Dolphins are often celebrated as symbols of ocean intelligence, known for their playful nature and complex social structures. This perception contrasts sharply with the reality of their behavior, which includes dark and aggressive tendencies observed in the wild. Scientists have documented behaviors that, when judged by human moral standards, would be described as brutal, coercive, and violent. These actions are a natural component of their complex social ecology and survival strategies.
Intraspecies Lethal Aggression
The most severe form of violence within dolphin groups is directed toward their own kind, particularly infanticide. Male bottlenose dolphins, often working in coordinated alliances, attack and kill the calves of unrelated females. This action is understood as a brutal, yet effective, reproductive strategy. The death of a dependent calf ends the mother’s lactation period, causing her to return to estrus and making her available for mating sooner.
Necropsies on young dolphin carcasses found stranded have revealed extensive injuries consistent with severe blunt-force trauma, including rib fractures and lung lacerations. These injuries suggest the calves were rammed or slammed repeatedly by adult dolphins, a tactic used to separate the calf from its mother and cause death. Male alliances are instrumental in these lethal attacks and in other aggressive displays against rivals. Aggression is also used to establish dominance, with dolphins using their teeth to rake or scratch each other’s skin, leaving visible parallel marks.
Targeted Attacks on Other Marine Species
Dolphins engage in aggressive, often fatal, interactions with other marine species, most notably the harbor porpoise. These attacks are puzzling because they are non-predatory; dolphins do not feed on porpoises. The violence appears driven by misdirected aggression, competition for shared food resources, or object-oriented play.
Porpoises have been found stranded with severe internal injuries consistent with being rammed or struck by dolphins. Researchers hypothesize this aggression may be practice for infanticide, or linked to elevated testosterone levels in young males during the breeding season. The attacks have been reported worldwide, sometimes involving dolphins throwing the porpoise carcasses or carrying them in their mouths. Dolphins have also been observed non-lethally harassing other marine life, such as sea turtles.
Coercive and Non-Reproductive Sexual Behavior
The reproductive strategies of male dolphins include highly coercive tactics, where alliances work together to control access to females. This behavior, known as “herding” or “consorting,” involves males isolating a female and preventing her from leaving the group for days or weeks. Coercion is maintained through aggressive actions, such as biting, chasing, and physically slamming into the female. Females frequently attempt to escape this forced association, but they struggle against the coordinated efforts of the male group.
Beyond reproductive coercion, dolphins engage in a wide variety of non-reproductive sexual behaviors, a trait linked to their high intelligence. This includes homosexual behavior, masturbation, and the use of objects, such as dead fish or eels, for sexual stimulation.
Understanding the Biological Drivers
These aggressive and coercive behaviors are not expressions of morality, but rather the result of intense evolutionary pressures and a complex social structure. Dolphins live in a dynamic fission-fusion society where alliances and dominance hierarchies are crucial for survival and mating success. The stable alliances formed by male dolphins are a successful strategy for maximizing reproductive opportunities, even if it requires aggression. These behaviors are fundamentally tied to the dolphins’ drive for reproductive success and resource competition, where aggression becomes an adaptive tool. Their high intelligence allows for the development of complex social learning and sophisticated strategies to achieve dominance.