Who--Or What--Is Killing These Great White Sharks?

Fagjun | Published 2017-12-02 08:13

Though great white sharks are known to be one of the most formidable predators in the sea, something else has been turning them into prey.


What's turning these predators into prey? [Photo by Jim Abernethy, National Geographic Creative]

 

 

Predators getting preyed on may sound somewhat ominous, but that’s what’s happening to great whites washing up dead on the shores of South Africa. However, what’s killing these sharks isn’t some rarely-seen sea monster from the deepest waters, or a known sea animal with an unknown prey drive. Instead, the thing that’s been preying on the sharks is actually an animal that’s quite well-known to us:

 

Orcas.

 

Orcas have reportedly been preying on great whites in the waters off the South African coast. Some of us may not think that orcas are more formidable than great whites, but orcas are actually considered to be an apex predator in most of the world’s oceans. Orcas are also larger than great whites, and they also hunt in groups instead of alone.



Surgical Precision


Several great white sharks have washed ashore, with their livers missing. [Photo by Tami Kaschke/Marine Dynamics via AP]

 

 

In May of this year, researchers found that orcas had taken the livers from three great whites, whose liver-less corpses then washed ashore in South Africa. White shark biologist Alison Towner wrote that the orcas had removed the livers with “surgical precision”. One of the sharks was also missing a heart, which was also probably taken by an orca. Before this discovery, however, there were no indications that orcas preyed on sharks in the region. There have been reports of orcas preying on sharks, but reports were rare and came from elsewhere. Since May, however, there have been more reports of orcas preying on great whites in the same waters.

 

Researchers say that the orcas targeted only the livers of the sharks, disposing of the rest of the carcass after they got what they wanted. Shark livers are rich in the hydrocarbon squalene, which is important in the production of hormones and steroids. Researchers have also found that white shark livers are able to store more energy than whale blubber.

 

Orcas are usually quite specific in the targeting of organs in their prey. They don’t really consume the entire carcass of the animals they hunt. Orcas have been known, for example, to eat only the tongues of the whale calfs they managed to successfully hunt. Researchers think that this may be because some organs give more energy than other parts of the body.

 

It’s also possible that going after only one organ may be a way for the orcas to conserve energy. Then again, just going after great white sharks may already be energy-consuming in itself, so it’s possible that this isn’t the reason why orcas usually go after only one organ in their prey.



Tonic Immobility


Orcas on the hunt [Photo by Paul Nicklen]

 

 

Orcas may be bigger and better hunters than great whites, but orcas aren’t one for resting on their laurels. Since they’re intelligent creatures, it’s possible that the orcas have learned how to make great whites more vulnerable.

 

Back in 1997, researchers watched as an orca flipped a great white over and then attacked it. The shark then became immobile, making it easier for the orca to consume. Something similar was observed years later in 2014, this time with a tiger shark.

 

What’s happening was a natural phenomenon called tonic immobility, which is a state of paralysis that occurs when sharks are flipped upside down, with their ventral side positioned upward. If great white sharks in particular are kept too long in this state, they effectively drown. It’s therefore possible that orcas have learned this weakness and are exploiting it to be able to efficiently hunt great whites.

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