When it comes to numbers, some things are innate, like our ability to judge how many items we see.
This ability is present even in newborn babies, who can tell the difference between the same and different amounts of objects, but it's not just a human skill.
Such sensitivity to numbers that doesn't involve abstract thought or learning is called numerosity, and has been seen across the animal kingdom, from monkeys, to fish and bees.
With the help of some canine friends, a research team from Emory University has now found that the parts of our brains involved in numerosity have been preserved at least since dogs and humans took different evolutionary paths some 90 to 100 million years ago.
"We went right to the source, observing the dogs' brains, to get a direct understanding of what their neurons were doing when the dogs viewed varying quantities of dots," says cognitive psychologist Lauren Aulet. "That allowed us to bypass the weaknesses of previous behavioural studies of dogs and some other species."
The researchers trained 11 very good boys and girls, including Bhubo, Daisy and Truffles, to enter and sit motionless in a functional MRI. But they gave them no clues on what to do once there. This is a key point of difference to past animal numerosity studies, where subjects were trained and rewarded for performing number tasks.
They then observed how the dogs' brains responded to seeing altering numbers of dots on a screen.
Eight of 11 of the dogs' parietotemporal regions lit up more intensely when the ratio of changing dots on the screen was greater, for example 2:10 vs 4:8.
But their brains didn't respond in the same way when only the size and positions of the dots changed, while the ratio remained the same.
This shows the dogs were responding specifically to a difference in quantity. The brain regions that activated where similar to those seen when primates - including humans - process amounts, which suggests our approximate number system is a conserved neural mechanism.
Example of the dotted screens the dogs saw. (Aulet et al., Biology Letters, 2019)
"Part of the reason that we are able to do calculus and algebra is because we have this fundamental ability for numerosity that we share with other animals," said Aulet.
Past research has shown dogs can count up to five and have a basic understanding of simple maths. For example, they can tell when something doesn't add up, like 1+1=1 or 1+1=3.
And this new study confirms that our canine companions really can perceive numerical amounts, without human training.
"Our findings suggest that the ability to represent numerosity and the mechanisms supporting this system are deeply conserved over evolutionary time, perhaps owing to a role in foraging or predation," the researchers concluded.
You can read their full study in Biology Letters.