Did Grey Seals Mutilate Two Harbour Porpoises?

Here’s the face of a grey seal. Aw. Doesn’t it look adorable? Remember, however, that seals evolved from bear-like ancestors and are part of the (mostly) flesh-eating group of mammals called carnivorans. If you look inside its mouth, you’ll find strong canine teeth. And Jan Haelters from the royal Belgian Institute of Natural Sciences thinks that those teeth did this to a harbour porpoise:

This poor animal was one of two harbour porpoises that washed up onto the Belgian coast in September 2011, three weeks and thirty kilometres apart. Both were freshly dead and still bleeding from huge wounds. Witnesses saw the strandings and reported them to local authorities. Haelters was at the site within an hour.

Both porpoises were young males with extensive injuries. The first, shown in the photo above, is obviously missing its throat. The second (click if you really want to see) was even more badly injured, with skin and blubber ripped from its belly. Both animals were also riddled with a series of smaller cuts and punctures.

None of these wounds are consistent with a man-made cause, like nets, fishing gear or boats. An animal, then. There are plenty of sharks in the area, but none of the wounds looked like a shark bite. Dolphins have been known to batter porpoises to death but they do not kill to eat. Their porpoiseless slaughter (I’m very sorry) results in pulped internal organs, rather than substantial missing flesh.

Instead, Haelters thinks that the pairs of short, parallel, curved punctures on the animals were inflicted by canine teeth. Foxes prowl the beaches but only at night—these porpoises washed up during the day. Dogs fit the bill too, but Haelters argue that there are no stray dogs in the area and that domestic ones aren’t allowed on the beach at that time of the year. Besides, people saw the stranded animals and officials got to the bodies quickly. Someone would have noticed if a dog was gnawing away at the carcass.

That leaves just two culprits, both of them seals. The gap between the paired punctures—four to five centimetres wide—rules out the smaller harbour seal. By process of elimination, Haelters thinks the grey seal must have killed the porpoises.

Male grey seals are formidable animals that can weigh up to 350 kilograms, and Belgian lifeguards did spot several “very large seals” close to the shore in the summer of 2011. These animals eat fish, crabs, squid, and even sea birds on occasion. They grab onto large fish like salmon and cod with the claws of their front flippers while flaying skin and flesh with their teeth—a feeding style that could easily have produced the injuries seen on the dead porpoises.

If Haelters is right, there’s something oddly poetic about the otherwise grisly story. Whales and dolphins evolved from deer-like hoofed animals while seals, as I said, are carnivores that have taken to the sea. A seal eating a porpoise is a modern marine episode in a longstanding conflict between a group of (mostly) predators and a group of (mostly) prey.

But Kathryn Ono, who studies marine mammals at the University of New England, says that the case is circumstantial, and that Haelters has too readily dismissed the possibility that dogs inflicted the wounds. “Is it possible that grey seals attacked the porpoises? Yes. Is it conclusive? No,” she says. “They would need either an eyewitness, or seal DNA in the wounds. [Even] with DNA, they cannot be sure if the porpoises were bitten post-mortem or were killed by the seals.”

So, no habeas porpoise for the grey seal yet (I’m really very sorry). Ono also points out that harbour porpoises are much faster than grey seals. Haelters raises the same question, but notes that porpoises often rest or move slowly at the sea surface, and grey seals can sometimes act as ambush predators. The two porpoises were also not in the best of health—their blubber was thin and they had lots of parasites. Perhaps they were too weak to react quickly to a surprise attack.

However, Haelters cautions: “We would like to warn against blindly extrapolating this cause of death to all other cases of heavily mutilated harbour porpoises found recently along southern North Sea shores.” Indeed, if grey seals really did kill the porpoises, it seems they only started recently. More than 600 harbour porpoises have been stranded on Belgium’s beaches since 2000, and not a single one has the same pattern of injuries as these two individuals. There are some cases from the Netherlands that fit the bill, but only since 2006. Haelters notes that the grey seal population in the North Sea has gone up dramatically in the last decade, so increasing competition could have spurred them to try out new delicacies.

Erich Fitzgerald from Museum Victoria in Melbourne agrees that the evidence is circumstantial. “It points to the possibility that grey seals in the North Sea have a much wider dietary repertoire than typically thought,” he says. “The next step in testing this hypothesis is a wider examination of other mutilated small cetacean carcasses as well as at-sea observations of grey seal feeding behaviour.”

“I would not make a big deal out of it until someone actually sees an attack, and if it is a common occurrence, then that will come soon,” adds Ono.

Update: Haelters got back to me with some responses to the criticisms. On the issue of whether the animal could have been bitten after death, he says that they found several signs of haemorrhaging, “indicative of wounds inflicted on a live creature”. He adds, “Cutting or biting into a dead animal does not provoke blood being pressed into the surrounding tissue – this was the case at several locations in both animals, and was clearly evidence of wounds being inflicted on a live porpoise.”

On the suggestion that he tests for seal DNA, he says, “We have at least thought of this, but after consulting with specialists, it became clear that – as everything happens in an aquatic environment – that any trace of seal DNA from the salive of the seal would have been washed away in the water before stranding, and that it would be very unlikely to find some.”

Reference: Haelters, Kerckhof, Jauniaux & Degraer. 2012. The Grey Seal (Halichoerus grypus) as a Predator of Harbour Porpoises (Phocoena phocoena)? Aquatic Mammals http://dx.doi.org/10.1578/AM.38.4.2012.343

Hat tip to Justin Gregg for alerting me to this paper – he has also blogged about it, with amusing reference to the Mayans 😉

More on marine forensics:

• For 150 years, no one had seen a full spade-toothed whale. Then two washed up on a beach.
• Death in the octopus’s garden
• Scientists solve millennia-old mystery about the argonaut octopus