When being extra sweet doesn’t pay off

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A new study reveals how competitiveness among bats, as well as relative perceptions of sweetness when tasting nectar, shape the evolution of nectar-producing plants. The results shed light on a curious paradox: why nectar-producing plants tend to produce dilute nectar, even though nectar-feeding pollinators tend to prefer nectar with higher concentrations of sugar. To gain more insights into this paradox, Vladislav Nachev et al. studied bats with a computer-controlled array of 23 artificial flowers. Each flower was equipped with sensors and connected to a nectar pump system that dictated the sugar concentration and volume of nectar. In the simulation, upon every feeding, each bat was assumed to transfer virtual pollen to the next flower and generate a virtual offspring seed there. In this way, the researchers were able to track the influence of nectar sweetness on multiple generations of virtual plants. When manipulating the number of bats present, the researchers found that higher numbers of bats (i.e., higher competition) resulted in consumption of less sweet nectar. Therefore, when faced with competition, bats favored increases in volume instead of concentration, shifting the balance toward more dilute nectars. Furthermore, the data on bat preferences suggest that an influence called the Weber effect may be at play, where increases in sweetness offer diminished returns in perceptions of sweetness. In a related Perspective, Hamilton Farris compares this to humans' perceptions, where if the number of bulbs lighting a room is increased from one to two, an observer is likely to notice the difference in brightness. Yet if the increase is from 50 to 51 bulbs, many observers will struggle to notice the change, he explains. Thus, pollinators based their choices on small differences in nectar sweetness, which led to selection for less sweet nectar overall.

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