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The thought that counts

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Professor Giorgio Vallortigara and colleagues have investigated whether very young chicks have a spontaneous tendency to map numbers into space, something humans do naturally when they form mental number lines. The procedure used was simple. The three-day old chicks were trained to walk around to behind a panel to find a food reward (one mealworm). On the front of the panel was displayed a certain number of elements such as dots. At each different training trial the appearance but not the number of these elements changed (for example in density or size). Therefore, the only constant relationship to the food and the panel was the number of the elements.


Then the chicks were presented with two panels, one to the left and one to the right, both displaying the same number of elements. These were either smaller or larger than the number used in the training sessions (and this time no reward was available behind either panel). When the numbers displayed on both panels were smaller than the training number the chicks chose to walk behind the left hand side panel.

However, when the pair of numbers was larger they chose to walk behind the panel on the right hand side. This behaviour was completely spontaneous, the chicks reacting to the difference in the number of elements offered. Their choice was made with no differential reward on offer. It is a demonstration of the 'mental number line' which is well known for humans but that is shown here for the first time in a non-human species. Professor Vallortigara's group has previously shown that very young chicks not only have a good understanding of the ordinal properties of numbers (two is more than one but less than three or five) but display a strong preference to 'count' the ordinal position on a line of items from the left to the right. Chicks can carry out simple mental addition and subtraction. When (say) seven imprinted objects were moving behind one or other of two identical opaque screens, the chicks correctly kept track of which screen was hiding more of the imprinted objects and, when free to approach, reliably chose to approach that screen...... a safety in numbers preference which is mirrored in real life. Chicks can also count. When trained to feed from a particular container, say the third, from a row of ten, chicks could correctly identify that container no matter how long the row was. This remained true even when the distances between the containers was increased, proving that they were using ordinal numbers.

Separate groups of chicks were trained to feed from (say) the third, fourth or sixth bowls in a series of bowls. The graph of results for the three groups, shows clearly that the chicks were able to correctly identify the appropriate ordinal bowl number whatever the length of the row.    

The researchers were able to demonstrate that it was the position number (for example bowl four) that the chicks were determining and not simply the distance of the bowl from the chick.  Here, most of the bowls were removed, so that (eg) now the bowl that is fourth in line from the chick is in the distance location of the original bowl ten in the series (see diagram). The overwhelming preference by chicks was for the correct serial position number and not simply the distance location. That is they chose the fourth bowl in line away from them.

Do chicks 'count' from left to right? By taking the above studies one step further (and changing the orientation of  the line of bowls with respect to the chicks) the researchers discovered that chicks have a strong preference for counting from left to right.  The experimental set up and results are summarised below, together with a video clip that demonstrates the set up and findings.    

Do only humans count from left to right?    
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Chicks consistently choose the bowl that is third from the left hand end of the line. While humans also show a preference for left to right counting, this is thought to reflect our cultural and language experiences, which obviously do not apply to chicks. What is more likely is that the chicks' behaviour shows that chicks have the same left eye/right brain preference - brain lateralisation - that humans display. This is very common among vertebrate species and is not something that reflects the supposedly superior cognitive abilities of humans. All of these displays of complex cognition add to the evidence of thoughtful birds in action. The three-day old chicks were making a judgement based on the context of the previous information they had been trained for.


For example, a chick trained for five elements would choose the right panel when shown eight elements (eight is larger than five). A chick trained for twenty elements would choose the left panel when shown eight elements (eight is smaller than twenty). Such choices reflect the left/right arrangement in space of quantities, a phenomenon well-known in humans. This left/right bias in areas like reading and writing can be modified by cultural practices, as shown in studies on young Palestinians and Jews but it does not explain the original bias. Clearly the three-day old chicks reflect this, their having been no chance for any such cultural change, so the bias must be biological in the spontaneous mapping of numbers. This is most likely caused by brain asymmetry, the different functions of the left and right hemispheres of the brain. The right hand side (linked to the left eye) will be in charge when numerical judgements are called for, perhaps accounting for the left/right bias. This bias has also been seen in monkeys, although the numerical task was not the same and there was not any estimation of magnitude involved but only of ordinal position.


Brain asymmetry is a general organising principle in all bilateral creatures, having been observed in all vertebrates (including fish) as well as insects. This asymmetry allows the brain to process information more efficiently. Research findings for chicks by Professor Lesley Rogers showed this to be true. Given the two tasks of looking for food while also checking the sky for predators, those chicks with better brain asymmetry did better, essentially subdividing the task between the two hemispheres simultaneously. It is the right eye (the left brain) that is in charge of the food discrimination task, a process that is begun in the egg when chicks are exposed to light in the last three days before hatching, whereas the left eye (right brain) is in charge of the predator detection task.

This is direct evidence of the possible link between light stimulation, brain asymmetry and number discrimination tasks. There is a survival advantage for all animals that have quantity discrimination and quantity computation abilities, the amount of food available being one example. Chicks have also been shown to have ordinal discrimination (see first, second, third as shown in information boxes above) when it comes to food sources in experiments. Research by Professor Srinivasan of the Queensland Brain Institute has demonstrated ordinal discrimination abilities in bees.

There is now a hypothesis among cognitive scientists that the brain has a general magnitude estimation mechanism that deals with the estimation of quantity, a mechanism that could apply to the different domain of numbers, space and (very likely) time. This linkage in the brain could well explain why these domains are associated in behaviour and so widely observed across species. How such cognitive abilities evolved is not yet known although there is evidence of divided brain function not only across most vertebrates but now, also among various invertebrates.

Such evolutionarily distant species may well have evolved brain asymmetry independently in order to give themselves advantages or it may be related to some genetic mechanism that is common to vertebrates and invertebrates. This can only be determined when the genetic basis of brain asymmetry is understood. It seems that, just like humans, it really is the thought that counts. .

Professor Giorgio Vallortigara was interviewed by Ruby Vincent for A Question of Balance. All images from Professors Vallortigara and Lesley Rodgers. Summary text by Victor Barry, May 2015.

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