When photographers Alister Kemp and Jamie Muny first saw this dolphin chucking a small, silvery marine animal about they thought the mammal was likely hunting a salmon. Upon closer examination, the duo realized the bottlenose dolphin was actually attacking its cousin species, the harbor porpoise.
Researchers at Sea Watch Foundation say such attacks, which occurred in two separate incidents earlier this month, are rare and why the aggressive behavior is happening has yet to be fully understood.
“The cause of these incidents is not clear, but aggressive interactions could be due to the high co-occurrence of the two species which can result in competition over a shared food resource, occasionally leading to the death of the smaller species, although other theories have included misdirected infanticide,” said Chiara Giulia Bertulli, sightings officer for Sea Watch Foundation.
“In the hundreds, if not thousands, of hours I’ve spent photographing the dolphins, I have never witnessed such an attack,” said Kemp.
Dolphins committing acts of “porpicide” are rare, but not unheard of. In 2011, researchers reported an unusually high number of intentional killing of porpoises by dolphins off the California coast that they believe were both territorial and sexual.
“Necropsies showed multiple fractures from blunt-force trauma,” read the report. In some of the instances, the bite marks and scrapes appeared around the genital areas of the porpoises. The “porpitrators” were either known or suspected young males.
Two years earlier, a group of 16 male California coastal bottlenose dolphins attacked two harbor porpoises over a brutal two-hour period, eventually killing one after beating it for 20 minutes.
Previous research has indicated that in some areas of the world porpoises and dolphins will reside in the same areas, which has the potential to create more conflict over territory. If you're feeling particularly sadistic, you can view other examples of savage porpoise killings here, here, and here.
Giulia Bertulli offers another explanation for the May porpicides.
“The changing testosterone levels in male dolphins could also influence the extent and seasonality of these attacks,” said Giulia Bertulli. “The waters in the UK are incredibly rich for whales, dolphins, and porpoises, and reporting sightings, whether they are of species displaying unusual behaviors or not, is a really important for Sea Watch as they are way to learn more about the many cetacean species occurring around the UK.”