Dolphins have a carefully cultivated reputation as the ocean’s friendliest creatures, but the reality is far less charming. Behind the permanent “smile” (a fixed feature of their jaw structure, not an expression of happiness) lies a catalog of behaviors that range from unsettling to genuinely violent. From killing smaller porpoises for no apparent reason to forming male gangs that coerce females, dolphins engage in some of the most aggressive social behavior documented in any marine mammal.
The “Flipper Myth” Problem
NOAA Fisheries has specifically warned the public about what it calls the “Flipper myth,” the idea that wild dolphins are friendly and safe to interact with. Dolphins are wild animals, and the curved shape of their mouths creates an illusion of a permanent grin that has nothing to do with their mood. This anatomical quirk, combined with decades of marine park shows and pop culture, has built a public image that doesn’t match the animal’s actual behavioral profile.
They Kill Porpoises for No Clear Reason
Bottlenose dolphins routinely kill harbor porpoises without eating them. This behavior, sometimes called “porpicide,” was first documented in the Moray Firth off Scotland’s northeast coast, where researchers witnessed four violent dolphin-porpoise interactions. Most porpoises that washed ashore in that area had multiple skeletal fractures and damaged internal organs. The same pattern has since been recorded in Cardigan Bay in Wales, Monterey Bay in California, and the Baltic Sea off Germany.
In one case from 2016, a single male bottlenose dolphin stayed in a region of the Baltic Sea for three months, and several porpoises with unusual blunt force trauma stranded during that period. Social media videos from the area showed dolphins actively harassing porpoises in the surrounding waters.
Researchers have looked into whether dolphins attack porpoises over food competition, but the two species have very little dietary overlap. The more supported explanation is darker: in every California case where the attacking dolphin’s sex was identified, it was male. Most attacks happened at the peak of breeding season, leading researchers to suggest that porpoise killing is a form of aggressive play by frustrated males with high testosterone levels and limited access to mates.
They Kill Their Own Young
Post-mortem examinations of stranded bottlenose dolphins in northeast Scotland found that five out of eight young calves had been killed by other bottlenose dolphins. Researchers also directly observed an adult bottlenose dolphin behaving aggressively toward a dead calf. This provided the first strong evidence of infanticide in any whale, dolphin, or porpoise species.
The size range of porpoises that dolphins attack closely matches the size range of dolphin calves that show signs of being killed by adults. This overlap suggests that porpoise attacks and calf killing may stem from the same underlying impulse. In many mammal species, males kill unrelated young to bring females back into a fertile state sooner, and a similar motivation likely applies here.
Males Form Alliances to Coerce Females
Among bottlenose dolphins in Shark Bay, Australia, males form alliances that harass, guard, and forcibly escort females to obtain mating access. This is one of the few examples of coordinated male sexual coercion documented in any mammal. Most mammals that practice coercion involve single males or temporary partnerships. Dolphins stand out because their alliances are stable, strategic, and involve multiple males working together against individual females.
The physical and behavioral costs to females are well documented. Females alter their movement patterns when adult males are nearby, shifting away from their preferred habitat and ranging farther from their usual home areas. These shifts are most dramatic when more than one male is present, regardless of whether the female is fertile. Researchers describe this as spatial sequestering: males effectively force females into areas they would never choose to occupy on their own. Adult females spend most of their time alone or with other females, but when cycling, they end up with multiple males roughly a third of the time.
Studies of captive dolphins confirm the same dynamic. In one facility, aggressive contacts between male-female pairs actually exceeded the number of friendly contacts. Females were consistently the main targets of male attacks in both captive groups studied, fitting the sexual coercion pattern observed in the wild.
They Brutalize Their Food
Dolphins don’t simply catch and swallow their prey. When eating octopus, they shake or toss the animal repeatedly until its head falls off, it breaks into pieces, and its arms stop moving. Another technique involves flinging the octopus into the air over and over. Individual dolphins tend to have a preferred method, or they combine both, repeating the process around a dozen times over several minutes before eating.
There is a practical reason for this: octopus arms can suction onto a dolphin’s throat and cause choking, even after the animal is dead. Tenderizing the prey makes it safer to swallow. But the result looks indistinguishable from an animal playing with its food, and in some cases, dolphins do appear to toss fish and other prey around recreationally before consuming it.
They Pose Real Risks to Humans
Dolphin injuries to humans, while uncommon, do happen. A retrospective analysis of marine animal injuries at a large hospital system found 11 dolphin-related emergency department visits. Seven of those involved being struck by a dolphin, three involved bites, and one was classified as other contact. Injuries occurred at multiple body sites, including the head, legs, and midsection. Women accounted for most of the dolphin-related injuries, outnumbering men roughly 8 to 3.
These numbers are small compared to other animal injuries, but they matter because most dolphin encounters happen when people deliberately approach wild dolphins to swim with them or feed them. The animals that injure people are often the same ones that have been habituated to human contact, which is precisely why NOAA advises people to admire dolphins only from a distance.
Hormones Drive Seasonal Aggression
Male dolphins and their close relatives experience dramatic hormonal swings tied to breeding season. In closely related finless porpoises, testosterone levels ranged from nearly undetectable to nearly 100 nanograms per milliliter, a massive swing controlled primarily by seasonal changes in daylight. High testosterone periods typically ran from roughly March through July, aligning with the breeding window.
When one captive porpoise was kept under constant artificial light, its testosterone cycles drifted and became erratic, with high-testosterone periods shifting earlier each year. Once natural light cycles were restored, the animal’s hormone patterns returned to a normal 12-month rhythm within a couple of years. This confirms that the aggression-linked hormonal surges in these animals are driven by environmental light cues, and it helps explain why porpoise killings and other violent behaviors tend to cluster during specific months.
The connection between these testosterone spikes and the seasonal timing of porpoise attacks is hard to ignore. Males with surging hormone levels and no access to receptive females appear to redirect their aggression toward smaller, vulnerable targets, whether those are harbor porpoises, dolphin calves, or each other.