Harbour porpoises are being killed in increasing numbers by bottlenose dolphins around British coasts, possibly due to competition for food.
Lack of food resources may be turning the cetaceans on each other
The evidence comes from counts of porpoises washed up on coasts and from post-mortem examination of the animals.
A lack of fish may be turning the dolphins on their cetacean relatives, according to some scientists.
The findings, from researchers based at London's Natural History Museum, appear in BBC Wildlife magazine.
Acting on their own, or with others, bottlenose dolphins ram the porpoises with their beaks, causing multiple injuries that include internal bleeding, rib fractures, ruptured lungs dislocated spines.
The porpoises are less than two-thirds the size of the dolphins.
Annual reports show a steady rise in the number of porpoise strandings, with 40 in 1995 and more than 120 in 2004. All of the strandings in the study occurred on the Welsh coast.
Of the 120 harbour porpoises stranded on the Welsh coast, 48 porpoises were in a condition suitable for a full post-mortem examination. Out of the 48 examined, 28 animals were identified as having died through attack by bottlenose dolphins.
This represents a three-fold increase in four years, according to the Strandings Co-ordination Group which is based at the Natural History Museum.
Rod Penrose, a spokesman for the group commented: "A decline in harbour porpoise bycatch over the years in Wales has been replaced by this violent interaction between these two species.
A bottlenose dolphin on its best behaviour for the cameras
"I personally believe the cause is competition for food."
Analysis of the stomachs of the porpoises suggested they had all eaten recently and had taken some unusual fish species, perhaps indicating that they were having difficulty finding their normal prey.
Mr Penrose told the Guardian that more research was needed to assess fish declines in coastal waters.
One theory suggested the violence might be re-directed aggression when access to females was limited. But female dolphins have also been observed taking part in the attacks on porpoises.
Another idea is that the behaviour might help to develop hunting skills, even though the victims are never eaten.