Zebrafish seem to be able to count when they are just four days old

Just a few days after hatching, zebrafish larvae can discriminate between bigger and smaller numbers, suggesting they may have an innate numerical ability.

Zebrafish larvae may possess some primitive counting ability just days after hatching.

Zebrafish appear to have numerical abilities at a very early age
blickwinkel / Alamy Stock Photo

Several animals, including honeybees and salamanders, have been shown to have some form of numerical skill. For example, lab experiments suggest rhesus monkeys may be able to count up to six.

Numerical ability has also been seen in very young humans, with one study showing that number sense in 6-month-old babies is linked to better scores in maths tests three years later.

Tyrone Lucon-Xiccato at the University of Ferrara, Italy, wanted to find out if this ability could be found in a vertebrate at an even earlier age.

“We were interested in studying the development of numerical abilities,” he says. “We especially wanted to focus on the really early undeveloped brain to see if this numerical capacity is something that is somehow intrinsic in the brain.”

To do this, Lucon-Xiccato and his colleagues focused their efforts on zebrafish (Danio rerio), which hatch from eggs just three days after fertilisation. “A newborn zebrafish is nothing more than an embryo,” says Lucon-Xiccato.

The team bred zebrafish larvae in a fish tank with black vertical bars painted on all sides in a regular pattern. The larvae were allowed to get used to the vertical bars for four days after hatching.

On the fourth day, the larvae were moved into a new tank in which two sides had differing numbers of vertical bars painted on to them, ranging from one to four; the other sides were blank.

For the first 6 minutes that the larvae spent in the new tank, the researchers measured the amount of time they spent closer to one side or the other.

Overall, the larvae spent close to twice the amount of time nearer the side with the larger number of bars compared to the other side.

“We were expecting them to select the area with more bars because it is more similar to their early environment,” says Lucon-Xiccato.

However, numerical ability isn’t the only factor that could explain these results. For example, the fish could be distinguishing between the two sides by observing the bars’ surface area or how dark they are overall.

To account for these factors, the team repeated the experiment varying these conditions and found that the larvae still tended to swim closer to the side with more bars.

Lucon-Xiccato says these findings don’t prove that numerical ability is hardwired in zebrafish but gets very close to it. “We believe that at least some of this ability or some of the circuits might have been there since the origin of the vertebrate,” he says.

“We have known that fish have numeracy skills for a rather long time,” says Culum Brown at Macquarie University in Sydney, Australia. But by testing very young fish, this study suggests that this ability could be present at hatching, he says. “The findings strongly suggest that fish have an innate capacity for numeracy skills,” he says.

Journal reference:

Communications BiologyDOI: 10.1038/s42003-023-04595-7

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