In May 2020, a sailor crossing the Strait of Gibraltar felt his small yacht being pushed from below. Something was striking the rudder repeatedly, bending and scraping the blade that controls the vessel’s direction. When he looked over the side, a black-and-white shape roughly the length of his boat was looking back.

Over the following five years, scientists recorded more than 760 similar encounters involving boats off the Iberian coast. Six vessels sank, and more than 250 sustained damage. Marine biologists across Europe have studied the incidents, trying to understand why a small population of critically endangered killer whales would deliberately target the rudders of sailing yachts.

The animals became known as the Gladis, a name derived from the word gladiator. The label reflected how the events were first understood: as an organized, combative campaign by the whales against humans. Research since then has produced a more complicated picture. The interactions appear to be learned social behavior, but what is being passed between the whales, and why, remains an open question.

The whales responsible belong to a small population of fewer than 40 Iberian killer whales that live in the Atlantic waters near the Strait of Gibraltar. They feed mainly on bluefin tuna. Researchers have identified around 15 individuals that have participated in the boat interactions, with the same whales appearing repeatedly in encounter reports.

The first recorded interaction took place on May 25, 2020, when an orca made contact with a boat in the strait. For several weeks, it appeared to be an isolated incident. In July of that year, a more disruptive encounter occurred, in which an orca struck the boat repeatedly until the vessel could no longer be steered. After that, reported incidents multiplied.

The encounters have followed a consistent pattern. The orcas approach from behind, target the rudder, and strike or bite it until pieces break away and the boat loses steering. The interactions have involved midsize sailing yachts, roughly 10 to 15 meters in length, with exposed single-spade rudders. Larger vessels, fishing boats, and motorboats have not been the focus of the behavior.

Throughout the recorded encounters, no human has been seriously injured. Researchers point out that the whales could cause far worse harm if they intended to. An adult orca is capable of attacking prey far larger and more dangerous than a person on a sailboat. The absence of any human injury has become central to the debate over what is driving the behavior.

The leading theory centers on a female whale researchers refer to as White Gladis. According to this account, she may have experienced a traumatic event, such as a collision with a vessel or an entanglement in fishing gear, at some point in her past. The encounter is described by researchers as a critical moment of agony. The theory suggests that this experience led her to target the rudders of yachts, parts of boats that may resemble the source of her injury.

Other whales appear to have learned the behavior from her. Orcas are known to transmit behaviors socially, with hunting techniques and other habits passed between generations and among pod members. The same process may explain how the rudder-targeting behavior spread among this population.

A competing explanation holds that the behavior is not a response to trauma but a form of play. Some researchers argue that the orcas have turned the rudders into playthings. The resistance and movement of the rudder, along with the sound and vibration it produces, may be stimulating to a highly intelligent animal. Younger whales appear to be learning the behavior by watching older ones, which would make it a cultural practice rather than an expression of aggression.

Claims that the orcas have sunk a thousand boats or are coordinating a deliberate campaign against humans are not supported by the evidence. The confirmed number of vessels sunk is six. The reported number of interactions is over 760, spread over five years along the coasts of Spain and Portugal. In October 2025, an interaction occurred off the coast of Portugal near Peniche, indicating that the behavior has spread beyond the Strait of Gibraltar area.

Both the trauma theory and the play theory agree on one point: the behavior is learned and socially transmitted. It has spread through the population as an idea rather than as an instinct, and it continues to be passed along to younger whales.

The Iberian killer whale population is critically endangered, with fewer than 40 individuals remaining. The continued interactions have generated calls for measures to protect both sailors and the whales. Researchers and authorities have explored ways to reduce encounters, such as advising boaters to avoid known areas and changing routes.

The question of whether the behavior stems from trauma or from play remains unresolved. The evidence confirms that the behavior is learned, social, and non-lethal. What it means, and where it will lead, is still uncertain. The whales have not communicated an answer, and they are not likely to.