To test cephalopod number sense, Yang and Chiao conducted a series of prey quantity preference tests using juvenile cuttlefish that were reared from eggs collected in the waters of Keelung and Nanilao, Taiwan. The researchers presented the young cuttlefish with various types of shrimp: large, dead, alive and small. The test subjects had to select between one of two options.
The cuttlefish generally went for larger quantities, and could even be seen mulling over similar choices.
Chiao told Discovery News that he and his colleague "think that cuttlefish think about their choices for a while before making a decision, and this delayed time is task difficulty dependent."
Over time, the researchers realized that the cuttlefish could distinguish number differences above the ratio of 1.25. This means that their number sense is at least equivalent to that of young humans and other primates.
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As for why 1.25 is significant, the researchers explained that the figure is just a rough estimate, but that differences below this amount would not matter much to cephalopods.
"Although we are not very sure, we think that number sense could benefit cuttlefish, not only in prey choice, but also in mate choice and navigation," Chiao said.
The scientists also determined that when the cuttlefish were particularly hungry, they would choose "fast food," such as one large shrimp versus multiple smaller ones. If they were not as hungry, they would eat the smaller shrimp.
The findings indicate that all animals with such number sense, including humans, integrate both internal and external information when making quantity and quality decisions about food. Previous research has shown that hunger level alters quantity-related decision-making in humans.
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It's believed that many animals have some level of number sense, but it could be particularly advanced in cephalopods. There are several reasons for this.