Here’s a longstanding mystery. Many bats have a sweet tooth, which allows plants to recruit them as pollinators by rewarding them with sugary nectar. Given a choice, these animals prefer their nectar as sickly sweet as possible, with up to 60 percent sugar. But the plants typically offer them a more dilute concoction, with just 20 percent sugar. That’s weird: Plants that produce sweeter nectar ought to be more attractive to pollinators, and so produce more offspring. Over time, nectar should evolve to be exceptionally sweet. But it hasn’t. Why not?
Vladislav Nachev and York Winter from Humboldt University in Germany have just conclusively solved this mystery, using a set of extraordinary evolutionary experiments that took more than six years to pull off.
The team worked in the Costa Rican rainforest, with free-flying, long-tongued bats that had been fitted with radio tags. They presented these animals to a field of 23 artificial “flowers”—jury-rigged Kodak film canisters connected to a pump that dispensed sugar water. Each canister was equipped with light sensors that could detect the presence of a visiting bat, and a receiver that could identify the radio tags. “Sitting in front of our computers, we could watch which animals were going to which flowers,” says Winter.
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.
And the simulations revealed one last twist: Nectar evolved to be even more dilute if there are lots of bats around. Winter’s colleague Kai Petra Stich confirmed this explanation by capturing the wild bats, and releasing groups of either three or nine of them onto the field of artificially evolving flowers. Sure enough, more bats meant more dilute nectar.
Here’s why. If there are many competing bats, each individual is more likely to visit flowers that have been recently drained, and so contain low volumes of nectar. And Weber’s law applies to volume too. A little bit of extra nectar feels like nothing if stocks are high, but seems like a feast if supplies are low.
So in a forest full of bats, plants benefit more by investing in nectar volume rather than concentration. Doing so inevitably dilutes the nectar even further, but the bats don’t care. Thanks to Weber’s law, “it’s so much more desirable for them to get a little extra volume that they don’t care about the concomitant loss of concentration,” says Winter. It’s not about what’s actually valuable, but what feels valuable. Essentially, the bats’ own senses betray them, driving the evolution of their floral partners towards less rewarding rewards.
The team’s efforts are all the more impressive because “the evolution of nectar reward is very difficult to study,” says Kathleen Kay, who studies plant evolution at the University of California, Santa Cruz. To study nectar in real flowers, scientists have to extract it, which can destroy the fragile blooms. Some have used artificial flowers, “but seldom in a way that would capture the high variability found in natural flowers or the real-time responses of wild pollinators.” Nachev and Winter solved that problem with their innovative blend of field studies, lab studies, and simulations.
In doing so, they “thoroughly nailed down an evolutionary puzzle,” says Nathan Muchhala from the University of Missouri-St. Louis, who studies pollinating bats. He notes that other scientists have put forward several other plausible explanations: The pollinators might need lots of water; the low sugar keeps insects away; high sugar levels would make nectar too viscous. “This study puts these to rest, convincingly demonstrating that the evolution of dilute nectar is in fact due to an interplay between tradeoffs plants face and how [bats] perceive the rewards,” he adds.
And since Weber’s law applies to much of the animal kingdom, “I think this effect will be universal,” says Winter. “I’m sure people will find similar examples in human decision-making. If we were looking for nectar, we’d do exactly the same thing.”