Other Environmental Themes

An Acoustic Nectar Guide

Bats account for one out of four mammalian species, and several hundred plants in Central and South America depend on these creatures for pollination. Since most bats rely on sophisticated sonar systems for navigation—some precise enough to distinguish between objects a fraction of a millimeter apart—it makes sense that tropical flowers might try to exploit these echolocating capabilities to lure potential pollinators. But how exactly is that done? The answer, at least in the case of Mucana holotonii, is for plants to develop "acoustically conspicuous structures" that make them easier to detect by their pollinators, glossophagine bats.

This conclusion was reached by Dagmar and Otto von Helversen, researchers at the University of Erlangen in Germany, based on their studies of M. holotonii—a flowering vine that abounds along the river banks and forests of Central America. The flower has a built-in, concave "mirror" formed by a single, raised petal called the vexillum. This structure reflects acoustic signals back to the pollinators that send them, leading the animals directly to the nectar-laden flowers.

When the vexillum was removed in one set of experiments, the number of bats visiting the flowers dropped four-fold. In separate experiments, during which the vexillum's reflectivity was dampened with cotton wool, visits by bats again dropped four-fold. Ideas about the vexillum's acoustic properties were confirmed in studies that involved beaming artificial, bat-like sounds to the floral mirror, which were then strongly reflected back to the source.

Interestingly, Mucuna species pollinated by a different kind of bat, which lacked echolocation abilities, do not have a raised and concave vexillum. This suggests that the reflective structure's geometry evolved as an adaptation to the glossophagine bats.

And there's no reason to think M. holotonii is unique in this respect, the von Helversens submit. Other bat-pollinated species may have made similar adaptations, resulting in the development of their own "acoustic nectar guides."