ANIMAL COGNITION
Number-space mapping in the
newborn chick resembles humans’
mental number line
Rosa Rugani,1,2 Giorgio Vallortigara,2 Konstantinos Priftis,1 Lucia Regolin1
Humans represent numbers along a mental number line (MNL), where smaller values are
located on the left and larger on the right. The origin of the MNL and its connections
with cultural experience are unclear: Pre-verbal infants and nonhuman species master a
variety of numerical abilities, supporting the existence of evolutionary ancient precursor
systems. In our experiments, 3-day-old domestic chicks, once familiarized with a target
number (5), spontaneously associated a smaller number (2) with the left space and a
larger number (8) with the right space. The same number (8), though, was associated with
the left space when the target number was 20. Similarly to humans, chicks associate
smaller numbers with the left space and larger numbers with the right space.
Number knowledge and processing are fun- damental to everyday life. There is now considerable empirical evidence that num- bers may be represented along a contin- uous, left-to-right–oriented, mental number
line (MNL) (1); however, the origin of this orientation is debated. In humans (2) and nonhuman
animals (3, 4), numerical judgments become
easier as the difference between the numbers
increases (the distance effect) and harder as the
magnitude of numbers increases (the size effect).
Interspecific similarities suggest a continuous
and analogical nonverbal representation of nu-
merical magnitude (3). This indicates that nu-
merical competence did not emerge de novo in
linguistic humans but was probably built on a
precursor nonverbal number system (1, 5).
The size and distance effects, though, are not
informative about the origin of the orientation
of the MNL. Indeed, the MNL has been, up to
now, demonstrated solely among humans (6–8),
where its orientation may be influenced by cultural factors, such as reading direction. People
primarily educated in Arabic show an inverted
spatial-numerical association of response codes
(SNARC) (9) effect (10), whereas people with mixed
reading habits (such as Israelis) show no SNARC
at all (11).
It remains unclear whether the MNL orientation is simply modulated or entirely produced
by educational factors. Seven-month-old infants
prefer increasing (e.g., 1-2-3) to decreasing (e.g.,
3-2-1) magnitudes displayed from left to right,
(12), showing that spatial-numerical association
does exist before mathematics and linguistic
education. A tendency to count from left to right
has also been found in domestic chicks (13), adult
Clark’s nutcrackers (14), and adult Rhesus macaques
(15). In these studies, animals were trained to
identify a target element in a sagitally oriented
series of identical elements. When required to
repeat the task with an identical series of elements rotated by 90°, animals identified as correct the target positioned from the left end (14).
However, this left-sided preference could depend on a general bias in the allocation of spatial attention (16). Both humans (17) and birds
(18, 19) primarily attend to objects in the left
side of space, a phenomenon termed “
pseudoneglect.” When a different paradigm was used,
adult chimpanzees were trained to touch in
ascendant order Arabic numerals (1 to 9) randomly displayed on a computer screen. At testing, they were presented with only two numerals
(1 and 9) displayed horizontally, one on the left
and the other on the right; chimpanzees responded faster to the left-right (1-9) than to the
right-left condition (9-1) (20). However, these
results are not conclusive concerning the spontaneous mapping of magnitudes onto space, because apes required intensive sequential learning
during training.
The spatial arrangement of numbers is highly flexible in humans: A fundamental characteristic of the human MNL is its relativity. In the
1-9 range, for instance, responding to 9 is faster
when responses are executed on the right; but in
534 30 JANUARY 2015 • VOL 347 ISSUE 6221 sciencemag.org SCIENCE
1Department of General Psychology, University of Padova,
Padova, Italy. 2Center for Mind/Brain Sciences, University of
Trento, Rovereto (Trento), Italy.
*Corresponding author. E-mail: rosa.rugani@unipd.it
Fig. 1. Experimental settings of experiment 1.
Chicks were trained to circumnavigate a panel, located in the center of the apparatus, depicting
5 identical elements (i.e., the target number). (A) In
all experiments, we used 20 different training stimuli,
differing in the spatial disposition of the elements.
The training finished whenever the chick circumnavigated the screen and reached the food reward
20 consecutive times. After training, each chick underwent two tests in random order: a small number
test (2 versus 2) (B) and a large number test (8 versus 8) (C). In all experiments, each test consisted
of five nonreinforced trials (a novel pair of stimuli
was employed on each trial). On each test trial, we
scored the panel first inspected by the chick and
computed the mean percentage of choices for the
left panel.
