Environment

How Humpback Whales Save Other Mammals from Being Hunted

Astonishingly, on two occasions, mother humpbacks with calves joined other humpbacks to chase away killer whales from sea lions. They even put their calves at risk.

One killer whale from a pod hunting a crabeater seal on an ice floe when a pair of humpbacks (one in the foreground) charged in and drove off the killer whales. Credit: Robert L. Pitman

One killer whale from a pod hunting a crabeater seal on an ice floe when a pair of humpbacks (one in the foreground) charged in and drove off the killer whales. Credit: Robert L. Pitman

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

The seal slid from the ice floe on which it had been perched precariously. A moment earlier, a pod of killer whales, also called orcas, swam in a line to create a wave large enough to wash over the ice floe. The desperate seal swam towards a pair of humpback whales that were nearby. One humpback rolled over on its back and caught the seal on its chest. Intent on their prey, the killer whales followed. The larger whale rose out of the water and the seal rose in the air, out of reach of the predators. But the hapless little animal slid down the slope of the behemoth’s body. The whale shoved it back to the safety of its belly with an enormous flipper. After the frustrated predators left, the seal made its way to a large ice floe. The humpback’s solicitous behaviour intrigued a human observer, the marine biologist Robert Pitman of the National Marine Fisheries Service, California, US.

This was the third such incident Pitman had witnessed. During the preceding week, he had seen a pair of humpbacks prevent killer whales from making a meal of two seals off the southern tip of South America. Best known for their haunting songs, the humpbacks warded off the predators by bellowing loudly and slapping the water with their flippers and tails. Initially, the killer whales, which are half the length of humpbacks, appeared to be harassing them as is their wont. Only on reviewing video footage of the first encounter did Pitman spot a seal jammed between the two humpbacks.

Killer whales are formidable predators; their coordinated hunts earning them the sobriquet ‘the wolves of the sea.’ They don’t prey on seals alone; they also target the calves of large whales such as humpbacks. In some populations, up to 40% of the 15-metre giants had scars from confrontations with orcas. In the North Pacific, the marine wolves take as many as 18% of humpback whale calves before they turn a year old. To safeguard their young, humpback whale mothers take circuitous routes through areas generally avoided by killer whales. Often other humpback whales, most probably males, accompany mothers with calves. In the event of an attack, they surround the calves, boxing them in. Or, the adults may enclose the calves within a formidable ring by pointing their heads in and tails out.

When killer whales target humpback calves, the large slow-swimming whales cannot outrun their pursuers. They put up a fight, slapping their enormous tails and flippers on the surface, charging at the predators, and bellowing. Mothers hoist their calves out of the water and perch them on their heads or backs. The humpbacks that rescued the seals behaved similarly.

Was the behaviour of rushing to the aid of other species widespread?

Pitman and 13 colleagues compiled 115 cases of humpbacks confronting killer whales documented by at least 54 observers between 1951 and 2012 in the Pacific and Atlantic Oceans.

Numerous observations indicate humpback whales in the vicinity rush to the rescue of their own kind. On one occasion, three humpbacks that were 6 to 7 kilometres away converged on a pod of killer whales. They couldn’t possibly see their antagonists or their quarry from that distance. They most probably hear their calls, say the researchers.

Killer whales are silent hunters since their prey have an acute sense of hearing. But once they have cornered their prey, they call to each other to coordinate the attack or summon reinforcements.

However, humpbacks don’t react to any killer whale call. Orcas are either predominantly mammal or fish eaters and the calls of these two groups are distinctly different. Humpbacks seem to know this and surge in the direction of meat-eating orcas, where they harass and sabotage their hunts. When they meet fish-eating killer whales, they get along amicably.

Since they are reacting to the calls of the attackers, they probably don’t know the species being attacked until they arrive on the scene. If the prey animals were not humpbacks, they didn’t usually abort their attack. They persisted in chasing away the predators, but several moved away or hung back. One male humpback was photographed rescuing Stellar sea lions on two occasions separated by 15 years.

In their rescue attempts, humpbacks often work in pairs. However, in one case, as many as 13 of them ganged up on a pod attacking a humpback calf. Even if they are alone, they still think nothing of taking on pods of killer whales 10 or more strong. Nearly 90% of their rescues were of other species, including other whales, seals, sea lions, and an ocean sunfish.

Each instance of aid may last up to an hour. On occasions, their efforts can last as long as 7 hours. They forgo opportunities to feed, rest, and socialise. But the humpbacks were not always successful in saving others’ lives. In a few cases, they arrived too late to prevent the preys’ deaths.

How did other researchers miss this unique behaviour for so long?

“Maybe two things,” Pitman told The Wire. “Whales are still recovering from near extinction due to 20th century whaling. Perhaps there have been just too few around for people to observe. The other thing may be observer bias: when people, even researchers, see spirited interactions between humpbacks and killer whales, they usually assume that the killer whales are attacking. I think that people will look at these these interactions with a different perspective from now on.”

Astonishingly, on two occasions, mother humpbacks with calves joined other humpbacks to chase away killer whales from sea lions. Why do they put their calves at risk?

“It is curious to me that not all humpbacks seem to be interested or willing to interfere with attacking killer whales,” says Pitman. “Perhaps it is only those individual humpbacks that were involved in killer whale skirmishes with their mothers when they were young. Teachable moments. My hope is that with this new perspective on humpback-killer whale interactions we will have answers to some of these questions before too long.”

Humpbacks don’t have any teeth. Instead, they have baleen plates that help to sieve their prey from the water. Nor is their large size alone a deterrent. The flippers of these giants measure 5 metres, a third of their body length, and weigh a ton. They are the largest of any marine mammal. Male humpbacks, unlike other baleen whales, use their flippers to compete with other males for access to females. They use their disproportionately large limbs to advantage in chasing killer whales away, say the researchers. In addition, the barnacles encrusted on the leading edge can gouge out the flesh of their opponents. Their tails are larger still. The gigantic crustacean-studded flippers in the front and the powerful fluke at the rear are the humpbacks’ best weapons.

“These ponderous appendages are potentially a lethal threat, and killer whales keep their distance around excited humpbacks,” says Pitman. “This aggressiveness might account for the fact that humpbacks are the only baleen whale known to accost killer whales.”

No doubt humpbacks whales deprive orcas of prey by interfering with their hunts. The much smaller orcas are adept predators and the sabotage probably doesn’t cost too much.

Humpbacks don’t target killer whales alone, they also mob other predators like false killer whales and pilot whales.

Are humpbacks being altruistic? Why do they rush to save humpbacks that may not be related? What do they gain by harassing killer whales? Humpbacks may mob a predator, much like birds target raptors. This isn’t unknown in the undersea world. Seals and sea lions chase sharks while dolphins gang up on sharks and killer whales.

Although humpbacks are generally solitary, they may form relationships with others when they congregate at feeding grounds. Going to the rescue of others of their own kind may build reciprocal relationships. If one whale helps another, others may come to its rescue when the time comes.

However, what makes them rush to the rescue of other species at considerable risk and cost in time and energy? The authors speculate it may be a spillover of the same behaviour, but this needs more research.

The study was published in the journal Marine Mammal Science on July 20, 2016.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.