Are fish born with the ability to count, or do they learn to do it? This is the question that researchers from the University of Padova sought to answer in a study published in a recent issue of the journal PLoS ONE.
Angelo Bisazza, Laura Piffer, Giovanna Serena and Christian Agrillo investigated the development of number discrimination in guppies (Poecilia reticulata).
The authors carried out a series of experiments using the spontaneous tendency of guppies to join a larger shoal to study the limits of number discrimination in newborn and juvenile fish.
In the first experiment, the authors divided a tank into three using transparent partitions, and placed shoals of newborn guppies of different sizes into each end compartment while placing a newborn guppy into the central compartment and recording its preference for joining either of the shoals presented.
Twenty fish were tested in a two versus three comparison, 20 in four versus eight and 20 in four versus 12. The authors additionally tested guppies in comparisons between one number in the small quantity range and one large number: twenty in the two versus five and twenty in three versus eight.
The authors found that the fish would choose the larger shoal when the choice in numbers was in the range of small quantities (two versus three), but not when the choice was between two larger quantities (four versus eight, four versus 12). This suggested that the guppies had an innate capability in discriminating small quantities while the capacity to discriminate large quantities should emerge later in development.
However, the fry were able to choose the larger shoal when they had to discriminate between one number in the small quantity range from one in the large range (two versus five and three versus eight).
In the second experiment, the authors investigated the exact limit of the newborns’ ability to discriminate between small shoals differing by one unit by presenting them with the following choices: one versus two, two versus three, three versus four, four versus five, and five versus six (testing 20 fish per comparison).
The experiments showed that newborn guppies could discriminate between sets differing by one unit up to four, beyond which they were unable to do so.
This was similar to results of previous studies, which showed that adult fish could count up to four.
The authors sought to examine the development of the ability to discriminate between larger numbers in guppies by testing 144 fish at three different ages (1-, 20- or 40-day-old) in the third experiment.
Furthermore, to assess the role of learning developing the ability to count, the authors reared half the fish in large groups (with the possibility of seeing shoals of variable numbers), and the other half in pairs (where the fish were never exposed to groups of large numbers).
By testing the ability of the fish to discriminate between groups of four and eight, the authors found that the fish raised in pairs were able to discriminate between larger numbers just as well as the fish raised in large groups, but were slower to pick up this ability (the fish raised in pairs were able to discriminate between four and eight only at 40 days old, whereas the fish reared in groups were already able to do so at 20 days old).
Lastly, the authors examined the true counting ability of the newborn and young guppies by eliminating the non-numerical cues associated with the previous experiments (e.g. total area occupied by the fish). This was done by testing the ability of 48 newborn guppies to discriminate between two and three, but without the ability to see all of the individuals in the stimulus groups at once (the tank was set up with a series of screens that allowed the fish being tested to see only one fish at a time).
A similar experiment was conducted using 36 20-day old guppies reared in groups and 36 40-day old guppies reared in pairs to test for the ability to discriminate between four and eight.
The authors found that the fish consistently chose the larger stimulus group, even though they were able to see only one fish at a time.
The authors concluded from their series of experiments that guppies have an innate ability to discriminate small numbers that is displayed immediately at birth, while discrimination of large numbers emerges later as a result of both maturation and social experience. This suggests that fish, like primates, might have separate systems for small and large number representation.
For more information, see the paper: Bisazza A, L Piffer, G Serena and C Agrillo (2010) Ontogeny of numerical abilities in fish. PLoS ONE 5, e15516. doi:10.1371/journal.pone.0015516