When grey seals become killers

What predator has been attacking porpoises? The animals have increasingly been found washed up on North Sea beaches bearing unusual bite marks. The specialists who have been addressing this question for the last few years have identified the culprit as grey seals, which have never been known to attack these kinds of animals before. They were given away by their DNA which was detected in the bite marks as Thierry Jauniaux and Mutien-Marie Garigliany both of whom are researchers at the Animal Pathology Laboratory of the University of Liege have shown. But the reasons remain to be determined.

Skin punctures porpoiseThe wounds are reminiscent of bite marks from a large dog or slash wounds from an unscrupulous fisherman. Parallel strips of flesh were cut deep into the skin and fatty tissue (the sub-cutaneous layer of fat) of the porpoises. In a career spanning more than twenty years, Thierry Jauniaux has never seen anything like this despite the fact that he carries out autopsies on more than 150 marine mammals per year. “So evidently, I will notice any new phenomenon immediately".

But up to 2011, the bodies of these cetaceans he examined had died from an infectious disease, such as a virus, or a complications due to a parasite or simply because they had been caught in a fisherman’s net and had then been released.  

Then the first case of an animal bearing these somewhat unusual cutaneous bite marks occurred. Then another and another and so on. The theory of an aggressive dog roaming the beaches or a fisherman slashing the bodies of animals that were caught by accident in order to ensure that they sank into the ocean were not very convincing because these strange bite- marks, observed on the neck or tail of the animals were inflicted before death. “Also, the marks were not clear”, explains Thierry Jauniaux while cutting a piece of paper with a scissors and then with his fingers by way of demonstration. “In the first case, the edges are smooth, in the second case, they are slightly jagged. It is this second type of bite that we have observed on the porpoises so this could clearly not have been caused by a knife”.

In order to identify the author of the bites, the distance between the edges of the wounds was measured and compared to the interdental spaces of different species. A match was found with grey seals.

Grey seals !!?? These animals had never been known to attack porpoises which are sometimes called the ‘pigs of the sea’. The video of an attack, filmed by an ornithologist on observation duty in Cap Gris Nez (North of France) who had had the presence of mind to put his smartphone on long-view, confirmed the suspicions. But the distance of the observation was quite considerable and doubts still remained. In short, it was necessary to prove the phenomenon scientifically. 

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