It’s not a story anyone would have believed if it weren’t so thoroughly documented. A pod of killer whales in Antarctica is chasing a Weddell seal that’s taken refuge on a small ice floe. The orcas swim shoulder to shoulder to create a wave that washes the seal off the ice and into the water. Death looks certain. And then, without warning, a pair of humpback whales arrives. The seal, panicking, swims towards them. A wave tosses it onto the chest of the closer humpback, which is by this point rolling belly-up at the surface. The humpback arches its chest out of the water to keep the seal high and dry, above the reach of the frustrated killer whales circling below. When the seal starts to slip off, the humpback gently pushes it back onto its chest with a flipper. The seal eventually escapes onto another ice floe. The orcas, denied their meal, swim off.
The marine ecologist who watched this happen in 2009 was, on his own subsequent account, so bewildered that it took him years to work out what he’d actually seen. And what he ended up finding, once he started asking other researchers whether they’d witnessed anything similar, was that it wasn’t a one-off. It’s a pattern. And it involves species the humpbacks have no obvious reason to help.
What the researchers actually documented
According to a 2017 review paper by Dr Robert Pitman of NOAA’s Southwest Fisheries Science Center, with Volker Deecke, Christine Gabriele and eleven other co-authors from research institutions around the world, published in Marine Mammal Science under the title “Humpback whales interfering when mammal-eating killer whales attack other species: Mobbing behavior and interspecific altruism?”, the team compiled 115 documented interactions between humpback whales and killer whales over a sixty-two-year period from 1951 to 2012. The interactions came from researchers, whale-watch operators and wildlife photographers across multiple ocean basins.
The numbers, once you sit with them, don’t quite behave the way you’d expect. In 57 per cent of the interactions, the humpbacks were the ones who started it. They swam towards the killer whales, not away. Ninety-five per cent of the orcas involved were mammal-eating forms rather than fish-eating ones, meaning the humpbacks were reliably distinguishing between the two ecological populations and only intervening when the killer whales were hunting warm-blooded prey. And in the interactions where humpbacks approached killer whales that were actively attacking something, 87 per cent involved a kill or a hunt in progress.
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The bit that made the paper genuinely surprising is what happened next. When the humpbacks arrived at the scene of an attack, only 11 per cent of the time was the prey another humpback. The other 89 per cent of the time, the killer whales were hunting something else entirely. A seal. A sea lion. A porpoise. A grey whale calf. A minke whale. A sunfish. Across the full data set, humpbacks were documented interfering with orca attacks on ten different species, including six kinds of pinniped, three other cetacean species, and one bony fish.
The interventions weren’t casual. Humpbacks travelled up to two kilometres to reach an ongoing attack. They approached bellowing and trumpeting. They positioned themselves between the killer whales and the prey. They tail-slashed. They struck orcas with their flippers, which on a fourteen-metre humpback are capable of doing real damage. They stayed for hours in some cases, refusing to leave until the killer whales gave up. In one incident in Monterey Bay in May 2012, described in reporting by National Geographic on the specific case, at least fourteen humpbacks converged on a group of killer whales that had killed a grey whale calf, and spent hours preventing the orcas from feeding on the carcass. One humpback stationed itself directly next to the dead calf, head pointed at it, tail slashing every time a killer whale approached.
None of this makes obvious sense.
Why they might do it, and why nobody’s quite sure
The problem, on any straightforward reading of animal behaviour, is that helping unrelated members of other species carries real costs and no obvious benefit. Humpbacks spend energy on the intervention. They put themselves at some risk, though adult humpbacks are large enough that killer whales rarely try to attack them directly. They forgo whatever they were doing before the fight broke out. And the animal whose life they end up saving has no way of returning the favour. Grey whale calves don’t grow up to rescue humpback whales. Seals don’t remember. Sunfish do not, in any meaningful sense, participate in a reciprocal ecosystem of mutual aid.
Pitman’s own explanation, as he put it in an interview with Science Magazine at the time the paper was published, is that the humpbacks are probably not doing this deliberately in the sense we’d usually mean by that word. His hypothesis is that they’ve evolved a very simple rule. Something like: when you hear a killer whale attacking, go and break it up. The rule almost certainly developed as a defence of humpback calves, because mammal-eating killer whales do sometimes hunt young humpbacks, and adult humpbacks who intervene can drive them off. Over evolutionary time, humpbacks who intervened saved more of their own calves and left more descendants. The rule got locked in.
What the rule doesn’t do, on Pitman’s reading, is distinguish between an attack on a humpback calf and an attack on anything else. The humpback hears killer whales going after prey. The humpback goes to break it up. If the prey turns out to be a seal or a grey whale calf, so be it. The humpback has already committed to the intervention by the time it arrives, and the underlying behavioural rule doesn’t have a subroutine for backing out once the species involved turns out not to be a relative.
Which is, if you accept it, a genuinely deflationary explanation. It’s not altruism in the noble sense. It’s a simple decision rule that happens to produce altruistic-looking outcomes because the rule was never sophisticated enough to distinguish who was and wasn’t worth saving.
Some researchers find that explanation completely satisfying. Others don’t. Alisa Schulman-Janiger, one of Pitman’s co-authors, has suggested that some individual humpbacks who intervene have themselves been attacked by killer whales in the past, and may be responding to something closer to remembered trauma than to a simple hard-wired rule. Others have pointed out that the level of coordination in some interventions, particularly the Monterey Bay incident with fourteen humpbacks arriving from different directions, is hard to explain by a rule that fires purely on hearing a killer whale acoustic signature. And Pitman himself has conceded that the possibility of something like inadvertent altruism, in which the humpbacks aren’t intending to help but end up doing so as a byproduct of a behavioural rule, doesn’t quite rule out the possibility that something closer to intention is also in the mix.
What the paper actually concludes is unusually careful. Interspecific altruism, in the paper’s wording, could not be ruled out. Which is not a claim that humpbacks are behaving altruistically toward other species. It’s a claim that after 115 documented interactions across sixty-two years, the researchers couldn’t cleanly reduce the pattern to any simpler explanation. Something is going on out there. It looks a lot like one species deciding, reliably and at cost to itself, to help members of other species who have nothing to offer in return. Whether that’s the same thing humans mean when they use the word altruism is a question nobody, on the current evidence, can quite answer.
Kiran Athar writes about the ordinary corners of the natural world where science and questions of meaning intersect. This piece draws on peer-reviewed research in Marine Mammal Science and contemporary coverage in Science Magazine and National Geographic.
