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| An orca of Moctezuma's pod with whale shark carrion in her mouth. (Kelsey Williamson) |
Here comes a sobering reminder. A pod of orcas (Orcinus orca) that patrol the Gulf of California have devised collaborative hunting techniques to take down the biggest fish in the ocean: the magnificent, filter-feeding whale shark (Rhincodon typus).
On multiple occasions, the pod – including a male named Moctezuma – have been observed hunting the gentle giants in a manner scientists believe may be similar to how another, unrelated group halfway across the world pursues and kills the white sharks off South Africa to slurp up their delicious, oily, fatty livers.
"It's not really surprising that they hunt the whale sharks, as well," marine biologist Erick Higuera Rivas of Conexiones Terramar
"What's surprising is seeing the interaction between two such charismatic species of the oceans and also how this specific pod knows how to adapt and has acquired different types of strategies to hunt different prey. In the case of hunting whale sharks, this pod uses teamwork efforts and specific hunting techniques to immobilize the sharks and extract the internal organs, specifically the liver."
All the world's orcas are classified as the same species, but groups in different regions around the world are relatively distinct from one another. There are physical differences between the groups, and nor do they intermingle. Some marine scientists believe that we are observing speciation in action.
Another way these groups of orcas, known as ecotypes, differ is in their hunting and dietary strategies. Some groups predominantly feed on fish such as salmon; others on mammals such as seals, or even large whales.
The orcas in the Gulf of California are not assigned an ecotype, but instead are considered generalists, hunting a wide range of prey that includes marine mammals, turtles, and fish.
One intriguing pod, however, is known to hunt elasmobranchs, the group of cartilaginous fish that includes sharks and rays. Since the 1990s, the formidable predators have been observed hunting multiple species, including devil rays, bull sharks, and, yes, whale sharks too.
As the number of sightings increased, scientists began to notice that the orcas hunting elasmobranchs were the same individuals, indicating a specialist hunting strategy.
Now, led by marine biologist Francesca Pancaldi of the Interdisciplinary Center for Marine Sciences in Mexico, a team of scientists has observed four whale shark hunts and seen that strategy in action for the biggest fish in the seas.
In three of the four hunts, Moctezuma was involved, and female orcas of his pod also made repeat appearances, suggesting that it is the one family group that engages in whale shark hunts.
The orcas' attack is highly coordinated. First, they ram the whale shark with their heads with some force until the whale shark is stunned and immobilized. Then, they flip the whale shark upside-down, and start tearing with their teeth so the whale shark bleeds copiously, focusing on the belly of the whale shark near the location of the liver.
No observations were recorded of the orcas eating the whale shark's liver; but it's not a big leap to assume that the liver is at least part of the orcas' focus. Shark livers are where they store all their fat, which presents an enticing treat to a hungry orca.
"I have personally seen the same orca pod featured in the paper eating only the liver of other species of sharks in the Gulf of California. We know from other publications that orcas target that organ (like in South Africa) because it is rich in lipids, omegas and other nutrients," Pancaldi
"Orcas could eat other organs like the stomach and intestines, but we don't know 100 percent. As in the case of the great white sharks in South Africa …, [the orcas] are also only interested in the whale shark's liver. The whale shark's body is mostly made up of cartilage and muscle, so it is not very rich in nutrients for a top predator mammal like orcas are."
The researchers don't believe that the behavior is new, driven by low prey availability, nor that it poses a danger to whale shark numbers. Whale sharks have been visiting the Gulf of California for decades. They are also slow moving and pretty defenseless.
And orcas are cunning. They learn from each other, and work together, making them an unstoppable force when they put their minds to something.
"Orcas specialize in hunting the prey that is available in their area or region. Learning is a key component for a variety of behaviors, including those related to foraging," Higuera Rivas said.
"Relatives, including mothers, often play a key role in this learning early in an orca's life, although individuals that engage in at least some foraging behaviors continue to increase their foraging efficiency throughout much of their life. These learning processes can result in the cultural transmission of behaviors that spread among close associates within a population or even throughout an entire population."
It's a bit rough on the poor whale sharks, but it's a fascinating puzzle about orcas. It could mean that Moctezuma's pod constitutes an emerging ecotype with an elasmobranch specialization that is not shared by the other orcas inhabiting the same patch of sea.
Future studies involving genetic and isotope analysis would help the scientists better understand the complex, invisible dynamics between the orcas of the Gulf of California.
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