A solitary male bottlenose dolphin is believed to have killed six harbor porpoises in a spate of attacks in the Baltic Sea in 2016. What made the dolphin carry out this killing spree is unclear and scientists are now trying to understand what was behind the behavior.
Bottlenose dolphins have been documented acting aggressively towards harbor porpoises since the late 1990s. Scientists first noticed the behavior off the coast of Scotland, where dolphins would attack porpoises, causing fractures to their skeletons and damaging their internal organs. Tooth marks were also found on several porpoise carcasses. After this, similar cases were reported off the coasts of Wales, California and North Carolina.
In a study published in BMC Zoology in September, researchers have looked at events that took place in the German Baltic Sea over three months in the fall of 2016. This region is not generally inhabited by bottlenose dolphins, so when three—one pair and a solitary male—turned up over 2015 and 2016, it made national headlines.
The male came back for three months in 2016 and, during this time, six harbor porpoises washed up dead with varying degrees of trauma inflicted to their bodies—unusual events for the region. The dolphin was recognizable through "eyelash-like scars" around his left eye and scars on the side of his blowhole.
Scientists led by Stephanie Gross, from the University of Veterinary Medicine Hannover, Germany, performed autopsies on the dead porpoises. They found all of the animals had experienced hemorrhages. Four had suffered fractures to the ribs, and in one case the animal's lung had been lacerated. Gross told Newsweek that heart failure was the presumed cause of death in most cases.
The team says it is likely the solitary male was responsible for the deaths. The necropsy findings fit with previously reported bottlenose attacks on porpoises. They also note that public sightings show there was a correlation between the dolphin sightings and the stranding of fatally injured porpoises.
"Despite the fact that no attack has been witnessed in German waters to date, our findings indicate the first record of lethal interactions between a bottlenose dolphin and harbor porpoises in the German Baltic Sea," they concluded. "Furthermore, to our knowledge, this is the first report of porpoise aggression by a socially isolated bottlenose dolphin."
Gross said they do not know why the solitary male arrived in the German Baltic, saying it may relate to range expansion or exploring new feeding grounds.
She said the team does not know why this dolphin would have attacked the porpoises. "Without witnessing the attack and having more information about the whole situation it is very difficult to say," she said in an email.
Based on analysis of previous attacks, the team say there are a number of different possibilities as to why dolphins are killing porpoises. These include territorial disputes, food competition, interference with feeding and defense of group members.
Another possibility is that the attacks relate to "object-orientate play." This includes the dolphins practicing infanticide or the aggression resulting from sexual frustration.
Both male and female dolphins are known to kill their young. In the study, the researchers say they may be using the porpoises, which are smaller than the dolphins, to "hone their skills for infanticide."
Discussing the possibility of sexual frustration, the researchers note that all the attacks in California were carried out by males. The attacks also took place in the fall, at the end of the main breeding season. "This might lead to the assumption that the attacks are linked to mating behavior and/or sexual frustration when female bottlenose dolphins are not in estrus anymore," they wrote.
Gross said the team plans to continue to record any dolphin sightings in German waters. They are hoping to find the dolphin and observe its behavior, ideally while interacting with a harbor porpoise. Where the solitary male from the study is right now is unknown, she said.
