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

Flowering plants have evolved an extraordinary variety of signalling traits to attract their pollinators. Most flowers rely on visual and chemical signals, but some bat-pollinated plants have evolved passive acoustic floral signals. All known acoustic flower signals rely on the same principle of increased sonar reflectivity. Here, we describe a novel mechanism that relies on increased absorption of the area surrounding the flower. In a bat-pollinated cactus (Espostoa frutescens) we found a hairy inflorescence zone, a so-called cephalium. Flowers solely emerge out of this zone. We measured the echoes of cephalia, flowers and unspecialized column surfaces and recorded echolocation calls of approaching bats. We found that the cephalium acts as a strong ultrasound absorber, attenuating the sound by -14 dB. 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.

Automated summary

Flowering plants have developed various signaling traits, including visual, chemical, and acoustic signals, to attract pollinators. This study focuses on the acoustic signals of bat-pollinated plants and describes a new mechanism where the flowers absorb ultrasound, thereby enhancing the acoustic contrast with the background echo. The researchers found that a hairy inflorescence zone in a bat-pollinated cactus acts as a strong ultrasound absorber, attenuating the sound by -14 dB, particularly at the echolocation call frequencies of approaching bats.