Echolocation

What is echolocation?

With echolocation, an animal first emits sounds in its environment, listens to the echoes of the sound waves bouncing off the surroundings, then gains a sense of where different objects are located. Belugas are classified as a species that uses echolocation because they use this process in order to forage and hunt, navigate, and avoid predators (Au, 2008). Belugas and other odontocetes perform echolocation specifically through clicking sounds. This echolocation mechanism provides the beluga and other odontocete cetaceans a higher acuity of their environment than any current man-made sonar system (Lammers & Castellote, 2009). This is also an effective strategy because sound travels five times faster underwater than in the air.

How do belugas echolocate?

Just like making whistle calls, belugas create biosonar clicks using the upper nasal passages. These short, broadband pulses are produced using the same MLDB complexes for whale calling. The highly directional pulses are also transmitted through the melon, a vestibular sac that rests dorsally in the upper part of the head. The sound that bounces off of the beluga's surroundings is received in the inferior colliculus and a mental image is produced. In a study involving a captive beluga, Lammers and Castellote (2009) were amongst the first to provide empirical evidence suggesting that the beluga had control over both signal generators, which would enable the whale to produce combined calls and greater echolocation clicks. At several axes, Lammers & Castellote (2009) observed two distinct pulses at different 15-degree intervals (Fig. 1). As there were no other internal or external surfaces which the sound could bounce off of to produce the other pulse, the only explanation could be that the beluga possessed two pulse generators.

On top of gathering evidence that belugas could control both signal generators, Lammers and Castellote (2009) also proposed that the two generators were separated by air. Horizontal separation between the phonic lips of about 8.7 to 10.4 centimeters, they theorized, would allow for the beluga to produce both sounds and within a distance that fits within the beluga's head. If the signal were transmitted through the air, this would make sense since the MLDB complex is bound by the melon, which is a large sac filled with air. As the ends of the phonic lips reach the floor of the vestibular sac, the lips both were vibrating when taking an endoscope down the blowhole to the vestibular sac. The observations thus led Lammers and Castellote (2009) to conclude that the beluga used both phonic lips to produce pulses simultaneously or very close to each other. Any intervals are thought to be the transmission delays through the air space of the vestibular sac, though this is not confirmed.

Figure 1: These are the on-axis and off-axis sonar clicks measured at different angles.

Having independent control over both phonic lips allows for a number of advantages for belugas. Firstly, the sum of the combined signals has a greater output than each signal individually. The higher amplitude of this sum of signals can travel over longer distances, hence allowing for a bigger target range. Secondly, the differences in energies that each generator transmits would allow for a broader spectrum click sound if they were combined. Changes in amplitude would also permit more control over energy distribution at varying frequencies. This advantage would also aid in target detection and identification.

Lastly, the ability to possibly use time delays to change the width and orientation of the echolocation beam would be a useful adaptation (Lammers & Castellote, 2009), especially while swimming amongst a lot of ice that creates irregularities in the environment, which in turn influences the acoustic soundscape.

Curiously, Abramson et al. (2013) conducted a study testing the beluga's capability of making relative quantity judgments visually and while using echolocation. Along with bottlenose dolphins, the beluga whales were able to visually discern and choose which of the two transparent boxes carried more fish. When using echolocation to choose the hidden box with the higher quantity of fish, the beluga succeeded. The ability to discern the shape of an object behind an opaque screen (or box) is unique to belugas, as no other animal or man-made technology is known to do so (Cormier, n.d.).

Picture Credits and Licenses - Title Image (scaled to fit into column): Greg Hume (Greg5030), CC BY-SA 3.0, via Wikimedia Commons