Diversity of echolocation

Hackett et al. 2013, echolocation calls of 13 species in the Arava Desert, Israel.

When I first saw the figure above, I realised how incredible the diversity of bat calls was. Of course, I had noticed this diversity before, without actually never taking the time to wonder why that was. Yet, this figure only shows a fragment of the diversity found in bat echolocation calls.

If echolocation always serves the same purpose, why such a diversity? Does it actually always serve the same purpose?

But first, what is echolocation?

Echolocation: definition

Echolocation is a sense, just like sight and hearing, helping the animal to perceive its environment. However, unlike those two senses where signals come from the environment (light and sounds), an animal must actively produce a sound to detect objects in its surroundings. So it’s an active sensor. Echolocation has several purposes: spatial orientation, identifying the animal’s surrounding habitat (linked to the presence of certain prey) and finding food (Denzinger and Schnitzler, 2013)

Bats aren’t the only animals using echolocation. There are three other groups capable of doing so: Odontocetes (a fancy word to say dolphins and relatives), some birds and…humans…though we need fancy technology for that!

Bats and Cetaceans (Dolphins and whales) are very different. The first fly, are usually tiny and feed on insects, fruit, nectar and what not, whereas dolphins and whales rely on water for their survival, feed mostly on fish and krill and can grow to be the biggest animals ever to have lived on this planet. Bats are also significantly more diverse than cetaceans with just under 1400 species described so far compared to around 70 species. Yet, both groups have developed this unique ability. Why would two very different animal groups need the same highly specific sense? The answer: visibility, or lack thereof.

Bats are mostly nocturnal so visibility is very poor when they’re at their most active. Many Odontocetes dive deep, beyond the reach of sunlight or live in murky waters such as in rivers. In those conditions, sight is pretty much useless so an alternative is needed to locate prey. Echolocation allows them to forage in a wide range of less easily navigated habitats.

There are very few species of birds capable of echolocating, namely the Oilbird (Steatornis caripensis) and a few species of Swiftlets (Aerodramus spp.). This ability allows those species to roost deep inside caves, far from most predators. (Brinkløv et al 2013)

Not all bats echolocate. Until recently, bats were split in Microbats and Megabats (Figure 1B).

The distinction was mainly based on size but it also happened that only the former group echolocates. The currently accepted classification is Yinpterochiroptera and Yangochiroptera (Figure 1A).

Interestingly, this leads to two different hypotheses on the evolution of echolocation. Either it evolved in the common ancestor of all bats and Pteropodids (fruit bats) subsequently lost it (and some Rousettus developed a new way to achieve a similar result), or it evolved twice, once in the Rhinolophoid group and once in the Yangochiroptera group (Springer 2013).

Figure 1:  Currently recognised bat taxonomy (1A, top) and its previous iteration (1B, bottom)
Figure 1: Currently recognised bat taxonomy (1A, top) and its previous iteration (1B, bottom)

Amongst the Pteropodidae, the biggest family of non-echolocating bats, several species of the genus Rousettus are known to produce clicks using their tongue (Roberts 1975).

Recent research has also shown that some members of the family are capable of echolocating with their wings! (Boonman et al. 2014)

That’s all great but why such a diversity?

I’m certain all of you have at one point or another in your life, have found yourself in a busy shopping mall, or café. How easy is it to have a normal conversation in there? Not that easy at all! That’s because all the sound waves are interfering with each other, resulting in a despicable noise.

Now, replace the humans in the shopping centre with bats. And turn off the lights. All those bats have to find food, listening to their own echo to picture their environment. Wouldn’t be easier if each bat has its own characteristic echolocation call?

While that’s not the case, strong variability between species allows different species to find different kinds of prey in the same environment, without disturbing one another.

Small bats will usually have a call of high frequency because it gives them the resolution required to find small prey whereas larger species will usually look for larger prey, thus having a lower call.

The specialised calls of Hipposideridae and Rhinolophidae work in a different way. They have a constant frequency and the bats listen for modulations in that frequency in the echo. Those modulations are the result of the Doppler effect. If you don’t know what that is, you’re far from the only one. But also, you do know what it is. When you hear an ambulance, you know if it’s in front or behind you. The sound changes as it goes past you. That’s the Doppler effect, in very short. If you want to know more, the internet is a much better teacher than I am.

This highly specialised method allows them to perceive the flutter of the flying insects, enabling them to not only distinguish a flying, living prey from a falling leaf but also to distinguish between different types of prey.

 
What about bats of the same species then?

Well… They also want to find food and avoid collision. What bats of the same species will usually do is slightly modulate their frequency to avoid interference. They will also emit “social calls” aimed at reminding the intruder that it’s their turf. Not so social after all.

A few interesting examples

What’s that funny looking nose for?

Ever looked at a photo of a Horseshoe bat or a leaf-nosed bat and wondered what that weird, somewhat ugly nose was for? Well…as said earlier, those bats have developed their echolocation differently than most other “microbats”. The sounds are still produced in the larynx, nothing new under the sun. What’s different though is that the sound then goes through the nose where it is focused into a narrow beam. This gives extreme precision, but also a low visibility range.

Tricking the moths that don’t want to be tricked

Picture this, bats hunt moths. Moths develop hearing to avoid bats. Bats find way to trick the moths into thinking the danger is much further than it actually is. Isn’t that amazing? Well, that’s the somewhat simplified story of the Barbastelle bat, Barbastella barbastellus. Instead of producing a series of single calls that moths with “ear drums” can easily locate, Barbastelle bats produce pairs of low intensity calls, at different frequencies. Because of the low intensity, the moths aren’t alarmed until it’s too late. It’s tricked into “thinking” that there are two distant bats instead of one Barbastelle right on its six. The combined information gathered from the two calls makes up for the lower intensity.

Form follows function

In Europe, two common species are the Common Noctule (Nyctalus noctula) and the Common Pipistrelle (Pipistrellus pipistrellus). Their calls vary significantly. Noctules hunt above the canopy and are fairly large so are mainly looking for larger prey. Pipistrelles hunt below the canopy, in more cluttered habitat and are very small so are looking for really small prey (mosquitoes!). A Noctule’s call is long and quite low in frequency, giving it plenty of range, but poor resolution. A Noctule also won’t be emitting the calls too quickly one after another as the collision risk is very low. A Pipistrelle’s call on the other hand needs to have high resolution and be emitted frequently to avoid collisions, so it’s much higher in frequency and much shorter.

On the diversity of accents

I’m sure we’re all familiar with slight language variations between countries, or even between regions inside a country. These variations are known to use as accents. Similar variation has been described in birds, such as the Chaffinch, Fringilla coelebs (Slater et al. 1984). Interestingly, geographical variation has also been described in bats, especially in constant frequency call emitting species such as Rhinolophus spp. and Hipposideros spp. At this stage, it’s not yet known whether it could be cryptic diversity (species where genetic data can show more diversity than morphology alone) or if it is indeed, geographical variation. Hopefully, future research will be able to answer that question! 

Sonogram of examples of calls of all echolocating groups of animals (and humans, who don't echolocate) (Brinkløv et al 2013).

Further reading

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