Cuttlefish Pass the Marshmallow Test in Study

A 2021 test of cephalopod intelligence offered another reminder that humans should be careful not to underestimate the minds of other animals.

Cuttlefish took part in an adapted version of the marshmallow test, with results suggesting that more may be happening inside their unusual little brains than previously realised. Their capacity to learn and adapt, researchers said, could have evolved to give them an advantage in the unforgiving marine world, where animals must eat or be eaten.

How the marshmallow test measures self-control

The marshmallow test, also called the Stanford marshmallow experiment, has a simple premise. A child is left in a room with a marshmallow and told that, if they resist eating it for 15 minutes, they will receive a second one and can eat both.

Delaying gratification in this way demonstrates cognitive abilities such as planning for the future. The original experiment was designed to examine the development of human cognition, specifically the age at which a person becomes capable of waiting for a better outcome.

Its simplicity makes it possible to adapt the test for animals. An animal cannot be told that waiting will bring a better reward, but it can be trained to understand that better food will arrive if it does not immediately eat what is in front of it.

Some primates have shown that they can delay gratification, as have dogs, although not consistently. Corvids have passed versions of the marshmallow test too.

In 2020, cuttlefish passed an adapted test as well. Scientists found that common cuttlefish (Sepia officinalis) could resist a morning meal of crab meat after learning that a much preferred dinner of shrimp would follow.

However, a research team led by University of Cambridge behavioural ecologist Alexandra Schnell noted that it was hard to establish whether self-control was responsible for that change in foraging behaviour in response to prey availability.

The cuttlefish marshmallow test

To investigate further, the team devised another test involving six common cuttlefish. Each was placed in a special tank containing two enclosed chambers with transparent doors, allowing the animal to see the food inside. One chamber held a less desirable piece of raw king prawn; the other contained a much more tempting live grass shrimp.

The cuttlefish had been trained to recognise symbols on the doors. A circle indicated that a door would open immediately, while a triangle meant it would open after a delay of between 10 and 130 seconds. A square, used only in the control condition, signalled that the door would remain shut indefinitely.

Under the test condition, the prawn was available behind the door that opened immediately, but the live shrimp could only be reached after a wait. If a cuttlefish chose the prawn, the researchers removed the shrimp straight away.

In the control condition, by contrast, the shrimp was kept out of reach behind the door marked with a square, which did not open.

Every cuttlefish in the test condition chose to wait for its preferred food, the live shrimp. They did not wait in the control condition, when the shrimp was inaccessible.

“Cuttlefish in the present study were all able to wait for the better reward and tolerated delays for up to 50-130 seconds, which is comparable to what we see in large-brained vertebrates such as chimpanzees, crows, and parrots,” Schnell said in 2021.

Learning, foraging and delayed gratification

The second part of the experiment assessed how well the six cuttlefish could learn. Each was presented with two visual cues: one grey square and one white square.

When an animal approached one square, the other was removed from the tank. A “correct” choice earned it a snack. Once the cuttlefish had learnt to link one square with the reward, the researchers reversed the cues, making the other square the one associated with food.

The cuttlefish that adapted most quickly to the switch were also those that could wait longest for the shrimp reward.

The findings suggest that cuttlefish can exercise self-control, although the reason remains uncertain. In parrots, primates and corvids, delayed gratification has been associated with tool use, which calls for planning ahead; food caching; and social competence, because prosocial behaviour, such as ensuring everyone has food, benefits social species.

As far as scientists know, cuttlefish neither use tools nor cache food, and they are not particularly social. The researchers instead think their capacity to delay gratification could be linked to how they forage.

“Cuttlefish spend most of their time camouflaging, sitting and waiting, punctuated by brief periods of foraging,” Schnell said at the time.

“They break camouflage when they forage, so they are exposed to every predator in the ocean that wants to eat them. We speculate that delayed gratification may have evolved as a byproduct of this, so the cuttlefish can optimise foraging by waiting to choose better quality food.”

It is a fascinating illustration of how very different species, living very different lives, can develop similar behaviours and cognitive abilities.

Researchers have also found evidence of “episodic-like memory” in cuttlefish, and in 2024 scientists reported the first observation of the animal forming false memories.

The team noted that future research should investigate whether cuttlefish really are capable of planning for the future. Their study was published in Proceedings of the Royal Society B.

A version of this article was first published in March 2021.