Abstract
Flowering plants have evolved an extraordinary variety of signaling traits to attract and guide their pollinators. Most flowers rely on visual and chemical signals, but some bat-pollinated plants have evolved reflective surfaces to acoustically guide echolocating bats. All known echo-acoustic flower signals rely on the same basic acoustic principles of increased sonar reflectivity. Here we reveal a novel mechanism through which plants acoustically communicate with bats, a principle that relies on increased absorption of the area surrounding the target flower, thereby enhancing echo-acoustic contrast. In a bat-pollinated columnar cactus (Espostoa frutescens) from the Ecuadorian Andes we found a hairy inflorescence zone, a so called lateral cephalium. Flowers of this cactus solely emerge out of this hairy zone. We measured the ultrasound echoes of the hairy zones, the flowers and unspecialized column surfaces with a biomimetic sonar head and recorded echolocation calls of approaching bats. We found that the hairy inflorescence zones act as strong ultrasound absorber, attenuating the sound by −14 dB compared to other parts of the column. The absorption was highest around the echolocation call frequencies of approaching bats. Our results indicate that, instead of making flowers more reflective, plants can also evolve specific structures to attenuate the background echo, thereby enhancing the acoustic contrast with the target. Similar sound absorbing mechanisms may be found in other species that interact with bats across a wide range of ecological contexts.
