Zoology: Numerical abilities may be hardwired in newly-hatched zebrafish

Zebrafish (Dania rerio) larvae can recognise differences between numbers of black bars within 96 hours of hatching, according to a study published in Communications Biology. The findings suggest that numerical abilities may be hardwired in newly-hatched zebrafish, according to the authors.

Previous research has demonstrated numerical abilities in newborn humans and newly hatched guppies and chickens — species whose newly-hatched brains are highly developed. However, prior to this study there has been little understanding of numerical abilities in non-human species that are born at a larval or foetal stage, with underdeveloped brains.

Tyrone Lucon-Xiccato and colleagues tested whether recently hatched zebrafish larvae could recognise differences in the quantities of vertical black bars printed onto the white walls of a tank. Larvae were initially raised 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 observed whether larvae swam towards the larger or smaller quantities of black bars, or remained in the centre of the tank, and timed how long larvae spent in each tank section. In control tests with 120 larvae, the authors repeated the experiment while controlling for differences in the density, surface area, and overall shape of different quantities of bars.

The authors found that 63.1% of larvae 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 (56.9 seconds vs 32.0 seconds). Among 30 larvae given the choice between walls featuring one and four bars, 66.5% preferred the larger quantity of bars. In tests controlling for bar density, surface area, and overall shape formed by different quantities of bars, 63.2% larvae preferred walls featuring larger numbers of bars. Together, these findings indicate that zebrafish larvae can recognise differing quantities of black bars and that they prefer larger quantities.

The authors suggest that the presence of numerical abilities in both newborn humans and newly hatched zebrafish larvae — two distantly related species — could indicate that numerical abilities are a hardwired, ancient feature of the developing vertebrate brain.