Orcas recently made headlines for moving into new Arctic territory, but it seems their push for ocean domination is just as strong in the South. Marine biologists working off the Australian coast observed the resourceful top predators hunting rare beaked whales, a behaviour that has never been seen until now.
This article contains images that may be disturbing to some readers.
The events took place near Bremer Bay Canyon, which sits 70 kilometers off the coast of Western Australia. The bay is a known biodiversity hotspot, one that's said to harbour one of the most unique deep-water ecosystems on the planet. And yet spotting a beaked whale (genus Mesoplodon) in the area is rare.
Beaked whales, which can reach 43 feet (13m) in length and weigh as much as an Indian elephant, are deep divers, with some descending to 9,800 feet (3,000m) in search of food. They also keep an extremely low profile when surfacing, which makes seeing one difficult in the best of conditions. In fact, most of the 20-odd species of beaked whales are known from dead specimens only.
But perhaps more interesting is that orcas are also seldom seen in these parts, despite being among the most iconic ocean dwellers. In a move to learn more about the whales that cruise the Aussie coastline, a team of researchers led by Curtin University marine biologist Rebecca Wellard visited Bremer Bay over a three-year period. The plan was to observe and photograph the whales – but the team got more than they bargained for when they witnessed not one, but four whale-on-whale predations.
"Very little is known about killer whales in Australian waters," says the team. "While encounters are typically rare and unpredictable, the area offshore from Bremer Bay appears to support abundant killer whales during the summer, and provides an opportunity to study this little-known population."
Of course, killer whales killing whales isn't exactly news. Many orca pods are known to hunt their marine-mammal brethren, and some even show preference for certain species or body parts (energy-rich grey whale tongue seems to be a favourite, for example). But the elusive nature of this particular prey sets the Bremer Bay events apart. The first incident was a two-hour battle involving a pod at least 20-strong.
Much like wolves, orcas are cooperative hunters, and what the team witnessed in Bremer was something of an orchestrated ballet. For the first hour, five core attackers closed in on the beaked whale, while the rest of the pod formed a loose perimeter. "[One] beaked whale broke off and headed towards our boat," recalls the team. "[But it] was quickly intercepted by another killer whale that was previously with the larger, dispersed group."
Depending on where – and what – they're chasing, killer whales will opt for a specialised strategy to get the job done. Stingray-hunting orcas in New Zealand, for example, will select a single "point whale" to pull rays from rocky hiding spots, and then lock the animals in place for other pod members to kill and dismember. Seal-eaters make waves to push prey off pack ice, or use their powerful tails to punt their target high into the air. California's shark-eating orcas, on the other hand, work together to drown their prey deep beneath the surface.
When it comes to landing a whale of a meal, the chosen tactic is something of a combo move. After closing in a second time, the Bremer pod continuously flanked the beaked whales, ramming and biting them to the point of exhaustion. They continued these synchronised flanks and attacks until the animal was tired enough to push underwater. The youngest members of the pod were even permitted to play a part, holding up the front or rear under the watchful eyes of their mothers.
"[After the kill], the killer whales exhibited social behaviour, including breaching and tail slapping," adds the team. Once the skin was removed, the blubbery carcasses were ready to eat.
Goriness aside, these images do hold interesting clues for the scientists involved, including rare intel about which species of elusive beaked whales move through the area. Because they lacked a signature erupted tooth and adult markings, the team suspects these were female or juvenile Gray's beaked whales (Mesoplodon grayi) and strap-toothed whales (Mesoplodon layardii).
But we're only just scratching the surface of what these predations might mean. Are the Bremer Bay orcas specialist hunters of beaked whales? Or do they turn to mammalian prey only when other food is scarce? These are the kinds of questions the team hopes to answer going forward.
"There are other accounts of Bremer killer whales potentially feeding on sunfish and an unidentified large species of squid," they explain.
The images could also have implications for what we know about orcas elsewhere in the world. The "sea wolves" have been seen feasting on the remains of dead beaked whales in both the Mediterranean and in the waters off Norway's coast, and at least one species of beaked whale – the Blainville's (which you might remember as the "crocodile-dolphin hybrid" – actively avoids orca calls. So it's possible that other pods are chasing down the rare leviathans as well.