When Lonneke IJsseldijk saw the dead pilot whale, she nearly jumped for joy. “I couldn’t believe my eyes when I saw a tail protruding from the blowhole.”
This was the evidence she needed to prove to her colleagues that, yes, it is possible for a pilot whale to choke to death on a fish. Her surprising (and gruesome) findings were recently published in the journal PLOS ONE.
IJsseldijk is the Cetacean Project Coordinator at Utrecht University’s Faculty of Veterinary Medicine, and has spent years performing post mortem examinations (necropsies) on the bodies of whales, dolphins and porpoises.
After a badly decomposed pilot whale washed up dead on a Dutch beach in December of 2014, IJsseldijk examined the body at her lab to determine the cause of death. Unexpectedly, she found a fish lodged in the whale's nasal cavity, half of it protruding into the animal’s oesophagus.
It was a sole – one of those odd-looking flatfish that plaster themselves to the seafloor to avoid detection. They're not typical pilot whale prey, which made this a bizarre discovery. But just what the sole was doing in the nasal cavity was an even bigger mystery.
Porpoises and dolphins (like killer whales, pilot whales are members of the dolphin family) have evolved structures in their throats to prevent water and food from entering their airways. Their larynx is elongated, forming a kind of spout-shaped plug called a goosebeak that separates the path to their lungs from their oesophagus. A series of strong muscles holds the plug in place.
Dolphins can, however, shift their larynx to the side (the plug is not permanently kept in place). Which brings us back to the curious case of the dead pilot whale. It seems the animal had relaxed its laryngeal muscles a bit in an ambitious effort to swallow the oversized sole. And that was a fatal mistake.
The fish apparently used this opportunity to try to escape through the newly opened path to the whale’s nasal cavity, where it could wriggle its way up and out of the blowhole to freedom. Things ended badly for both: its airway plugged, the whale quickly suffocated to death; the sole never managed to escape.
At least, that was IJsseldijk’s hypothesis. The problem? Nobody had ever found a pilot whale that had choked on a fish before, so IJsseldijk’s colleagues had their doubts. “A lot of people were sceptical [of] this,” she notes. Could the sole have slipped into the airway only after the whale’s death? Given its advanced state of decomposition, it was difficult to draw concrete conclusions.
Then, a second pilot whale was found dead on a Dutch beach just a month later in January of 2015 (what are the chances?), the tail of a sole protruding from its blowhole. A necropsy confirmed that this fish – which was lodged squarely in the airway – was the cause of death.
IJsseldijk had been right.
“Everyone I asked was very surprised about a pilot whale dying of asphyxia,” says IJsseldijk. But this time, the evidence was much more convincing.
Sole appear uniquely qualified to attempt a mad dash for freedom through a dolphin’s blowhole if they find an opening. “[They're] very agile fish, that may roll up in any direction when handled,” IJsseldijk and her co-authors note.
But there's still more to this mystery. What strange series of events led to not one, but two pilot whales choking on sole just a month apart? We don't have clear answers, but there is a likely scenario.
Just six weeks before the first dead animal washed up, a pod of a few dozen pilot whales was spotted in the North Sea, along the coasts of the UK and Belgium. They were far from home – pilot whales are almost never seen in this part of the Atlantic.
Whatever the reasons for their unusual appearance, it's clear the whales had found themselves in foreign territory. Perhaps unable to find familiar food, they turned to hunting sole – maybe for the first time in their lives. With no flatfish-eating experience, it's possible they hadn’t mastered the right swallowing techniques. “It seems a risk to switch to unknown feeding sources and at least it was fatal in these two cases,” suggests IJsseldijk.
In the end, proving that a pilot whale can choke on a fish has taken luck, persistence and willingness to get your hands quite dirty. “A lot of colleagues wondered why I was spending time to get such a rotten carcass to the faculty as post-mortem research is really difficult in such a state of decomposition,” said IJsseldijk. “I am now very happy and proud that this worked out so well.”
Top header image: squallidon, Flickr