Counting Cuttlefish

It turns out that children and cuttlefish have a significant characteristic in common: the ability to count to five.

According to new research published this summer by Tsang-I Yang and Chuan-Chin Chiao[i] at the National Tsing Hua University, cuttlefish are capable of making some pretty impressive foraging decisions.  As a relative of squid and octopus, cuttlefish boast one of the largest invertebrate brain-to-body-size ratios[ii]— but you still might not suspect that this color-changing cephalopod is smarter than your dog (or, at least, better at discriminating quantities).[iii]

[1] Two juvenile cuttlefish at the Georgia Aquarium.

While adult cuttlefish usually hunt fish and crabs for dinner, juveniles—such as the ones used in this study—eat primarily shrimp.  Building off of this fundamental preference, the authors were curious as to whether the animals would choose a group of shrimp to hunt based on the number of prey items in it and, if so, to what extent they could discriminate between groups of different amounts.

Yang and Chiao started by showing that cuttlefish show significant preference when presented with two very different groups (one versus five shrimp).  They then made the task substantially harder by reducing the numerical difference between the groups to a single prey item.  Even so, the cuttlefish still chose the larger group a significant portion of the time, up to the ultimate decision of four versus five shrimp.  Additionally, as the groups got larger, the ratios between them increased (two versus three, three versus four, and four versus five) and the animal took longer to decide which set to go for, suggesting that the cuttlefish was actually processing the difference and making an intentional foraging decision.

Furthermore, this study’s results support the hypothesis that cuttlefish use the same cognitive mechanism to discriminate between quantities as humans (specifically, analogue magnitude: numerical perception based on the ratio between two quantities [iv]).  One study even suggested that rhesus macaques, an old world monkey species, are only able to distinguish groups up to three versus four food items, while the cuttlefish still showed significant discriminatory abilities through four versus five shrimp. [v]  Not bad for a mollusk!

However, cuttlefish won’t always go for the larger number of prey items.  Yang and Chiao also showed that the cephalopods almost always choose one live over two dead shrimp—talk about a picky eater!  Furthermore, just like humans make riskier decisions when hungry,[vi] some of the foraging choices a cuttlefish makes are state-dependent: given a choice between one large or two small shrimp, the cuttlefish commonly went for the single “jackpot” shrimp when food deprived, but when satiated it typically preferred the two smaller, easier-to-catch ones instead.

[2] Once they decide which prey item to go for, cuttlefish hunt by protruding their cuttlebone (yes, that is the name of that thing) and using it to grasp and suction their victim.  The author assumes no liability if this visual gives you nightmares.

While we may be distantly related to cuttlefish on the phylogenetic tree (compare molluscs and chordates), their incredible number sense and decision-making capabilities have revealed fascinating parallels between humans and cephalopods.

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References:

[i] Yang, Tsang-I., and Chuan-Chin Chiao. “Number Sense and State-Dependent Valuation in Cuttlefish.” Proc. R. Soc. B 283, no. 1837 (August 31, 2016): 20161379. doi:10.1098/rspb.2016.1379.
[ii] Wells, Martin John. Brain and Behaviour in Cephalopods. Stanford University Press, 1962.
[iii] Ward, Camille, and Barbara B. Smuts. “Quantity-Based Judgments in the Domestic Dog (Canis Lupus Familiaris).” Animal Cognition 10, no. 1 (August 29, 2006): 71–80. doi:10.1007/s10071-006-0042-7.
[iv] Brannon, Elizabeth M. “The representation of numerical magnitude.” Current Opinions in Neurobiology 16, no. 2 (March 20, 2006): 222-229. doi:10.1016/j.conb.2006.03.002.
[v] Hauser, M D, S Carey, and L B Hauser. “Spontaneous Number Representation in Semi-Free-Ranging Rhesus Monkeys.” Proceedings of the Royal Society B: Biological Sciences 267, no. 1445 (April 22, 2000): 829–33. doi:10.1098/rspb.2000.1078.
[vi] Symmonds, Mkael, Julian J. Emmanuel, Megan E. Drew, Rachel L. Batterham, and Raymond J. Dolan. “Metabolic State Alters Economic Decision Making under Risk in Humans.” PLOS ONE 5, no. 6 (June 16, 2010): e11090. doi:10.1371/journal.pone.0011090.

Media Credits:

[1] Photo by the author.

[2] Photo by Klaus Stiefel.  https://www.flickr.com/photos/pacificklaus/15330640186/in/photolist-mBPc9n-qectGp-poKpor-88T7Ef-pcG3r1-pmHxdw-mFbPig-oGvvPL-oj2Vd6