Picture writing the numbers 1 to 5 in a horizontal line, from smallest to largest. Where did you put 1? If you placed 1 on the left and 5 on the right, you share this preference with most humans.
Authors
- Scarlett Howard
Lecturer, School of Biological Sciences, Monash University
- Adrian Dyer
Associate Professor, Department of Physiology, Monash University
Humans are not alone in this preference. Some other primates , and even some birds , also order small-to-large quantities from left to right. Although, some animals do prefer to order quantities from right to left .
This is known as the mental number line, and it shows how brains typically organise information. But why do our brains do this?
To investigate how and why brains order numbers, it's sometimes useful to step back in time. How did a common ancestor of humans and insects order information? To find out, we can compare the results of humans and bees: we last shared a common ancestor more than 600 million years ago.
Two recent studies on bees have revealed a lot about how tiny brains order numbers.
One study , conducted by a team in Europe, showed that bees prefer to order lower numbers on the left and higher numbers on the right, just like many humans. Our new study , led by Jung-Chun (Zaza) Kuo and her supervisory team, has explored how numbers and space interact in the bee brain.
'Number' and 'space'
As humans, we link the concepts of "space" and "number" . This means there is a logic to how we order numbers (typically from left to right in ascending order: 1, 2, 3, 4 … and so on).
Studies have shown humans may also have a vertical - bottom to top - preference when processing numerical information efficiently .
There may also be educational influences, especially due to language and writing direction. Some languages, like English, write from left to right. Others, such as Japanese, Chinese, Korean, Hebrew or Arabic, can be written in other directions. Writing direction can influence how we prefer to order numbers.
Meanwhile, honeybees are efficient learners and show evidence of being tiny mathematicians. In past research, they have been shown to add and subtract , understand the concept of zero , use symbols to represent numbers , order quantities , categorise numbers by odd or even , and show evidence of linking numbers to spatial information like size .
The competency bees show around numbers makes them an ideal animal to look at how number and space interact in a miniature brain.
How did we test bees in our study?
We gave freely flying bees sugar water for visiting an image of three circles printed on a card: this was our "reference number". The card was hung in the centre of a large circular screen, with a drop of sugar water on a platform underneath it.
As the bees repeatedly visited the reference number, they learned an association between the number three, the centre of the circular screen, and a reward. In between visits, bees took the sugar water back to their hive to be made into honey.
After bees had learned to associate number (three) and space (middle) with a reward, we tested them on numbers higher and lower than three, to see if they had linked space and number.
We showed bees images of a higher number (four circles) and a lower number (two circles). Two identical images of four shapes were shown simultaneously on the left and right sides of the screen. If bees preferred the larger number on the right, they would fly to the quantity of four presented on the right more than when four was presented on the left.
We did the same for the smaller number of two shapes. If bees preferred four circles on the right and two circles on the left, that would reveal they have a left-to-right mental number line, like humans.
We also tested if bees had a preference to order numbers upwards or downwards, and found no preference for linking space and number vertically. However, bees did prefer options that were towards the bottom of the circular screen.
So, how does the bee mental number line work?
The study by the European team found bees have a consistent left-to-right mental number line. This means they prefer to order lower numbers on the left and higher numbers on the right.
Our study has confirmed bees prefer to order higher numbers on the right. But we also found bees preferred to visit the right side of the screen. The preference of bees to order numbers from left-to-right and to visit the right side of the circular screen interacted in an intriguing way.
The bees in our study showed a preference for higher numbers on the right, but not for lower numbers on the left. This could be because the right-side bias we observed cancelled out the preference for smaller numbers on the left.
Taken together, the findings of both studies confirm that bees do possess a left-to-right mental number line and also that they have a bias towards the right side of their visual space.
Our team suggests such biases - for example, how most humans are right-handed - may be an important part of how brains make sense of ordering information in the world.
The birds and the bees (and the apes)
By looking at the behaviours of animals, we can sometimes learn more about ourselves.
These two recent studies on bees show there is a complex interaction between ordering numbers and how spatial relationships are processed by an insect brain.
We now know that the preference to order numbers from left to right exists in several very different animal groups: insects, birds and apes. Perhaps evolution has landed on this preference as an advantageous way to process complex information.
Scarlett Howard has received funding from the Australian Research Council and Air Force Office of Scientific Research.
Adrian Dyer receives funding from the Alexander von Humboldt Foundation, the Air Force Office of Scientific Research and the Australian Research Council.
