Beaked whales’ stealth behavior gives clues to mystery of mass stranding

In March 2000, U.S. Navy destroyers were conducting routine training exercises in the Bahamas; they swept an underwater canyon with sonar at 265 decibels to uncover the locations of two decoy submarines. Less than 36 hours later, 17 marine animals, including 14 beaked whales (family Ziphiidae), were found stranded on the nearby Abaco Islands. The Navy would later admit that the use of sonar caused the whale deaths. What remained a mystery was why it happened.

A new paper published in Scientific Reports suggests adaptations that have evolved in the whales over millions of years to escape predators could explain why they are prone to mass stranding. The research shows that the diving behavior of these enigmatic mammals is shaped by an overwhelming desire to avoid hungry killer whales (Orcinus orca) that roam shallow waters.

“We still did not understand what in the behavior of beaked whales explains why they react so strongly to killer whale and sonar sounds,” said study co-author Natacha Aguilar de Soto from the University of La Laguna in the Canary Islands, Spain. “Now, this paper solves the puzzle. Predation avoidance has been such as strong pressure in the evolution of beaked whale behavior that any sound resembling killer whales will elicit an intense stress response.”

These elephant-sized whales inhabit areas far ashore and spend a significant chunk of their time in the deep sea. In the few minutes that they spend at the surface, the whales inhale enough oxygen to sustain them during deep dives that can last two hours. They can plunge a kilometer deep (about 3,300 feet) — the equivalent of three Eiffel towers stacked on top of each other — in search of food. Their bodies are forged to survive the frigid depths and crushing pressure, which can reach hundred times what they experience at the surface.

In the lightless depths, they get by on sound, or echolocation. Their most common vocalization is a click-click that allows the whale to create a snapshot of its surroundings based on the echoes that bounce back to it. These short spurts of sound quicken to a buzz when it spots something of interest, essentially transforming the mental images into a video feed.

But this noise calls attention to the whales, and the killer whales that prey on them have a keen sense of hearing, allowing them to home in on beaked whale clicks from miles away. They lurk in shallow waters and are known to set upon their target silently. Beaked whale pods are small, so they don’t have the numbers to ward off an attack. If a killer whale detects a beaked whale, the encounter is unlikely to end well for the smaller whale.

There are 22 species of beaked whale, but members from only a handful of species have ever been seen alive. To map the diving behaviors of the elusive whales, the researchers analyzed biologging data from 26 tagged wales: 14 Blainville’s beaked whales (Mesoplodon densirostris) and 12 Cuvier’s beaked whales (Ziphius cavirostris). They were tagged off the coast of El Hierro in the Spanish Canary Islands, the Gulf of Genoa in the Mediterranean Sea, and off Terceira in the Azores, a Portuguese archipelago in the Atlantic Ocean. Aguilar de Soto led the fieldwork in the Canary Islands, and co-author Fleur Visser led in the Azores.

The data from the tags painted a picture of stealthy, synchronized behavior driven by a single motivation: to evade predation.

The data revealed that whales in a pod synchronize their deep dives. And they only start vocalizing once they reach at a depth of 500 m (1,640 ft). They time their dives so that there’s almost 99% overlap in their foraging, and thus the time during which they’d be making noise would be limited to about five hours in a day. If they all foraged at different times and were noisy throughout, they’d be easy game for any killer whales within earshot.

In their deepwater foraging grounds, they’re safe from the killer whales, which can’t hunt at those depths. Here, the beaked whales can forage and communicate freely. When it comes time to surface again, either when they’re done feeding or are running out of oxygen, they regroup in the depths and then begin their ascent. On their way up, they go silent from about 700 m (2,300 ft) deep, all the way up to the surface.

That isn’t all. In the depths, they might be out of range for an attack, but if their position is known, a predator can lie in wait as they start ascending and intercept them. However, the biolocation data shows that the beaked whales don’t come up vertically. Instead, they ascend at an unpredictable angle, surfacing at a point that can be a kilometer away horizontally from the spot where they began their climb. The researchers suspect this is to ensure that predators have less of a chance to guess where they might surface.

All this coordination and stealth comes at a cost. Beaked whales’ foraging efficiency is reduced because of the longer time it takes for them to ascend and the fact that while foraging they are bound together by a kind of “acoustic leash” to other members of their group, including younger ones with limited diving ability. They need to communicate and regroup before they quietly ascend. Diving takes a toll on whales, and they must try and maximize their returns from foraging. But these animals spend 35% less time foraging because of their particular diving patterns.

This behavior led the researchers to conclude that the whales’ need to escape predation was pressing, and gives a clue to why they react sharply to even the slightest hint of a predator. The researchers say naval sonars signal danger to these whales, and they react in duress. Between 1874 and 2004, 136 mass strandings of beaked whales were reported; of those, 126 occurred between 1950 and 2004. A 2009 study found that this surge in strandings coincided with the advent and use of high-power mid-frequency active sonar (MFAS) — the same kind used in the Bahamas back in 2000. There are still unanswered questions about how sonars could be leading to mortality and how specifically they cause the whales to lose their way and end up on a beach. But it does show that even in the depths of the ocean, where they’ve evolved ingenious strategies to evade a school-bus-sized predator, beaked whales are still vulnerable to human interference.

Citations:

Aguilar de Soto, N., Visser, F., Tyack, P. L., Alcazar, J., Ruxton, G., Arranz, P., … Johnson, M. (2020). Fear of killer whales drives extreme synchrony in deep diving beaked whales. Scientific Reports, 10(1). doi:10.1038/s41598-019-55911-3

Schrope, M. (2002). Whale deaths caused by U.S. Navy’s sonar. Nature, 415(6868), 106-106. doi:10.1038/415106a

Malavika Vyawahare is a staff writer for Mongabay. Find her on Twitter: @MalavikaVy

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