In most of the animals that have been investigated, including humans, specific memories are lost over time. Due to the deterioration of the part of the brain that is responsible for processing and storing the what, where or when of an event, the events are blurred, distorted or lost without more. But in cuttlefish this so-called episodic memory remains unchanged until their penultimate days. And it seems to be related to mating.
Researchers from French, British and American institutions have been training several dozen cuttlefish to record the time and place in which they put their favorite food, shrimp. They did different variations of the tests with two large groups of specimens. Some, with an average of 12 months, were about to leave adolescence. The others, already between 22 and 24 months, had an age equivalent to that of humans of 80 to 90 years.
In the experiment, the results of which have just been published in the scientific journal Proceedings of the Royal Society B, first trained the cuttlefish to approach a specific spot on their tank marked with a black and white flag. Then, they reinforced this behavior by placing two of their usual prey before the flags. In places and moments, the flag was waved with a piece of shrimp. In others, the prey was live shrimp although of a similar size to the other piece. In addition to the different location, the researchers reduced the frequency in which they offered live seafood: prawns every hour, shrimp every three. And so they stayed for four weeks.
Cuttlefish with an age equivalent to 90 human years remembered where and when to go to eat their favorite food, shrimp
Thereafter, they studied whether the cuttlefish remembered what food, where and when it would be available. To make sure that what they learned was not a repeating pattern, the positions of the flags were unique each day. They found that regardless of age, the two groups of cuttlefish knew where and when to go if they wanted to eat shrimp. They were old or young, the hit ratio was the same, 8 out of 10 approaches with a prize.
Researcher Alexandra Schnell, from the department of psychology at the University of Cambridge and lead author comments in a press release: “Cuttlefish remember what they eat, where and when and use this information to guide their food decisions in the future.” But, Schnell adds, “What is surprising is that they do not lose this ability with age, despite showing other signs of aging, such as loss of muscle function or appetite.”
Other studies had shown that cuttlefish did lose the ability to remember skills learned long ago, long-term memory. It had also been verified that their semantic memory, that which refers to general knowledge not linked to a specific space-time event, remained unchanged. In both types of memory, these cephalopods are indistinguishable from other especially clever or intelligent animals, such as corvids, rodents, various marine mammals, apes, and humans themselves. But only cuttlefish seem immune to episodic memory impairment.
Cuttlefish only mate at the end of their life, although several times and preserving their episodic memory would prevent them from copulating with the same partner twice.
The key may be that all the other animals mentioned have hippocampus and cuttlefish do not. In birds and mammals, this part of the brain is key in space-time memory. Recent studies, for example, have shown that GPS and assisted guidance tools are affecting the hippocampus. But cephalopods, octopuses too, use their vertical lobe for cognitive processing skills that have to do with what, when and where they have eaten. And this part of their brain remains intact until a few days before they die. What is striking is that it also appears connected with mating, the last thing cuttlefish do in this world.
“Resistance to age-related deterioration could be the result of mating pressures experienced by cuttlefish,” Schnell explains in an email. And he details it: “They only reproduce at the end of their life, around 20 months of age, which is comparable to 90 years in human age. The last three months are crucial, as they need to mate with as many pairs as possible before dying at the end of their breeding season. Cuttlefish could spread their genes widely by remembering who they mated with, where and how long ago ”.
These results highlight the cuttlefish as an interesting species to investigate the natural mechanisms that protect complex memory from the impact of aging. And these findings, says Schnell, “could help advance research on human memory and how it declines with age.”