Beaked whales are elite when it comes to their deep diving, but their echolocating clicks expose them to a dangerous predator: orcas. New research shows that groups of beaked whales can reduce predation risk by coordinating deep dives and stealthy ascents.
For beaked whales, it’s an aquatic case of the hunter becoming the hunted.
These medium-sized toothed whales, of which there are over 20 different species, use echolocation to find their prey—a hunting strategy with the unfortunate side effect of alerting nearby orcas to their presence. Orcas, as apex predators, are more than happy to take full advantage. Personally, I like to avoid the term “killer whale,” but in all fairness, that’s exactly what they are—killers.
Understandably, beaked whales have a natural fear of orcas. A paper published today in Scientific Reports shows how this fear has led to an effective but costly strategy that essentially makes beaked whales invisible to orcas: highly synchronised deep foraging dives and silent, unpredictable ascents.
As they begin their epic coordinated dives to the ocean floor, the beaked whales enter into stealth mode, in which they refrain from making clicks. Once at depth, they’re free to use their echolocation skills, spending upwards of an hour hunting marine animals, including squid. The whales return to stealth mode during their slow, synchronised ascent, surfacing at a seemingly random location.
A primary motivation of the study, which involved marine biologists Natacha Aguilar de Soto from the University of La Laguna in Spain and Mark Johnson from the University of St. Andrews in Scotland, was to gain a better sense of how underwater sonar technology might be affecting beaked whales, who are notorious for mass strandings.
“When beaked whales started stranding following navy sonar exercises, we knew very little about their behaviour. But as we started learning about them, the weirder they seemed,” Johnson told Gizmodo. “Compared to other deep-diving whales like sperm whales, beaked whales dive in a way that doesn’t seem to make sense energetically. We wanted to understand what made them behave the way they do and to see if it helped make sense of their strong reactions to sonar.”
Trouble is, beaked whales are exceptionally difficult to study, as they live far from the coast in deep water and they’re difficult to spot in the open seas. The researchers had to find locations reasonably close to shore, a requirement that led them to the deep waters off the Canary Islands, the Azores, and the Ligurian coast of Italy.
The next challenge was to figure out a way of tracking these elephant-sized animals, who spend more than 90 per cent of their time underwater.
“To do that, we designed small electronic tags that record their sounds and movements and which attach with suction cups,” said Johnson. “The tags stay on for up to a day but record an immense amount of data in that time about behaviour.”
Indeed, the retrievable tags allowed the team to track the movements of the whales in exquisite detail, such as the steepness, depth, and duration of their dives and the even the sounds they made. In total, the researchers studied the behaviour of 26 beaked whales, of which 14 were Blainville’s beaked whales and 12 were Cuvier’s beaked whales.
The resulting data showed the whales performing their deep dives in concert with each other, at depths approaching a full kilometre, and total underwater durations lasting more than 45 minutes (yeah, that’s a long time to hold your breath!).
Leaving the surface together, the whales entered into their clickless stealth mode while still in shallow waters, where they’re vulnerable to orca attacks. Their vocalisations only began once they surpassed depths of around 450 metres, after which time individuals broke away from their social group to hunt independently. Safe from orcas, the whales used their clicks with impunity, pinpointing the location of prey. Foraging sessions near the seafloor lasted around 25 to 30 minutes on average, with individuals snatching as many as 20 to 30 small prey during a single dive, according to the research.
Echolocation, in addition to spotting prey, also allowed the whales to keep track of each other in the dark waters.
Incredibly, the vocal foraging time of individuals overlapped by over 98 per cent, which the researchers referred to as “extreme synchronicity” in the paper. Indeed, that’s an impressive degree of coordination, given that the whales hunted alone at depth. This strategy served to reduce their collective exposure to predators by over 25 per cent, according to the research.
The whales also performed a “coordinated silent ascent in an unpredictable direction,” wrote the authors in the paper. During these ascents, the whales rendezvous at depths of 760 metres, where they once again entered into stealth mode. Slowly rising to the surface, the beaked whales swam at a very shallow angle, during which time they traversed 1 kilometre of horizontal distance from their last “vocal position,” on average. By suddenly surfacing and appearing at a location far from the place where they last made a sound, the beaked whales became far more difficult for the orcas to track.
When asked if this behaviour might be due to something else, Johnson said, “nature is economical and many behaviours serve more than one purpose, so we cannot say that beaked whales’ diving strategy is only for avoiding predation.” That said, “other proposed explanations don’t hold water,” he added. Other scientists “have explored if the long ascents could help beaked whales avoid decompression sickness, as for a scuba diver, or if they somehow save energy, but neither explanation has been shown to fit,” said Johnson.
This survival tactic comes at a cost, however. The researchers calculated that these deep hunting dives, some of which last longer than an hour, truncate foraging time by over 35 per cent compared to the shallow diving strategies used by other toothed whales. At the same time, their diving “reduces by an order of magnitude the risk of interception by killer whales,” wrote the authors in the study. So the reduction in hunting time must be worth it—an evolutionary strategy that emerged on account of intense predatory pressure exerted by the orcas, according to the researchers.
In terms of limitations, the researchers were only able to tag two whales in the same group, owing to the difficulty of placing the devices on the whales (who spend an average of just two minutes on the surface between dives). Future research involving more beaked whales, and in different geographical locations, would improve the fidelity of the data.
In terms of how naval sonar might be affecting beaked whales, Johnson said the new results are a good sign that beaked whales are highly tuned to avoid predators and that sonar could have a negative influence on their behaviour.
“Beaked whales don’t want to take chances, and so any unusual sound that could possibly be from a predator may trigger a strong evasive behaviour,” Johnson told Gizmodo. “This is a strategy that has worked over millions of years, but the invention of sonar has brought a whole new set of sounds into the water that beaked whales cannot know are not from predators.”
Hopefully the groups responsible for polluting the oceans with sonar will work to reduce these sounds and limit the places in which sonar is used. But sadly, it’s yet another sign that human activities are messing with nature—and harming some of the most fascinating creatures to have ever appeared on the planet.