Synchronized Swimming: Or How “Fear of Killer Whales Drives Extreme Synchrony in Deep Diving Beaked Whales”

Introduction

Beaked whales are a unique species for various reasons—one of these is their diving mechanisms. Most deep-diving mammals will spend a great deal of time foraging at the bottom, employing typical biosonar-mediation techniques. Beaked whales, however, like to do the typical deep dives, but only use echolocation at the deepest depths possible for a short period of time. This deep dive is followed by several shorter ones.

While the strategy as a whole is unusual and taxing, their lack of echolocation usage is another thing altogether—researchers believe this is in order to decrease the risk of predation. One natural predator of theirs, the killer whale, employs acute hearing as one of its hunting strategies, meaning that animals such as the beaked whales will be more susceptible to being hunted if they are constantly using biosonar methods of foraging.

Background 

This study proposes that much of beaked whale behavior is strongly linked to their relationship with killer whales. Killer whales are an incredibly influential apex predator and are thought to have caused some species of toothed whales to utilize large groups as social defense, or even change the frequencies of their vocalizations. However beaked whale behavior is noticeably different from these other cetaceans. Beaked whales have developed an extreme-predator response which makes them incredibly sensitive to naval sonar and vulnerable to mass strandings. The results suggest beaked whales coordinate vocal productions and movements when returning from the safer deep waters to the surface. This study delves into the unique foraging and diving behavior of beaked whales, and the influence of predation by killer whales. 

Methods 

The depth and movement patterns of 14 Blainville’s beaked whales near the Canary Islands and 12 Cuvier’s beaked whales in the Gulf of Genoa and near Azores were tracked via digital acoustic recording tags (DTAGs) attached by suction cups for up to 20 hours before the tags were released and recovered. Recordings from the hydrophones on the DTAGs were analysed using the DTAG toolbox to identify beaked whale vocalizations: clicks, buzzes, rasps, whistles. 

Results 

Coordination

The team analyzed the data from literature and biologging beaked whale pairs during deep dives to tell them information about their spatial relationship and coordination efforts during their deep dives. Figure 1 shows the dive profiles of three pairs of beaked whales. The red triangles signal when each whale in the pair starts and ends making vocal noise. Since these are close together, it shows synchronization in vocalization, which results in the group being silent for approximately 80% of the dive, keeping them undetectable from predators. The blue lines at the bottom of Figure 1 are points at which the whale pair splits, and the result is the immediate loss of coordination at this point.

This deep dive coordination allows for approximately 5 hours a day of sound overlapping in foraging, resulting in groups of beaked whales being shielded from killer whale acoustic detection over 75% of the time.

Silent Ascent Swimming

The team also tested the predictability of beaked whale ascents, believing that they move horizontally in a way that makes it difficult for killer whales to track their location. The reconstruction of tracks from past ascents shows that beaked whales swim on average one kilometer from where their last sound was made to where they surface, and they do so in a way that is counter to that of predator expectations. This unpredictable ascent creates confusion for the killer whales as they now have a wide circle of possible surfacing locations to search, as well as yields a significantly lower rate of interaction between the two species when compared to more common vertical ascent behavior.

Significance/Future Impacts 

This study gives some insight on the impacts of human-generated sonar on marine mammals. These beaked whales sacrifice massive amounts of energy and foraging time for the sake of evading predators, as running and hiding are their primary means of combating predation. As a result, should they hear anything akin to the sound a predator makes (like naval sonar), they have no choice but to assume the worst. This anti-predator response may “push beaked whales beyond their physiological limits and in some cases lead to sonar induced mortalities” (Aguilar de Soto et al.). There have been efforts in past years to get the navy to decrease their use of sonar in areas with marine mammals, and some of have been successful, such as in 2015 when the navy agreed to ease their sonar testing in the ocean off the west coast of California, a popular spot for beaked whales. But now that we know exactly what the beaked whales are interpreting the sonar as, hopefully we can move towards a longer term solution than simply a ceasefire.

ORIGINAL ARTICLES: 

Beaked whales may evade killer whales by silently diving in sync

https://www.nature.com/articles/s41598-019-55911-3

Other sources/references: 

Killer Whale Predation:

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0166670 

Effects of sonar on beaked whales:

https://www.livescience.com/64635-sonar-beaked-whales-deaths.html 

Related/Original:

https://royalsocietypublishing.org/doi/10.1098/rspb.2018.2533

Related:

https://www.nrdc.org/onearth/please-silence-your-sonar

Blainville’s Beaked Whales:

https://www.fisheries.noaa.gov/species/blainvilles-beaked-whale

Image at top of post credit: https://www.fisheries.noaa.gov/species/blainvilles-beaked-whale

 

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