For the first time, a non-human animal has been found able to distinguish odd and even numbers – even when they have never seen specific numbers before. The masters of this impressive feat of abstract mathematics are honeybees, equipped with brains containing only a hundred-thousandth as many neurons as our own.
The concepts of "odd" and "even" numbers are so widespread in society that the words themselves are synonymous with "strange" and "balanced" respectively. We save time when counting the number of something by using two, four, six, eight rather than singly – but is this a uniquely human trait?
The study of other species' mathematical capacities has become widespread in recent decades, but when Dr Scarlett Howard of Deakin University searched the literature, she could find no studies exploring whether animals grasp the concept of odd and even numbers. In the journal Frontiers in Ecology and Evolution, Howard and co-authors have rectified that, starting with tiny-brained creatures.
Researchers placed cards with different numbers of shapes next to bee feeders.
One set of bees was trained to associate 2, 4, 6, and 8 shapes with sugared water and 1, 3, 5, and 7 with quinine, which bees hate. A second group was taught the reverse. Sadly, Howard told IFLScience these sets of insects were not respectively named “two bee” and “not two bee”.
Once the bees had demonstrated their capacity to choose the appropriate number most of the time, they were confronted with a choice between feeders marked with 11 and 12 shapes respectively.
The bees had never been shown cards with either number. Yet those who were taught to associate even numbers with reward predominately flew to the 12-shape feeder, while those taught to prefer odd numbers chose the one marked with 11 shapes. Neither group managed the tasks with perfect reliability, but success rates of around 70 percent were clearly statistically significant.
Howard told IFLScience the team does not know how the bees are doing this. Perhaps, like a dinner party host, they are pairing all the shapes up and noticing if there is one leftover. Alternatively, they might be dividing by the number two, or simply counting and categorizing the numbers, despite previous questions of whether bees can count past four.
It's also not clear if there is any evolutionary benefit to the skill.
“We thought maybe it is to do with numbers of petals,” Howard told IFLScience. Odd and even petal numbers might be one more way bees can distinguish flowers (along with color and smell) that help them know, or communicate to hive mates, the richest food sources. Alternatively, these may be the first bees ever to use this capacity, reflecting their rapid learning capacity.
The findings raise enough questions to populate a hive. Howard told IFLScience she hopes the work will inspire other researchers to test similar capacities in vertebrates. She added; “I'd love to explore how high [bees] can go.” Seeing whether they can distinguish which of 40 and 41 is even, for example, might shed light on how they are categorizing smaller numbers.
Inspired by the bees' capacity with just 960,000 neurons, the authors built a neural network capable of distinguishing even and odd numbers with just five neurons. They hope this may lead to efficient processing methods that require little computing power.
Like previous research suggesting bees can understand zero as a number the most important aspect of the work could be to shed light on a question Howard summarizes as; “Did humans discover mathematics, or did we invent it?”
A particularly puzzling observation is that the bees taught to favor odd numbers learned faster than those rewarded with even numbers. This contrasts with humans, who have been shown to be faster at classifying numbers as even than odd. Several explanations are considered in the paper, but at this stage everything is speculative.