Cuttlefish Pass the Marshmallow Test in a Remarkable Study

A 2021 test of cephalopod intelligence offered a reminder to humans that we should not underestimate the mental abilities of animals.

Researchers gave cuttlefish a revised version of the marshmallow test, and the outcome suggests that more may be happening inside their unusual little brains than previously understood.

The researchers said the animals' capacity to learn and adapt might have evolved to give them an advantage in the ruthless eat-or-be-eaten marine environment they inhabit.

How the marshmallow test works

The marshmallow test, also known as the Stanford marshmallow experiment, has a relatively simple design.

A child is left in a room with a marshmallow and told that, if they can resist eating it for 15 minutes, they will receive a second one and can eat both.

The ability to postpone gratification demonstrates cognitive skills such as planning for the future. The original experiment was designed to investigate the development of human cognition, specifically the age at which a person becomes capable of waiting for a better outcome later.

Its simplicity makes the test adaptable for animals. Although an animal cannot be told it will receive a better reward for waiting, 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 can delay gratification, as can dogs, though dogs do so inconsistently. Corvids have also passed the marshmallow test.

Cuttlefish and the marshmallow test

In 2020, cuttlefish passed a version of the test too. Scientists found that common cuttlefish (Sepia officinalis) could resist a morning meal of crab meat after learning that a much more appealing dinner of shrimp would follow.

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

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

The cuttlefish had been trained to recognise symbols on the doors. A circle signalled 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, indicated that the door would remain shut indefinitely.

During the test condition, the prawn was placed behind the door that opened straight away, whereas the live shrimp could only be reached after a delay. If a cuttlefish chose the prawn, the researchers immediately took away the shrimp.

In the control condition, by contrast, the shrimp stayed 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, the researchers found. In the control condition, where the shrimp was inaccessible, the animals did not bother waiting.

“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, self-control and foraging

The researchers also tested how well the six cuttlefish could learn by presenting them with two visual cues: a grey square and a white square.

When an animal approached one square, the other was taken out of the tank. A “correct” choice earned the cuttlefish a snack.

After the animals had learnt to associate one square with food, the researchers reversed the cues, making the other square the signal for a reward.

Notably, the cuttlefish that adapted most quickly to the switch were also those able to 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 requires planning ahead; food caching, for obvious reasons; and social competence, because prosocial behaviour, such as ensuring everyone has food, benefits social species.

As far as we know, cuttlefish neither use tools nor store food, and they are not particularly social. Instead, the researchers believe their capacity to delay gratification may be connected 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 ways of life in very different species can produce similar behaviours and cognitive abilities.

The team noted that future research should investigate whether cuttlefish are indeed capable of planning for the future.

The research was published in Proceedings of the Royal Society B.

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