The Dolphin That Kills And Never Eats
Bottlenose dolphins ram harbour porpoises until their ribs break, toss the bodies into the air, and never eat them. Post-mortems on Scotland's coast point to a darker explanation: infanticide.
The Smile That Kills For Nothing
The bottlenose dolphin is the animal people pay to swim with. Along the coast of Scotland, it is also the animal that batters harbour porpoises to death and leaves the bodies to wash ashore.
Broken Ribs on a Scottish Beach
In 1996, veterinary pathologist Harry Ross and biologist Ben Wilson published post-mortem results for harbour porpoises stranded around the Moray Firth. Most of them, roughly two in three, had not drowned or starved. They had died from trauma: multiple skeletal fractures and damaged internal organs.
The skin told the rest of the story. The cuts on the carcasses matched the spacing of bottlenose dolphin teeth, and the Moray Firth is home to a resident bottlenose population. Four violent attacks were later witnessed directly. Dolphins ram the smaller porpoises with their snouts, strike them repeatedly, and throw them clear out of the water.
Not for Food
What makes the behaviour strange is what does not happen afterward. The dolphins do not eat the porpoises. There is no feeding, no scavenging. The porpoise is killed and abandoned.
The same pattern has since been documented on the California coast, where researchers coined a word for it: porpicide. Coastal bottlenose dolphins there kill harbour porpoises in the same way, and again leave the bodies untouched.
The Answer Was in the Calves
A second Moray Firth study, published in 1998, examined stranded bottlenose dolphin calves from the same area. Five of eight young calves showed the same injuries as the porpoises, and an adult dolphin was observed attacking a dead calf. It was the first strong evidence of infanticide in any whale or dolphin species.
The porpoises that were killed fell in the same size range as those calves. The leading explanation is that porpoise killing is linked to infanticidal behaviour: the dolphins may be treating porpoises as they treat calves, or rehearsing the same attack. The reason is still not fully settled, but the pattern is clear. The dolphin kills the porpoise, kills its own calf, and eats neither.
Sources
- royalsocietypublishing.org: Violent interactions between bottlenose dolphins and harbour porpoises (Ross & Wilson, 1996)
- porpoise.org: Evidence for infanticide in bottlenose dolphins (Patterson et al., 1998)
- science.org: Flipper’s dark side
- researchgate.net: Porpicide: coastal bottlenose dolphins kill harbor porpoises along the California coastline
