests an importance for mating, for example to attract females.

Wrinkle-faced bats seems to form leks – clusters of males gathered in one area trying to attract choosy females. In these bats males gather in the night, hang from neighbouring trees, put on their masks and “sing” (that is, produce a series of different ultrasound noises). When a female lands next to a male, he takes off his mask and begins copulation.

But what is the purpose of the face mask? For now, it’s just speculation. Maybe together with the wrinkles it helps to direct the “song” ultrasounds? Maybe it helps to control of scent signals produced in the skin around their chin? The mask is whitish, so maybe it serves as a visual signal for nearby females? Maybe the shade of the masks says something about the quality of the male (like the condition of peacock influences the appearance of its feathers)? Although bats are active during the night, this species have relatively large eyes and sight may be an important sense.

One thing seems clear, the mask is not there to hide the wrinkled face from the eyes of the potential partner. Or to reduce the spread of the diseases…

You can find video recording of wrinkle-faced bats at the suspected lek here.

Photo and the main source of information: Rodrıguez-Herrera et al. (2020).

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