A fully grown humpback whale is a humongous, ornery adversary. It's perfectly capable of taking on killer whales with ferocious body slams and flipper slaps to defend its vulnerable calves. But recent research shows humpbacks do more than just protect their young: they often go on the offensive, actively seeking out and harassing killer whales  and even preventing them from attacking other species like seals, porpoises and sea lions.

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Killer whales attack a seal hauled out on an ice floe, with an agitated (bellowing) humpback in the foreground. Image: J Durban

Robert Pitman, a marine ecologist with the National Oceanic and Atmospheric Administration (NOAA), witnessed one of these interventions during a research expedition in Antarctica in 2009. After a pack of killer whales washed a Weddell seal from an ice floe and went in for the kill, a pair of humpbacks swam to the scene. One of the giants flipped onto its back and lifted the seal onto its enormous belly. As the killer whales approached, the humpback arched itself upwards, creating an artificial island for the pinniped prey. The whale gently nudged the seal back onto its chest when it started to slip into the water again. "I was shocked," Pitman told Science Magazine. "It looked like they were trying to protect the seal."

Pitman put out a call to colleagues to ask if anyone else had observed such perplexing behaviour. To his surprise, dozens of other scientists and naturalists had witnessed similar events.

The results of this survey were published recently in Marine Mammal Science. Pitman and his co-authors recorded 115 observations of humpbacks interacting with killer whales (also known as orcas) and 38 of these saw killer whales chased down and hampered during a predation attempt. The humpbacks usually acted in groups, resembling the kind of mobbing behaviour seen in birds when trying to drive off a predator. In 89% of these cases, the killer whales were targeting a species other than humpback whales. In other words, when humpback whales do get involved in a scuffle with orcas, it's usually to save a creature that is not a humpback whale.

But why risk potential injury (and possibly death) to help another animal? The question has been one of the stickiest sticking points in biology for centuries but it's even harder to explain when the animals involved belong to different species.

It's easy enough to understand why a humpback whale mother might risk her life to save her calf from a predator. Her job is, after all, to pass on her genes and perpetuate the species. Laying down your life for your children (or close relatives) seems a pretty straightforward way to ensure that the genes your family shares are preserved. This idea, called kin selection, helps to explain why social insects like bees will sacrifice themselves protecting a colony of their sisters.

The desire to help your unrelated friends can be explained by the concept of reciprocal altruism which is the scientific way of saying "if you scratch my back, I’ll scratch yours". Risk your life to save a friend and they might someday risk their life to help you.

But when explaining the cross-species salvation attempts of humpback whales, kin selection and reciprocal altruism don't fit. The seals (and other animals) are not family members, and they're never going to return the favour. Instead, Pitman suggests it's possible that the whales have evolved a simple behavioural rule: when you hear an orca attack, go break it up. If this rule saves a family member or friend just 11% of the time, it might still be worth it. And if it saves a seal or a porpoise the other 89% of the time, then that's just a happy accident. 

So what's going on inside the whales' minds when they see a seal targeted by a pack of orcas?

Some scientists and authors might argue that the humpbacks' behaviour could be evidence of empathy. In recent years, the idea that non-human animals experience empathy has been gaining traction. Primatologist Frans de Waal has argued that "there is increasing evidence … that animals are sensitive to the emotions of others and react to distress in others by attempts to ameliorate their situation or rescue them."

Esteban Rivas, director of the Institute for Animals in Philosophy and Science, suggests that the humpbacks' seal-rescuing behaviour can be "better interpreted in terms of empathy than an automatic mobbing response."

"[These reports show] not just aggressive behaviour toward the orcas, but also the gentle and helpful behaviour of the humpbacks toward the seal itself," he argues. In that light, the rescue of the Weddell seal in Antarctica was perhaps more than an accident. "[The whales] understood the distress and needs of the seal in that particular moment." 

According to Rivas, the humpbacks' behaviour is what experts involved in empathy research call targeted helping. "[It's when] an animal uses its cognition to take the perspective of another animal and is then able to give the specific help that is needed."

So what's driving these whales to the rescue? Is it a strong drive to thwart killer whale attacks to potentially save a fellow humpback? Or is some sense of empathy for another species at play here? Those are hard questions for animal behaviour scientists to answer. Either way, the seals aren't complaining.   

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Top header image: Alexandre Roux, Flickr