When an animal remembers its friends or its enemies, it may simply be remembering a kind of predator or prey relationship, even within its own species. Social relationships rely on cognition and sociality. Potential social alliances or threats therefore could be assessed in what is called a fission-fusion social system. How long do dolphins remember their relationships? Using the characteristic signature whistles, lifelong memory proved to be potentially present within the groups studied. At least 20 years of sociality were recorded, for the first time in any of the whale groups (cetaceans). Elephants have been found to possess this capability and there is evidence that some humans can manage this feat.

The suggestion is that cognition and sociality are related in LTSR (Long-term social recognition) and provide excellent survival benefits. Complex social environments require this obvious aid to maintaining the society. In total, crows , hyenas, elephants, 2 monkey species and humans have been studied. They are all species with fission-fusion social systems, but less socially-complex animals do have memories of various kinships and/or rivals.

In the tested dolphins, a significantly greater response to familiar whistles was found compared with that to unfamiliar whistles. The length of separation from the other dolphin was irrelevant to the response. Decay of memory in calves less than one-year old was indicated because they were less discriminating than adults or juveniles. The individual whistles remained stable throughout the study and spectrograms indicated they were still stable after 12 years

Jason N. Bruck of the University of Chicago publishes the paper in the Proceedings of the Royal Society B: Biological Sciences.