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Long-term Memory Is Not Just for Humans

Long-term Memory Is Not Just for Humans

Dolphins can remember friends even after 20 years apart.

Dolphins can recognise old mates’ whistles even after being apart for more than 20 years, says a new study published in Proceedings of the Royal Society of London B.

This represents the longest memory about other animals ever recorded for a non-human species. Actually, dolphins’ exceptional memory skills may even be more lasting than in humans, as recognising old friends becomes harder as human faces change over time.

To establish how well dolphins could remember former companions, Dr Jason Bruck, from the University of Chicago, chased around 53 bottlenose dolphins to record their whistles. The animals were located at six different facilities, including Brookfield Zoo, Chicago and Dolphin Quest, Bermuda, which are part of a breeding consortium that rotates their dolphins. Crucially, they kept records for decades about which animals lived together and for how long.

A familiar whistle.

Using underwater speakers, Dr Bruck then played the recorded whistles to dolphins, from either an unfamiliar animal or one that they had lived together with. “Most of the time dolphins ignored the unfamiliar whistles”, says Dr Bruck, but “when they heard a dolphin they knew, they would quickly approach the speaker playing the recording”. The vast majority of animals presented this kind of memory even after decades apart, leading Dr Bruck to conclude that dolphins maintain lifelong memories of each other’s whistles.

However, it’s hard to say how long it takes for dolphins to learn a mate’s call. “I have noticed dolphins are capable of repeating unfamiliar whistles they hear in less than half a second”, says Dr Bruck, but “whether the whistle is committed to memory at that point is a good question”.

Jason Bruck working with a dolphin at the Brookfield Zoo near Chicago.

The most interesting debate at this stage, however, is why dolphins need this kind of memory. For this Dr Bruck has a simple answer: “We don’t know yet”. In the wild, dolphins frequently separate from their group and need to form new relationships when they join another group. “It may be possible that animals need to remember social experiences from previous partners after extremely long separations”, he adds, “or this super-long term memory might be a simple carry-over effect of selection for generalized cognitive skill”.

When asked about future research, Dr Bruck wants to “explore whether or not signature whistles elicit mental images of the dolphin producing them in other dolphins’ minds”. It’s easy to assume that a dolphin signature call works the same way as human names, but so far no one has been able to test what these whistles represent for a dolphin.

Source: Bruck J (2013) Decades-long social memory in bottlenose dolphins. Proc R Soc B 20131726. http://dx.doi.org/10.1098/rspb.2013.1726.

Photo Credit: Flickr/pmarkham

Alex Reis

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