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

Abstract

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.

Keywords: Bat pollination; Cactaceae; Cephalium; Chiropterophily; Echolocation; Floral acoustics; Ultrasound absorption.

Fig. 1.

Spectral target strength of different morphological structures of Espostoa frutescens for different frequency bands. The spectral target strength was obtained from ensonification measurements at a distance of 15 cm. We measured unspecialized parts of the cactus column (green; n=6 columns, 10 measurements per column), isolated flowers (pink; n=6 columns, 10 measurements per flower from different angles) and the hairy cephalium zone (purple; n=6 columns, 10 measurements per column). Lines within boxplots are the median and whiskers show the non-outlier minimum and maximum.

Fig. 2.

A nectar-feeding bat approaching a flower and echo fingerprints of different cactus surfaces with flowers. (A) Image of a Geoffroy's tailless bat (Anoura geoffroyi) approaching a flower of E. frutescens, which is embedded in the hairy cephalium zone (photo credit: ©MerlinTuttle.org). (B) Echo fingerprints of acoustic scans along the cactus column. On the left is a natural column with cephalium and flower; on the right is an experimentally manipulated column. The flower was cut out of the hairy zone and fixed on an unspecialized part of the column. The intensity (spectral target strength in dB) of the echo is given in colour gradation (red indicates high intensity, blue low intensity).

Fig. 3.

A typical series of calls of a Geoffroy's tailless bat approaching an E. frutescens flower. The microphone was placed next to the flower, as shown in Fig. S3.