Scientists believe younger orcas are imitating the behaviour of an older, hostile female.

The whale, known as White Gladis, is thought to have been traumatised following a collision with a boat or being trapped in fishing nets.

"That traumatised orca is the one that started this behaviour of physical contact with the boat," said Alfredo Lopez Fernandez, a biologist at the University of Aveiro in Portugal who has co-authored a study on the mammals' behaviour.

He told Live Science the behaviour was "defensive based on trauma" and was being imitated by other whales.

Three orcas reportedly attacked the rudder and side of a sailing yacht off the coast of Spain on 4 May. The coastguard rescued all of the people aboard but the boat eventually sank.

Its skipper, Werner Schaufelberger told German publication Yacht magazine that two smaller whales appeared to copy the technique of a larger one by slamming into the boat.

Just two days earlier, six orcas rammed the hull of another yacht in the Strait of Gibraltar.

It followed a similar reported incident in November off the coast of Portugal when another vessel sank after its hull was cracked after being hit.

There have been at least 60 reported incidents between orcas and boats off the Iberian coast already this year, according to data from GT Orca Atlantica working group.

Reports of incidents began in 2020 leading Portugal's National Marine Agency to issue a statement warning sailors of the mammal's "curious behaviour".

It said the whales may be attracted to rudders and propellers and warned sailors to switch-off boat engines if the whales approach.

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A year later Spanish authorities banned small boats from sailing near the coast of Cape Trafalgar following 50 encounters with orcas, including 25 in which boats had to be towed to shore.

Iberian orcas are an isolated subpopulation of killer whales that can grow up to 21ft (6.5m) in length. They are classified as critically endangered by the International Union for Conservation of Nature.