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Forget general AI, apparently zebrafish larvae can count

Numerical abilities could be a hardwired, ancient feature of the developing vertebrate brain, study suggests

Researchers in Italy have discovered newborn zebrafish possess the ability to count, suggesting numeracy may be hard-wired into the vertebrate brain.

University of Ferrara behavioural biology assistant professor Tyrone Lucon-Xiccato and his colleagues showed larvae of zebrafish (Danio rerio), a freshwater fish from the minnow family, can recognize differences between numbers of black bars within 96 hours of hatching.

The researchers started their investigation after discovering the nascent creatures, usually around 4mm in length, preferred the tank walls featuring larger quantities of black bars and spent almost twice as much time near larger quantities of bars than smaller quantities.

Early numerical abilities have been discovered by scientists in a number of other species, including humans, guppies and chickens, whose newborns have relatively well-developed brains. But the study published in Nature journal Communications Biology this week is the first to show the apparent ability to count during a creature’s larval or foetal stage, when they have underdeveloped brains.


Adult zebrafish, which are usually 1.8cm to just under 4cm long in the wild

“We demonstrated that zebrafish larvae exposed to vertical bars at birth acquired an attraction for bar stimuli and we developed a numerical discrimination task based on this preference. When tested with a series of discriminations of increasing difficulty, zebrafish larvae reliably selected the greater numerosity,” the study said.

Lucon-Xiccato and colleagues initially raised larvae in tanks with walls covered in a pattern of vertical black bars. After 96 hours, 180 individual larvae were placed in the center of a separate tank, which featured two white walls with different quantities of up to four black bars printed onto them. The researchers studied the larvae’s preference for the larger or smaller number of black bars or stuck to the middle of the tank. They timed how long larvae spent in each tank section. Control tests accounted for the different densities, surface areas, and shapes of the bars.

The study showed 63.1 percent of larvae preferred the tank walls with larger quantities of black bars, a trend sustained despite varying bar density, surface area, and overall shape in control tests. The researchers concluded they had discovered the larvae could recognise differing quantities of black bars and wanted to be near the larger quantities.


Zebrafish larvae under the microscope

The finding is all the more remarkable given the relatively poorly developed nervous system of the zebrafish larvae at birth. They only display simple responses to stimuli such as pain, for example.

“Other behaviors, such as feeding and efficient swimming, will appear after the sixth-day post-fertilization and more complex behaviour such as social interactions occur only three weeks later. Given their peculiar developmental mode, the most likely explanation for the presence of numerical abilities in zebrafish larvae is an innate biological predisposition.

"In line with the fitness advantages of processing numerical information, such hardwired numerical abilities might prepare individual zebrafish for important behaviour but only in later life,” the study said.

Whatever the evolutionary driver for numerical ability, the study of zebrafish larvae shows it “might be a hallmark feature of vertebrates’ brains,” the scientists concluded. ®

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