An ultrasound absorbing inflorescence zone enhances echo-acoustic contrast of bat-pollinated cactus flowers

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 acoustic flower signals rely on the same principle of increased sonar reflectivity. Here we describe a novel mechanism through which plants can make flowers acoustically conspicuous, a principle that relies on increased absorption of the area surrounding the flower. In a bat-pollinated columnar cactus (Espostoa frutescens) we found a hairy inflorescence zone, a so called cephalium. Flowers solely emerge out of this zone. We measured the ultrasound echoes of cephalia, flowers and unspecialized column surfaces and recorded echolocation calls of approaching bats. We found that the cephalium acts 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 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.