Dolphins never forget the names of their friends

The signature whistle of acquaintances is still recognized even after 20 years of separation

Dolphin © SXC
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Dolphins have a better memory than many people: Even after more than 20 years, they still recognize the individual signal whistle from conspecifics, with whom they had ever had contact. It does not even take a year-long relationship: It is sufficient if the previous contact has lasted a few months, as a US researcher found out in the expierment.

They are unique in the animal kingdom: Only dolphins are known to have individual names - albeit in the form of special whistles. Recordings show that about half of all whistling sounds of marine mammals consist of these signature whistles - the animals almost regularly call their name out into the sea. Their conspecifics can identify them by means of these whistling sounds. The sea mammals not only signal their own identity, they also mimic the name whistles of conspecifics in order to get in touch with them.

Out of sight, out of mind?

One thing remained unanswered: What happens if dolphin companions separate after some time? Is it then "out of sight, out of mind" for the individual whistles? Or do dolphins remember the names of previous companions? Such a strategy would certainly make sense, as Jason Bruck from the University of Chicago explained. Because the own signature whistle accompanied a dolphin, as far as one knows, its entire life unchanged - similarly as with us our name.

In addition, dolphins live in a very complex social structure, a so-called Fission-Fusion system: Time and again, their group composition changes, for example, because young males leave their original school and temporarily merge into bachelor associations. Good social memory would therefore be of great use to the animals - after all, it is much easier to spot potential enemies or alliances if you know whether or not you already know your counterpart.

Whistles of former acquaintances as a test

For his study, Bruck conducted tests with 63 dolphins. Part of it was part of a breeding program that regularly exchanged animals between institutions, others came from a California institute. First, the researchers determined which animals knew which conspecifics and how long they had been held together in a pool. It took into account relationships of at least three months, the longest had lasted more than 18 years. The animals had also been separated for at least six months. The longest separation that Bruck took took 20.5 years. display

Afterwards, the researcher first played the whistles of unknown dolphins via underwater loudspeakers to his test animals, until the sea creatures became bored. Then followed either another unknown whistle or the whistling name of a conspecific, the test dolphins knew from their past. Decisive for Bruck was the reaction of the dolphins to these different whistles: Did you turn your head towards the loudspeaker? Did they approach him and if so, how close did they swim? Did they even try to make physical contact with the speaker?

Clear recognition even after 20 years

The result: with unknown whistles the dolphins did not react to very weak. If, on the other hand, the name of a former acquaintance were mentioned, the response was drastically greater, as the researcher reports. Surprisingly, it did not matter how long the animals had been in contact before, whether they were related and how long they had been separated even the more than 20-year separation seemed to be the memory of the familiar signature whistle had not impaired.

Since dolphins have a life expectancy of about 25 years in the wild, one can conclude that they keep the name whistles of their acquaintances in their head their entire life, resumes Bruck. The 20 years observed here are the longest period ever for which a social memory could ever be detected in animals.

What the behavioral scientist is next interested in is the question whether this type of memory is directly related to the intelligence of animals or even a prerequisite for higher cognitive abilities, For this purpose, the social memory capacity of other species would have to be investigated - above all, of course, elephants, which live in similar group structures as dolphins and of which there are also reports that they still be able to recognize conspecifics after ten years. (Proceedings of the Royal Society B, 2013; doi: 10.1098 / rspb.2013.1726)

(Royal Society, 07.08.2013 - ILB)