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Who’s Signaling Who? Animal-Plant
Acoustic Interactions
Given that plants play such an important role in transmitting animal signals, biologists have begun investigating whether plants have evolved mechanisms to enhance or repress animal acoustic signals. Although current evidence is fragmentary, what exists is intriguing. During a presentation at the ASA plant bioacoustics special session, Michael G. Schöner at the University of Greifswald (Greifswald, Germany) revealed that various bat species pollinate around 250 genera of tropi- cal plants. His close examination of several of these species has found that the petals and flowers of these plants generate strong reflections of bat ultrasonic signals over a broad range of aspect angles. Removal of these structures reduced the abil- ity of bats to locate and pollinate the plants (Schöner et al., 2016). As one particular example (Figure 1), a pitcher plant (Nepenthes hemsleyana) that serves as the mutualistic (mutually beneficial) host for a small bat species (whose feces fertilize the plant) has a concave structure in the back wall that serves as a reflector. A closely related pitcher plant species that does not host bats lacks this structure and is dramatically less acousti- cally reflective. Although these detailed measurements have been directly conducted only in a few plants, Schöner et al. noted that the fertilization by bat feces is not uncommon in other plant species, including trees. They also observed that many bat-interacting plants display flagellichory, the character- istic of exposing fruits by hanging them on a long peduncle, a structure would seem to enhance a bat’s ability to acoustically detect fruit.
Although his research has been confined to bats, Schöner et al. (2016) have speculated whether plants have evolved structures to enhance signals from other animal orders such as insects. Several ant species colonize plants and have alarm signals that are transmitted via knocking their bodies on the plant’s stem, alerting the entire colony. No one knows whether plants could have evolved efficient means of enhancing these signals.
However, entomologists have studied the phenomenon of “buzz-pollination” for decades, wherein pollinating insects (mostly bees) use vibrations to release and extract pollen from
flowers. Buzz-pollinated flowers occur in species from 65 dif- ferent plant families and are believed to have independently evolved multiple times. Although considerable research has examined the behavior and signals produced by pollinating insects, a recent review paper on buzz-pollination (De Luca and Vallejo-Marín, 2013) noted relatively little work has been performed on how the mechanical structures of flowers
Figure 1. Passive acoustic signaling in two nonrelated plant species. The neotropical vine Marcgravia evenia attracts bats, which pollinate its flowers. Exemplary spectral directional patterns of a dish-shaped leaf (a) and a foliage leaf (b) are shown. Dish-shaped leaf echoes were of high intensity, were multidirectional, and had an invariant echo signature compared with those of foliage leaves. The paleotropical carnivorous plant Nepenthes hemsleyana attracts bats that fertilize the plant with their feces. Exemplary spectral directional patterns of the back wall of N. hemsleyana pitchers (c) show that this structure is a similar multidirectional echo reflector as the dish-shaped leaves of M. evenia. Such structures are missing in other pitcher plant species such as N. rafflesiana (d), the closest relative of N. hemsleyana, that does not attract and host bats. Reprinted from Schöner et al., 2016, with permission from Elsevier.
may have coevolved to permit species-specific stimulation of pollen release.
Perhaps further surprising acoustic interactions between plants and animals lie in wait.
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