A Cephalopod Has Passed a Cognitive Test Designed For Human Children
Cuttlefish
have been put to a new version of the marshmallow test, and the results appear
to demonstrate that there's more going on in their strange little brains than
we knew.
Their
ability to learn and adapt, the researchers said, could have evolved to give
cuttlefish an edge in the cutthroat eat-or-be-eaten marine world they live in.
The
marshmallow test, or Stanford
marshmallow experiment, is pretty straightforward. A child is placed
in a room with a marshmallow. They are told if they can manage not to eat the
marshmallow for 15 minutes, they'll get a second marshmallow, and be allowed to
eat both.
This
ability to delay gratification demonstrates cognitive abilities such as future
planning, and it was originally conducted to study how human cognition
develops; specifically, at what age a human is smart enough to delay
gratification if it means a better outcome later.
Because
it's so simple, it can be adjusted for animals. Obviously you can't tell an
animal they'll get a better reward if they wait, but you can train them to
understand that better food is coming if they don't eat the
food in front of them straight away.
Some primates can
delay gratification, along with dogs, albeit
inconsistently. Corvids (e.g. crows), too, have passed the
marshmallow test.
Last year, cuttlefish also passed a version of the marshmallow test. Scientists showed that common cuttlefish (Sepia officinalis) can refrain from eating a meal of crab meat in the morning once they have learnt dinner will be something they like much better - shrimp.
As
a team of researchers led by behavioural ecologist Alexandra Schnell of the
University of Cambridge point out in a new
paper, however, in this case it's difficult to determine whether
this change in foraging behaviour in response to prey availability was also
being governed by an ability to exert self-control.
So
they designed another test, for six common cuttlefish. The cuttlefish were
placed in a special tank with two enclosed chambers that had transparent doors
so the animals could see inside. In the chambers were snacks - a less-preferred
piece of raw king prawn in one, and a much more enticing live grass shrimp in
the other.
The
doors also had symbols on them that the cuttlefish had been trained to
recognise. A circle meant the door would open straight away. A triangle meant
the door would open after a time interval between 10 and 130 seconds. And a
square, used only in the control condition, meant the door stayed closed
indefinitely.
In
the test condition, the prawn was placed behind the open door, while the live
shrimp was only accessible after a delay. If the cuttlefish went for the prawn,
the shrimp was immediately removed.
Meanwhile,
in the control group, the shrimp remained inaccessible behind the square-symbol
door that wouldn't open.
The
researchers found that all of the cuttlefish in the test condition decided to
wait for their preferred food (the live shrimp), but didn't bother to do so in
the control group, where they couldn't access it.
"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.
The
other part of the experiment was to test how good the six cuttlefish were at
learning. They were shown two different visual cues, a grey square and a white
one. When they approached one, the other would be removed from the tank; if
they made the "correct" choice, they would be rewarded with a snack.
Once
they had learnt to associate a square with a reward, the researchers switched
the cues, so that the other square now became the reward cue. Interestingly,
the cuttlefish that learnt to adapt to this change the quickest were also the
cuttlefish that were able to wait longer for the shrimp reward.
That
seems like cuttlefish can exert self control, all right, but what's not clear
is why. In species such as parrots, primates, and corvids, delayed
gratification has been linked to factors such as tool use (because it
requires planning
ahead), food caching (for
obvious reasons) and social
competence (because prosocial behaviour -
such as making sure everyone has food - benefits social species).
Cuttlefish,
as far as we know, don't use tools or cache food, nor are they especially
social. The researchers think this ability to delay gratification may instead
have something to do with the way cuttlefish forage for their food.
"Cuttlefish
spend most of their time camouflaging, sitting and waiting, punctuated by brief
periods of foraging," Schnell said.
"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's
a fascinating example of how very different lifestyles in very different
species can result in similar behaviours and cognitive abilities. Future
research should, the team noted, try to determine if indeed cuttlefish are
capable of planning for the future.
The
team's research has been published in Proceedings
of the Royal Society B.