2017-01-06 18:14:16
Trilobites: Bats Like Their Plant Nectar Sweet — Though Maybe the Plants Know Better

18:14, January 06 208 0

This is how we’re taught to count as children: 1, 2, 3, 4, 5. Yet our innate grasp of quantities isn’t linear. We’re more likely to experience the world in ratios, like 1, 2, 4, 8, 16. It’s easier for us to see the difference between one and two marbles than 15 and 16 of them.

In this way, we’re a lot like certain types of bats who have shaped the evolution of the plants they pollinate by making proportional judgments about what they’re willing to eat. That’s the finding of a study published in Science on Thursday. The study set out to answer an old evolutionary riddle: How do plants pollinated by bats get away with offering nectar in much lower sugar concentrations than the bats prefer? It turns out the plants are just giving the bats what they appear to want.

Given a choice through experiments, bats choose syrupy nectars, with 60 percent sugar — but the plants they pollinate in the wild produce watery nectars with 20 percent sugar.

“Maybe it’s the plants enforcing a dilute nectar that they prefer on the animals — or maybe it’s the bats exerting selection pressure,” said York Winter, a professor of cognitive neurobiology at Humboldt University, and one of the study’s authors.

To figure out which scenario was happening, his team ran evolution simulations in the field, on the computer and in the lab, assuming that the bats were choosing certain plants to pollinate over others.

Combining these simulations, the researchers found an explanation for how nectar-feeding bats end up preferring nectars that are less sweet — particularly when there’s fierce competition for a limited amount of nectar. The crux of that explanation? The same kind of nonlinear, proportional decision making that people use in many everyday judgments.

This pattern is encapsulated by the Weber-Fechner law, which says that animals — including people — commonly perceive physical stimuli in relative increments rather than absolutes.

Given one type of input, the Weber-Fechner law is relatively straightforward. But when you’re forced to deal with multiple considerations, like trying to simultaneously judge prices and sizes of cereal boxes at the grocery store, the effect becomes more complicated. Which factor will stand out more, and therefore carry more weight in your decision making?

Nectar-feeding bats also consider two things at the same time: the volume of available nectar, and its sugar concentration. They prefer high levels of both, but in the wild, nectar sugar concentrations are generally midrange, while volumes per bat are low, especially when many thirsty bats are competing for limited supplies.

The Weber-Fechner law dictates that the bats perceive increases in volume more acutely than they do increases in sugar concentration. In other words, they are more sensitive to changes in quantity than in quality.

“Gaining just a little bit more nectar causes a much stronger change in sensation — so they go to flowers where they get a little bit more nectar,” Dr. Winter said.

Over many iterations, the researchers’ simulations confirmed, the bats help pollinate plants that produce more nectar, even if that nectar is more watery and less sugary.

Dr. Winter is intrigued by the possibility of applying these findings to human behavior. How do people make decisions when they have to take into account multiple factors?

“Multiple dimensions can interact and cause nonrational decisions, where you are given two options and you actually take the lesser one,” Dr. Winter said.

Consider that a proportional sense of quantities is what sometimes leads people to expend more effort saving $5 on a $10 purchase than $10 on a $100 purchase.

So, the next time you find yourself defaulting to rough estimates, you may want to stop and disentangle your perceptual biases. Take this lesson from nectar-feeding bats: It’s all relative, until the only nectar that’s left for you to drink is watery.