Tuesday, August 20, 2013

Eavesdropping on a foraging pilot whale

The world of a deep-diving toothed whale is hard to imagine. However, it is even more challenging to imagine how the whales themselves perceive their underwater environment.

Frants Jensen analysing data from a
recently tagged pilot whale
Toothed whales depend on echolocation sounds for orientation and foraging. They send out high-amplitude, highly directional biosonar signals but have to listen for the tiny fraction of sound energy reflected off prey and other objects. Foraging requires both detecting and discriminating the faint echoes from prey through background noise but also closing in on and capturing prey up close.

The beauty of echolocation-based foraging is that we can use acoustic recording tags to eavesdrop on the foraging of tagged animals such as sperm whales or beaked whales, effectively tapping into their own biosonar system.

A time-frequency representation (spectrogram)
of a foraging buzz produced during prey capture
Like other toothed whales, pilot whales switch from relatively slow clicking when searching and approaching prey, to periods of very fast clicking (a foraging buzz) when they get close to prey. These periods of rapid buzzing can be used for identifying when or where tagged animals forage. This information can also be combined with the other sensors of the tag to identify specific kinematic maneuvers during prey capture, or determine whether prey was captured.

Listen in on a pilot whale while it searches for and captures prey

Here, we have taken a dive profile and marked down the different foraging events using some of the auditing software developed for these DTAGs. Five different sequences (labeled A to E) can be listened through by pressing the corresponding button:

A: Pilot whale starts searching for food using regularly spaced echolocation clicks.
A: Play sound

B: First foraging buzz in this deep dive.
B: Play sound

C: A handful of foraging buzzes occur near the bottom of the dive, some of them quite long, possibly indicating prey chases.
C: Play sound

D:  On its way back to the surface, the pilot whale is mostly silent, but occasionally calls out for group members.
D: Play sound

E: As the tag breaks the surface of the water, the animal catches its first breath of fresh air in 14 minutes.
E: Play sound


 Written by Frants Jensen

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