It's a Battle Royale; one of nature's great confrontations.

Under the waves cruises a pod of orcas, huge, sleek predators, each around 8 metres long and weighing some 6 tonnes. Each with big jaws, full of teeth.

They are hunting a great baleen whale, one of the largest animals that has ever lived.

Such life-or-death battles, between orcas and whales, have captured the popular imagination.

But the truth is more complicated.

For a start it has been unclear whether orcas, also known as killer whales, really hunt whales, and how often. Nor did we know how the whales themselves might react to such attacks.

Now for the first time, scientists have recorded orcas attacking and killing humpback whales, specifically young calves. The results are published in the journal Marine Mammal Science.

What's more, the humpback whales themselves aren't passive victims. They aggressively turn to battle the orcas, and even recruit "escort" whales to help fight off the attacks.

Rare sightings

Orcas have developed a reputation for preying on baleen whales, a group that includes blue, fin and humpback whales among others. They have been recorded attempting to attack almost every species, and also sperm whales, the largest species of toothed whale.

Many whales display tooth marks made by orcas on their tails and flippers, suggesting such attacks are common.

But with a few notable exceptions, including times when orcas have preyed upon grey whale calves, successful attacks by orcas on whales have rarely been documented.

For example, humpback whales are among the most studied large whales, being observed for countless hours at sea by scientists. More orca tooth marks are found on humpbacks than any other whale species. But until now there was no scientific record of an orca killing a humpback whale.

Research over recent decades has also revealed a range of orca populations around the world, each hunting different prey using different techniques. Many don't hunt whales at all. Some orcas for example, particularly those living in northern latitudes including the North Pacific and Antarctica, only hunt fish, while others exclusively hunt seals.

But now researchers have observed the action close up.

Robert Pitman, a marine biologist based at the US National Oceanic and Atmospheric Administration, in La Jolla, California and colleagues from the US and Australia, tracked orcas and humpback whales off the coast of Western Australia.

They observed orcas attempting 22 separate attacks on humpback whales.

On 14 occasions, the orcas attacked and killed a humpback whale calf.

To investigate further, the scientists attached a tracker to a female orca, which allowed them to use satellites to monitor her movements. They followed her for six days.

During that time, her pod attacked eight humpback whale calves. During the seven attacks in which the researchers witnessed the outcome, the orcas killed the humpback whale calf on three occasions.

That suggests that, for this population of orcas at least, humpback whales are a predictable and plentiful prey, although the orcas were only seen attacking calves and not adult whales.

But the story doesn't end there.

Baleen whales themselves are popularly thought to be large but generally unassuming, passive creatures. Pitman's study reveals another side to their character.

When chased by orcas, certain species of baleen whale are known to try to outswim their pursuers. Blue, fin and minke whales are thought to do this, sprinting at high speed so that the orcas can't keep up.

On some occasions, the humpback whales seen by Pitman's team sought out protection. They swam to shallow water, nearby reefs, or even under the researchers' boats. These tactics often curtailed the attack.

But at other times, the humpback whales decided to stay and fight.

As the orcas approached, the mother humpback would sometimes move her calf to her side, or lift it out of the water using her head or flippers. She also blew huge breaths of air to disturb the orcas, and lunged or charged at them, slashing and slapping her tail and flippers.

Perhaps most surprising, humpback whales also have adult "escorts" that try to protect calves that are not their own, joining the mother in defending the smaller whale. These escort whales either charged at the orcas, or placed themselves between the attackers and calf, thrashing their tails and flippers.

It is not yet clear whether escort whales are related to the calves and if not, how they might benefit from defending them.

Despite the efforts of the mother whales and escorts, the orcas were more often than not successful in their attacks. But the presence of the escorts did reduce how many times a whale calf was killed.

In light of their study, the researchers believe that orcas may congregate each year off the coast of Western Australia to prey on baby humpback whales.

Humpbacks usually give birth to one calf every two years, so each calf killed is a significant loss to a mother.

Follow Matt Walker and BBC Earth on Twitter.