Dolphins are widely admired for their intelligence, playful nature, and seemingly benevolent interactions. Their popular image often portrays them as graceful inhabitants with complex social structures and remarkable cognitive abilities. However, like many species with intricate social dynamics, dolphins exhibit diverse behaviors that challenge this idealized perception. This exploration delves into lesser-known aspects of dolphin conduct, including aggression towards other marine life, intricate intraspecies interactions, and problematic encounters with humans.
Aggression Towards Other Marine Life
Bottlenose dolphins display aggressive behaviors towards other marine species, notably harbor porpoises. This phenomenon, termed “porpicide,” involves dolphins attacking porpoises, often resulting in severe trauma like fractured ribs, broken jaws, and internal organ damage. Post-mortem examinations rarely show signs of consumption, indicating these attacks are not predatory. Initial well-documented instances emerged off Scotland in the late 1990s, with similar cases reported worldwide.
Several theories explain this behavior. One hypothesis suggests dolphins and porpoises compete for similar food resources, leading to aggressive interactions for dominance or to reduce competition. Another explanation proposes these attacks stem from misdirected sexual aggression, where male dolphins may mistakenly target porpoises. Territoriality can also play a role, with dolphins attacking porpoises that venture into their established territories. Some researchers also consider “practice killing,” where dolphins, especially males, might use porpoises to hone skills related to infanticide or fighting.
Complex Intraspecies Social Behaviors
Within dolphin pods, certain behaviors challenge the perception of dolphins as uniformly gentle. Infanticide, the killing of calves by adult male dolphins, is documented across various species. This behavior is often linked to sexual selection; a calf’s death ends the mother’s lactation, making her fertile sooner for mating. Male dolphins have been observed attacking newborn calves, attempting to drown them shortly after birth. Research on stranded calves provides evidence of violent attacks from males of the same species, resulting in fatal injuries.
Male dolphins also exhibit sexual coercion and aggression towards females. They form long-term alliances, sometimes lasting decades, to isolate and control females for mating access. These alliances, involving two to three males, cooperate to herd and guard females, sometimes leading to injuries. Aggression within pods, less commonly lethal than infanticide, can occur through ramming, biting, chasing, and tail slapping, often as part of dominance disputes or male-male competition. Tooth rake marks, visible as parallel stripes on a dolphin’s skin, indirectly measure these aggressive interactions, indicating common encounters.
Problematic Encounters with Humans
Dolphins, intelligent and adaptable, can develop behaviors leading to problematic interactions with humans. One behavior involves dolphins learning to steal fish directly from fishing lines or nets. This “depredation” impacts human livelihoods and risks injury to dolphins from hooks or entanglement in discarded fishing gear. Once dolphins learn humans are an easy food source, they may alter natural hunting behaviors and become more reliant on human activities.
While rare, documented instances show dolphins exhibiting aggressive or overly assertive behavior towards humans, particularly solitary or habituated individuals. Interactions can include pushing, biting, or preventing swimmers from leaving an area. In some cases, human injuries have included minor bites and even broken bones. Experts suggest such aggression stems from stress, territoriality, or misdirected social and mating behaviors, especially in lonely male dolphins seeking interaction. Extremely rare, documented cases also show solitary dolphins attempting sexual interactions with humans, which can be inappropriate and dangerous due to their size and strength.