Two monkeys have created a splash among experts after proving that apes can learn to do the chest stroke.
Cooper the chimp and Suryia the orangutan have caused a monkey puzzle that has left scientists scratching their heads.
Most land mammals instinctively doggy paddle in the water by paddling their paws but until now it was believed that great apes were very poor swimmers.
Monkeys usually flounder around in a flurry of limbs when they find themselves out of their depth and some have even drowned in zoos that use moats to keep them enclosed.
But these two have shown they can learn the chest stroke after being brought up by humans in the USA.
Scientists watched Cooper swim in his pool in Missouri where he enjoys retrieving objects from the bottom.
And Suryia was filmed at a private zoo in South Carolina swimming 12m independently.
Both used a leg movement similar to the frog kick used by humans to do the chest stroke.
Researcher Renata Bender said: “It was very surprising behaviour for an animal that is thought to be very afraid of water.”
Scientists believe that tree-dwelling ancestors of apes and humans might have lost the instinct to swim and over time developed other strategies to cross rivers, such as wading upright or using natural bridges.
But these apes seemed determined to prove them wrong.
Each ape had a slightly different style. Cooper moved his back legs together but Suryia kicked them out alternately.
Cooper eventually became so confident in the water that he could dive – helped by safety ropes – and taught himself to stay afloat.
And Suryia learned to swim under water with his eyes open and could move like human swimmers with his face immersed.
Researchers concluded: “The common opinion that apes are not able to swim due to an anatomical barrier is clearly rejected here.”