Writing in Marine Mammal Science, Mark Cotter and colleagues from the nonprofit group Okeanis say it’s unclear what would motivate the seeming good-nwatured dolphin species to use lethal force on another marine mammal that doesn’t appear to compete for food or territory.

The researchers observed three such attacks on lone porpoises between 2007 and 2009.

Mobs of dolphins pursued the porpoises at high speed, then rammed and drowned them.

Cotter said that during one attack, three dolphins corralled the victim and were joined by seven others who then rammed the porpoise to death.

He said that two dolphins lingered to “play” with the carcass and then pushed it toward the researchers’ boat.

New Scientist reports that Cotter commented: “It was like they said: ‘We’re done playing with it, here you go.’”

The marine scientists believe that high levels of testosterone may be responsible for the lethal aggression since the attacks, mainly by males, appeared to occur at the height of the breeding season.

Photos: Barbara Scoles/Mark Cotter - Okeanis