Bats like nectar sweet, but do plants know better?

CERTAIN TYPES of bats 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 new study.

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 nectars with 60% sugar — but the plants they pollinate in the wild produce watery nectars with 20% 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 Dr York Winter, one of the study’s authors.

Combining evolution simulations in the field, on the computer and in the lab, 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.

Nectar-feeding bats 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 behaviour. 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. This is why, sometimes, people expend more effort saving $ 5 on a $ 10 purchase than $ 10 on a $ 100 purchase. THE NEW YORK TIMES

Merlin D Tuttle/science Source, via The New York Times A bromeliad plant flower, with an orange nectar bat using its long tongue like a straw to sip nectar.