It was a complicated set-up. The team had to set up their flowers in the middle of the jungle. They had to install air-conditioned huts for storing the computers and sugar water. They decorated the cans with wild plants, and even discovered a scent that attracted the bats. “I never thought that we could place a little plastic thing in the forest and hope that wild animals would visit,” says Winter. “But it worked!”

And here’s the wonderful bit: Nachev and Winter allowed the flowers to evolve.

Each had a virtual genome—a set of four genes that determined the concentration of its nectar. If a bat moved between two of them, a computer assumed that it had transferred virtual pollen across, and combined the flowers’ genes to create virtual seeds. Over the course of an evening, the flowers that were best at attracting bats produced the most seeds. The next night, the computer recalibrated the field of flowers by randomly selecting 23 of the seeds, and using their genomes to set the nectar on offer.

From the bats’ perspective, little changed from night to night. But in the virtual world, a new generation of fake flowers bloomed with every new sunset. And Nachev and Winter could watch the evolution of their nectar in fast-forward.

They found that, as predicted, the flowers do not enter into a sugary arms race. Instead, blooms that begin with either dilute or concentrated nectars eventually evolve towards more intermediate levels within 10 to 12 generations. Middling nectar is simply better at attracting bats—but why?

The answer lies in a psychological quirk that’s common to bats, humans, and many other animals. Imagine standing in a room that’s lit by a single lightbulb, and switching on a second—you’d almost certainly notice the difference in brightness. But you’d struggle to notice the light of that extra bulb in a room that was already lit by 50. In both cases, the amount of extra light is exactly the same; it’s just more perceptible in the first context.

This is called Weber’s Law. It means that our ability to spot a difference between two sensations changes with the intensity of those sensations. The brighter a room, the more extra light you need to notice a change in brightness. The louder a sound, the more extra noise you need to notice a change in volume. And, as Nachev and Winter showed for long-tongued bats, the sweeter a nectar, the more extra sugar they need to notice a change in sweetness.

Weber’s Law means that plants get diminishing returns from sweetening their nectar. Once they get to a certain point, they’d need to chuck in a lot more sugar to make a difference to passing bats, and it’s energy-intensive to make sugar. Nachev and Winter confirmed this by setting up simulations in which virtual bats visited and pollinated virtual flowers. They showed that as long as the bats obey Weber’s law, the flowers inevitably evolve towards dilute nectar.