Killer whales display personality traits similar to those of humans, such as bravery, cheerfulness and stubbornness.
For the first time, researchers assessed captive whales using personality measures usually reserved for humans, asking their trainers to complete surveys ranking the mammals on a list of 38 traits.
They found the traits of killer whales - known as orcas - were similar to those of both humans and chimpanzees.
The whales were found to be playful, gregarious and sociable like humans but more like chimps when it came to being stubborn, protective, patient and peaceable.
"This is the first study to examine the personality traits of killer whales and how they relate to us and other primates," said lead researcher Yulán Úbeda, a doctoral student in psychology at the University of Girona in Spain.