Dolphins are capable of remembering incidents that took place as long as 20 years ago, the longest by any species in animal kingdom, a latest study reveals.

According to the researchers, a dolphin can remember its former partner's whistles even after two decades of separation.

So far, elephants were believed to have the longest social memory in the animal world.

Author of the study, Jason Bruck of University of Chicago's Institute for Mind and Biology, had conducted the research on the complex relationship patterns between 56 captive bottlenose dolphins for more than two decades. The study was published in the Proceedings of the Royal Society B: Biological Sciences.

The researchers housed certain dolphins together under separate waters and recorded their behaviours and signature whistles. Experts have earlier found out that each species of dolphin has its own signature whistle much like human names.

The responses of the dolphins were observed whether the dolphins were able to recall their former partners' unique whistles.

Bruck said, "Dolphins got bored quickly listening to signature whistles from dolphins they don't know."

The researcher continued, "When they are familiar with the call, the dolphins are more likely to approach the speakers for longer periods of time. They will maintain contact with the speaker - if they are unfamiliar with the call they are more likely to ignore what I am playing. It's unprecedented in the study of animal behaviour to find memories this long."

He also highlighted the example of two bottlenose dolphins named Allie and Bailey. The duo of female dolphins lived together when they were young. But, Bailey was able to recognise Allie's whistle even after several years, the researchers point out.

"I was expecting five, may be 10 years. Memory for over 20 years just has not been systematically shown in animals so I was amazed by what I found," said Bruck.

The average life expectancy of bottlenose dolphins is 20 years but some live as longer as 45 years.