Bottlenose dolphins are known for problem-solving when it comes to hunting difficult prey. Previously, researchers have seen the dolphins protecting their soft snouts with sponges while digging in the seafloor for fish. Octopus hunting requires other skills, however. First, the octopus has to be disarmed—literally. And then those arms have to be bashed into submission. Those who do not learn this trick are likely to die. In a paper published recently in Marine Mammal Science, Murdoch University marine biologist Kate R. Sprogis and colleagues report that they've come across two dolphins killed while trying to eat octopuses:
It is apparent that octopus handling is a risky behavior, as within our study area a known adult male stranded and a necropsy confirmed the cause of death was from suffocation from a large 2.1 kg octopus. The dolphin had attempted to swallow the octopus, however, the octopus was found almost intact, with the head and the mantle of the octopus in the dolphin's stomach and the 1.3 m long arms separated from the head and extending out of its mouth. Similarly, another [bottlenose dolphin] died from suspected asphyxiation due to an octopus lodged in its mouth and pharynx.
Essentially, the octopuses' tentacles keep fighting, blocking the dolphins' airways, even after most of their bodies have been swallowed. It's a terrifying way to die, but Sprogis and the researchers observe that octopuses must be such valuable prey that they are worth it. Over seven years of observation, she and her team watched 33 dolphins "handling" octopuses in ways that made them meal-ready. Typically, the encounter would start with the dolphin biting the octopus' head off, followed by tossing the legs into the air so that they smack hard into the water over and over. Dolphins would execute the grab-and-toss move about 10-15 times before they were satisfied.
It's not unusual for dolphins to shake or hurl other kinds of prey to break it up or soften it, but the process is especially important when it comes to octopuses. Dolphins have to disable defensive responses in the legs, preventing the octopuses from using suckers to adhere to the dolphins' faces or throats. Basically, to kill an octopus, you have to rip off its head, then kill each of its legs. Then you can feast.
Dolphins aren't born knowing all the ins and outs of octopus hunting. Sprogis and her colleagues found that adult females were the most likely to attack octopuses, and it appeared that this was a behavior that younger dolphins were learning from other members of their groups. Bottlenose dolphins are very social, and octopus attacks only happened in the very largest groups. More research will be needed to know for sure how dolphins are teaching each other the art of the octopus smackdown, but it does seem to be a strategy that's shared in these Australian dolphin groups. The researchers found no other reported observations of octopus hurling in the current scientific literature.
There's even a specific season for octopus hunting: winter and summer, when the water is warming and octopuses are deep into their mating season. The dolphins may hunt octopuses more often during this time because that's when the cephalopods are most vulnerable. After mating and laying eggs, octopuses become senescent, or show signs of rapid aging. They slow down and have a more difficult time with camouflage. It's a perfect time for the dolphins to strike.
It's still unclear why dolphins are willing to risk so much for such a small meal. Sprogis and colleagues muse that the nutritional value of an octopus must be "substantial." Or, as marine biologist Holly Bik points out at Deep Sea News, maybe it's just that "dolphins are a$$holes."
Marine Mammal Science, 2017. DOI: 10.1111/mms.12405
Listing image by Marine Mammal Science
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