Published: Wednesday, 18 August 2021 20:25
Remember that mouthwatering meal you had 20 years ago at that unforgettable restaurant? Old cuttlefish can remember that sort of thing, too. In a new study, researchers found the squid relative can recall the entire experience of its favorite meal—and, unlike in people, that ability gets better with age.
In human brains, some memories have more staying power than others. Recall of facts such as the location of the nearest grocery store or the date of the first Moon landing—what scientists call semantic memory—generally doesn’t fade with time. But we can also remember unique events in our own lives, which include not only where and when a thing happened, but also the specific sensations we experienced as it was happening. This “episodic memory” often dims over the years. Studies in jays, rats, and monkeys show they also have both kinds of memories. And 8 years ago, another animal joined the club: the cuttlefish.
To find out how cuttlefish memory might change over time, researchers first tested their semantic memory, or their recall of facts. Because they couldn’t ask the cephalopods about the first Moon landing, they tested them on something they could recall: where they got their food each day.
The researchers first trained 12- and 24-month-old cuttlefish (Sepia officinalis) to eat each meal of the day at specific, different locations in their tanks. After 3 weeks, they tested their ability to remember those locations. Even when the scientists didn’t deliver a meal, both the young and old animals showed up for breakfast, lunch, and dinner at the right place and time. That suggested they had learned valuable facts—and that their semantic memory was strong.
To test their episodic memory, the researchers added another variable: the animals’ personal preference. They offered the cuttlefish two meals—some rather dull prawn meat and their favorite live grass shrimp—at the same time in two different places in their tanks. Then they offered another two meals at the designated tank locations, but at different times: One hour after the first meal, the cuttlefish got prawn, or, 3 hours later, they were given both prawn and shrimp (see video, above).
After 4 weeks, the researchers let the animals “choose” their meal by swimming to the right part of the tank at the right time. If they swam to the prawn area 1 hour after their first meal, they’d get prawn—but they’d forfeit the shrimp. If they waited 3 hours and headed over to the shrimp area, they got both shrimp and prawn.
Eventually, both the young and old animals learned to wait longer to get their shrimp. That suggested they had formed complex, episodic memories that included not only where and when they had eaten, but also which meal was tastier, researchers report this week in the Proceedings of the Royal Society B. What’s more, the old cuttlefish achieved this recall much faster than the young ones, suggesting their ability to form these memories remains strong with age—and may even improve over time.
“It’s a clever study,” says Daniel Osorio, a neuroscientist at the University of Sussex who wasn’t involved in the work. “It shows that these animals are really good at learning complicated things.” Many studies make big claims about cephalopod intelligence, he says, but few demonstrate it in such a sophisticated way.
That this ability stays strong with age also speaks to “fundamental differences” in nervous system organization in cuttlefish versus mammals, says Marcos Frank, a neuroscientist at Washington State University, Spokane, who was not involved with the research. And it also shows these animals clearly have a sense of time, he says.
Although cuttlefish, like mammals, show signs of deterioration in their brain during aging, their vertical lobe—a structure related to learning and memory—appears to stay intact when they’re old. The fact that cuttlefish mate until very late in life, just a few weeks before dying, might have something to do with this, says Alexandra Schnell, lead author of the study and a comparative psychologist at the University of Cambridge and the Marine Biological Laboratory in Woods Hole, Massachusetts. “They go out with a bang,” she says. She speculates that because the main goal of the cuttlefish during the breeding season is to mate with as many partners as possible, the preservation of episodic memory helps them remember who they mated with—and where and when—so they don’t keep mating with the same individual.
For Schnell, the fact that neither type of memory declined with age in the cuttlefish was surprising. “I thought the older cuttlefish … just wouldn’t perform as well as the younger cuttlefish,” she says.
She and her colleagues are now interested in finding out whether cuttlefish are able to plan for the future, an ability that requires episodic memory. Earlier this year, they showed cuttlefish have good self-control, a sign that they learn to expect things that are coming later. But the jury is still out over whether this “mental time travel” is possible for them.
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