Dolphins are widely celebrated for their intelligence, complex social structures, and playful nature. However, scientific observation of these marine mammals reveals a much more nuanced and sometimes dark reality, including high levels of intraspecific aggression. Examining whether dolphins are cannibals requires analyzing the documented behaviors and the technical definitions that govern biological classification. The question demands a careful review of lethal aggression, infanticide, and the rare instances of consuming a member of their own species.
Defining Cannibalism in Biology
In biological terms, cannibalism is defined as the act of one individual consuming all or part of another individual of the same species as a food source. This definition emphasizes that the consumption must be a primary driver or a significant secondary motivation for the interaction. It is a common ecological interaction observed across more than 1,500 animal species.
The distinction between true cannibalism and simple necrophagy is important for this discussion. Necrophagy refers to eating a conspecific that has already died from other causes, such as illness, injury, or stranding. An animal engaging in necrophagy is scavenging a readily available carcass, which does not carry the same implications as actively killing a member of its own species for sustenance.
Documented Lethal Aggression and Infanticide
The most common aggressive intraspecific behavior observed in dolphins, particularly in common Bottlenose Dolphins (Tursiops truncatus), is lethal aggression, not consumption. Adult males are frequently documented engaging in brutal attacks directed at young calves, a behavior known as infanticide. Post-mortem examinations of stranded dolphin calves have consistently shown evidence of severe blunt trauma and extensive rake marks—lacerations caused by the attacker’s teeth—across their bodies.
These injuries, which include fractured ribs and internal hemorrhaging, are consistent with purposeful, violent interactions where the primary goal is clearly to kill the calf. In one study of stranded dolphins, a significant proportion of young calves were found to have been killed by other Bottlenose Dolphins. Direct observations of these attacks show adult males repeatedly ramming and isolating the calf from its mother. While these acts are lethal, the violence appears focused on causing death, not on procuring a meal.
Evidence of Consuming Conspecifics
When addressing the consumption aspect of cannibalism, the evidence for dolphins as habitual cannibals is extremely limited. Diet studies, which rely on analyzing stomach contents of deceased dolphins, reveal a diet consisting almost exclusively of fish and cephalopods, like squid and octopus. These analyses consistently show that dolphins are generalist predators focused on typical marine prey, not on their own kind.
Instances of consumption are rare and typically considered opportunistic necrophagy or an incidental consequence of a violent attack. If a dolphin is consumed, it is usually a secondary benefit following a lethal interaction, rather than the reason for the killing itself. The lack of conspecific remains in the stomachs of otherwise healthy, feeding dolphins suggests they do not systematically hunt and eat other dolphins.
Hypothesized Motivations for Dolphin Attacks
The primary scientific theory explaining dolphin infanticide is the sexual selection hypothesis. Adult males often kill a calf that is not their own to prematurely end the mother’s lactation period. Since a female cannot become pregnant while nursing a calf, the death of the infant causes her to return to estrus sooner, making her available for the male to mate with and potentially sire her next offspring.
This aggressive behavior is an evolutionary strategy to maximize reproductive success, driven by intense male-male competition for mating opportunities. Other contributing factors to lethal aggression include resource competition, where larger dolphins may eliminate potential rivals for food or territory. The motivation behind the attack is overwhelmingly reproductive or competitive, reinforcing the conclusion that the goal is not to secure a meal.