Dolphins can remember their old friend's signature whistle after being separated from them for 20 years.
The study found dolphins have the "longest social memory ever recorded [in] a non-human species," a University of Chicago press release reported.
Dolphin's long-term memory surpassed even other mentally sophisticated animals, such as elephants and chimpanzees.
"This shows us an animal operating cognitively at a level that's very consistent with human social memory," Jason Bruck, leader of the study who earned his PhD from the University of Chicago's Department of Comparative Human Development, said.
Bruck looked at data from 53 bottlenose dolphins from a breeding consortium. The consortium often rotates their dolphins' tank-mates and keeps a vigilant record of the shuffle.
"This is the kind of study you can only do with captive groups where you know how long the animals have been apart," Bruck said. "To do a similar study in the wild would be almost impossible."
Past studies have found dolphins have unique whistles (similar to names), and often answer or imitate their friend's call.
Bruck played recordings of a whistle the target dolphin had never heard before.
"Dolphins get bored quickly listening to signature whistles from dolphins they don't know," Bruck said.
After playing the unknown dolphin's whistle several times, Bruck let the study subjects hear the call of an old tank-mate they hadn't seen in years.
"When they hear a dolphin they know, they often quickly approach the speaker playing the recording," Bruck said. "At times they will hover around, whistle at it, try to get it to whistle back."
The study leader also played recordings of a dolphin that was around the same age and gender as the familiar caller to rule out other causes of the excitement.
In one case, Bruck played the whistle of a dolphin named Allie for her old tank-mate, Bailey. The pair hadn't lived together for over 20 years.
When Bruck played a recording of Allie's call, Bailey reacted to the whistle as she would any other familiar dolphin.
Bruck noticed a pattern; dolphin's reacted more enthusiastically to familiar calls than unfamiliar ones, even if they hadn't heard the call for decades.
He concluded that dolphins maintained a life-long social memory, in the wild their life expectancy is about 20 years.
"Why do they need this kind of memory? I'm not sure they do," Bruck said. "The cognitive abilities of dolphins are really well-developed, and sometimes things like this are carry-along traits. But to test whether this kind of social memory capacity is adaptive, we would need more demographic data from multiple populations in the wild to see if they experience 20-year separations."
The study also brings up the question: how close are dolphins' calls to human speech?
"We know they use these signatures like names, but we don't know if the name stands for something in their minds the way a person's name does for us," Bruck said. "We don't know yet if the name makes a dolphin picture another dolphin in its head."
Bruck hopes to answer these questions in further studies.
(WATCH)
