Forget about elephants - scientists say that
dolphins have the longest memories yet
found in a non-human species.
Researchers in the US say that even after 20
years of separation, dolphins could recall the
whistles of former companions.
The authors believe that these long-term
memories are a product of the complex social
connections that dolphins have evolved.
The research is published in the Proceedings
of the Royal Society B .
In the study, the scientists used information
on the relationships between 56 captive
bottlenose dolphins that have been moved for
breeding purposes between six different zoos
and aquariums in the US and Bermuda.
The records, dating back decades, showed
which of the dolphins had been housed
together.
Total recall
The researchers then played recordings to the
dolphins on underwater speakers of the
signature whistles of animals they had once
lived with, and measured their responses.
"When they are familiar with the call, the
dolphins are more likely to approach the
speakers for longer periods of time," said Dr
Jason Bruck from the University of Chicago
who carried out the study.
"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."
Dr Bruck highlighted the case of two female
dolphins called Allie and Bailey. They had once
lived together in the Florida keys when they
were very young.
The scientists say that the complicated nature
of their social groupings impact their ability to
recall whistles
Bailey now lives in Bermuda but when a
recording of Allie was played, she instantly
responded in even though it was was 20 years
and six months since they had been in contact.
Dr Bruck says this type of response was
typical. Compared to unfamiliar calls, there
was a clear pattern in the data where dolphins
responded significantly more to whistles from
animals they had once known, even if they
hadn't seen or heard them in decades.
To check that the dolphin's response was due
to recognition of a former companion, Dr
Bruck would also play a test recording of an
unfamiliar bottlenose that was the same age
and gender as the familiar animal.
Nuclear family
The researchers believe that the complex
nature of dolphin social systems is behind the
long term memory effect.
In the ocean, dolphins have a fluid social
arrangement that scientists term a "fission-
fusion" model. They may leave one group and
join others many times in their lifetimes.
"It is important for them to recall the calls of
dolphins they have had previous encounters
with, to decide whether or not that's someone
they want to approach when they hear that
whistle about a mile out, or whether they want
to avoid that individual," said Dr Bruck.
"Having a long term social recognition for that
ecological reason can be the difference
between an animal having a very negative
social interaction and a positive one."
According to the researchers, a dolphin's
abilities to recall events indicate that the
cetaceans have a level of cognitive
sophistication comparable to humans,
chimpanzees and elephants.
While elephants are also reputed to have
extremely long memories of up to 20 years,
there is little scientific evidence of their
abilities outside of family relationships.
In this research paper, the dolphins were able
to remember family members as well as
strangers.
In recent weeks, another study has shown
that dolphins have their own signature whistles
that appear to have the same function as
names do for humans.
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