When, 6 years ago, divers captured on video a cuckolding attempt among squidlike animals called cuttlefish, experts were stunned. “The violence was beyond anything we had ever seen in the laboratory,” says Roger Hanlon, an ecologist at the Marine Biological Laboratory in Woods Hole, Massachusetts, who had been studying captive cuttlefish for years. Now, by carefully analyzing the behavior of the two males involved, he and his colleagues suggest the stepwise escalation of their fight likely required more brainpower than many researchers thought invertebrates had, they report this week in American Naturalist. The video (above) first shows a common European male cuttlefish (Sepia officinalis) mating with a female. While he escorts her to where she will lay her eggs, a second male suddenly appears and chases him away. But the first male doesn’t give up, and as his rival starts to get fresh with the female, the scuffle gets ever more intense. The rivals squirt ink at each other and jet about. Then, their dark markings turn even darker, and they engage in a quick battle of biting, grappling, and cork-screwing that soon sends the intruder scurrying off. Now that the scientists know how such explosive situations come about, they hope to recreate those circumstances in the lab to study male rivalries more systematically.