Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.
Species: Physeter macrocephalus
In the chill darkness 2 kilometres below the surface of the Southern Ocean, one of nature's greatest battles is being fought. One of the combatants is a kraken – the largest invertebrate known to exist, a colossal squid, over 12 metres in length. The other is its predator.
Named after the creamy sperm-like fluid found in its head, sperm whales are one of the few animals that can hunt and kill adult colossal squid. It may be bad news for the Antarctic's krakens, but it is good news for us: the whales' deep-sea depredations function as a carbon sink, slightly easing the effects of anthropogenic climate change.
Almost all marine life is found within 200 metres of the surface, the so-called photic zone. In this sunny region there is enough light for microscopic plants called phytoplankton to photosynthesise, absorbing carbon dioxide. In turn, the phytoplankton support a network of animals that feed on them and each other.
Earlier this year it was shown that Southern Ocean baleen whales help keep this process going by releasing huge amounts of iron in their faeces.The Southern Ocean is short of iron, limiting the amount of life it can sustain, but these injections of iron help out.
Now Trish Lavery of Flinders University in Adelaide, South Australia, and her colleagues have gone a step further. They have found that while the baleen whales merely help keep the iron cycle going, sperm whales actually inject iron into it by hunting their prey at great depths and then defecating when they return to the photic zone. In effect they ferry iron from the depths of the sea to the surface, where the phytoplankton can use it.
Based on existing studies of sperm whale behaviour and anatomy, Lavery and colleagues calculated that the 12,000 sperm whales living in the Southern Ocean collectively eat 2 million tonnes of prey each year – including 60 tonnes of iron.
Of this, about 36 tonnes ends up in the photic zone. That keeps swathes of phytoplankton going there, taking in CO2 from the atmosphere, that otherwise couldn't make a living.
Of all the carbon taken in by the phytoplankton, between 20 and 40 per cent ultimately sinks to the bottom of the ocean as various forms of waste. Lavery's team calculated that 400,000 tonnes of carbon gets dumped in this way every year as a result of the sperm whales' activities – far more than the estimated 160,000 tonnes the whales release by breathing.
All of which raises the question, how do sperm whales bring down such monstrous prey? No one has ever seen a sperm whale attack a colossal squid, but we can still work some of it out.
The whales have one advantage: in the cold Antarctic, colossal squid are rather slow-moving, lying in wait and ambushing their prey rather than actively hunting them. A hungry sperm whale might not have to chase very hard.
Similarly, the rather less enormous jumbo squid of the eastern Pacific actually go deeper to cool off, and the local sperm whales may hunt them at depth because they are slower and thus more vulnerable down there. And even the sizeable giant squid is something of a weakling despite its fearsome reputation.
Still, a weapon or two might be handy. And this could be where the sperm comes in. The spermaceti fluid is used to focus loud clicks, which the whales use for echolocation. The clicks can be over 230 decibels, making them the loudest sound produced by any living thing. It's been suggested that the whales use these clicks to stun their prey – but when they were played to prey animals in the lab there was no effect.
The spermaceti also cushions the whales' heads, particularly in males, which have much more of the stuff than females do. This allows fighting males to ram each other (or, in Moby-Dick, ships). However, there is no evidence of them ramming prey.
Females have their own hunting method. They live in tightly knit groups called pods, communicating with loud clicks and sharing the care of their calves. A tracking study earlier this year suggested that females hunt in packs, herding jumbo squid into "bait balls" just as dolphins do with fish. Young males live in "bachelor" pods and may therefore do the same thing, but as they get older they become solitary.
But if we really want to find out how they win their titanic battles, we need to get a video camera down there.
Read previous Zoologger columns: Globetrotters of the animal kingdom, Judge Dredd worm traps prey with riot foam, Flashmobbing locusts have redesigned brains, Smart camo lets glow-in-the-dark shark hide, Attack of the self-sacrificing child clones, The most kick-ass fish in the sea, The most bizarre life story on Earth?, Keep freeloaders happy with rotting corpses, Robin Hood meets his underwater match, The mud creature that lives without oxygen.
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