Tuesday, April 2, 2019

Why do sperm whales use so many types of clicks?

Sperm whales are the most iconic cetacean species of the Azores. 

They spend most of their lives underwater and dive on average around 40 minutes, but studies say they can dive more than 1.5 hours to depths up to 3,000 meters (Whitehead, 2003). Whilst underwater, they developed ways to communicate and orient themselves, especially at depths below the photic zone where dark prevails. They use echolocation, a system used by odontocetes (whales with teeth), which consist on a series of clicks produced by the animal to find their prey and to communicate amongst each other. 

Since sperm whales are the largest odontocetes alive, with males reaching up to 18 meters in length and females up to 12 meters, they have a very large fatty organ in their head called spermaceti (see figure 1). This organ in the sperm whale takes up to 1/3 of the body length. Sperm whales can make many different types of echolocation consisting of clicks, slow clicks, creaks, trumpets, codas, etc. 

But why do they use so many types of clicks? Scientists use hydrophones to record those sounds and analyze them. Some of the different types of clicks are explained below.

Figure 1: how a sperm whale produces and receives sound (from the American Museum of Natural History, 2013

Clicks & Creaks

The first reason why sperm whales use echolocation is to locate their prey. As they dive, the sperm whales click ‘normally’ and almost continuously in a specific sequence. This sequence, when recorded, looks like in figure 2A (usual clicks). This system is used where sound gets produced by the sperm whale (see figure 1sound out’). This sound can also bounce back to the sperm whale (lower part of ‘sound in’ see figure 1) when the sperm whale encounters something, such as a squid. When they find a prey and get closer, the clicks are emitted more rapidly until it becomes a buzz or a creak” (see figure 2B - creak), which happens when they are closest to their prey. It can sound like this

Figure 2A. Usual clicks. B. Creak. Time in seconds in the horizontal axis (from Whitehead, 2003).

Gunshot

Sometimes, another type of echolocation is used while foraging, or searching for food. It is the loudest sound made on Earth in the entire animal kingdom, and it is called the “gunshot” which can be up to 223 dB (Mohl et al., 2001) which is more than the sound of a jet-plane taking off (150 dB) (Federal Agency, 1992). Although not scientifically proven, it is thought that this sound is so loud that it can stun its prey, such as squids, making it easier to capture them. 

Codas

Sperm whales also use echolocation to socialize and recognize each other. They have complex social structures where echolocation plays an important role in the bonds with their families. Sperm whale females travel in pods and remain with their calves and juveniles for many years. This allows them to develop ways to communicate in the pod. The social aspect of echolocation is what is called “codas” (see figure 3). Codas are a sequence of clicks of up to 20 clicks in a row (Whitehead, 2003) and studies have shown that these codas are shared in populations, or regions (Hoogenboom, 2016). It can almost be compared to a language or a dialect that humans speak in different parts of the world, where everybody develops a certain way of communicating amongst each other. Listen to this example of a recorded coda in exchange between two individuals which had a sequence, from the Dominican Republic. 
Figure 3: Coda. Time in seconds in the horizontal axis (Whitehead, 2003).

Slow clicks 

Another slightly mysterious way of communicating is the “slow click” (see figure 4). They are loud ringing clicks repeated in 5 or 8 seconds, and are associated with mating sperm whales. They seem to be louder than the ‘normal’ clicks of the sperm whales and can be heard from an average of 20 km distance, which is considered far compared to the ‘normal’ clicks of the sperm whales (4.5 km). These clicks are usually made by mature male sperm whales, and sometimes by females as well. They may use this to either attract females or chase away other males. Another theory is that the males use it to show masculinity, how large they are for example (Whitehead, 2003). 
Figure 4: Slow click. Time in seconds in the horizontal axis (Whitehead, 2003).
So… why do they use so many types of clicks? 
The above examples are only a few echolocation types of sperm whales identified by scientists. Over the years, scientists concluded that sperm whales are much more complex social animals than initially thought. Some of the most renowned sperm whale researchers in the world (e.g. Hal Whitehead or Shane Gero) explained that it is very important to study the sounds produced by sperm whales across the globe in order to understand how they group with each other and create a culture over the years they live together. If we don’t study their culture, we won’t understand their lives. So.. let’s study them!  



Bibliography


- Federal Agency Review of Selected Airport Noise Analysis Issues, Federal Interagency Committee on Noise (1992). Source of the information is attributed to Outdoor Noise and the Metropolitan Environment, M.C. Branch et al., Department of City Planning, City of Los Angeles, 1970. 
- Hoogenboom, M., (2016). The whales that speak in code. BBC. http://www.bbc.com/earth/story/20160120-the-whales-that-speak-in-code
- Mohl, B. (2001). Sound transmission in the nose of the Sperm whale. Physeter catodon: a post mortem study. J. Comp. Physiol
- Whitehead, H. (2003). Sperm Whales. Social Evolution in the Ocean. University of Chicago press. 


Other links used:

- Ocean Conservation Research. Creak of the sperm whale. https://ocr.org/sounds/sperm-whale/ 
- American Museum of Natural History. (2003). Whale biology.  http://www.juansanmartin.net/undecima.html
- BBC. Sperm whale call. 5r exchange at the onset of a dive. https://soundcloud.com/bbc_com/sperm-whale-call-5r-exchange-at-the-onset-of-a-dive

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