Perception of silent and motionless prey on vegetation by echolocation in the gleaning bat Micronycteris microtis

Abstract

Gleaning insectivorous bats that forage by using echolocation within dense forest vegetation face the sensorial challenge of acoustic masking effects. Active perception of silent and motionless prey in acoustically cluttered environments by echolocation alone has thus been regarded impossible. The gleaning insectivorous bat Micronycteris microtis however, forages in dense understory vegetation and preys on insects, including dragonflies, which rest silent and motionless on vegetation. From behavioural experiments, we show that M. microtis uses echolocation as the sole sensorial modality for successful prey perception within a complex acoustic environment. All individuals performed a stereotypical three-dimensional hovering flight in front of prey items, while continuously emitting short, multi-harmonic, broadband echolocation calls. We observed a high precision in target localization which suggests that M. microtis perceives a detailed acoustic image of the prey based on shape, surface structure and material. Our experiments provide, to our knowledge, the first evidence that a gleaning bat uses echolocation alone for successful detection, classification and precise localization of silent and motionless prey in acoustic clutter. Overall, we conclude that the three-dimensional hovering flight of M. microtis in combination with a frequent emission of short, high-frequency echolocation calls is the key for active prey perception in acoustically highly cluttered environments.

Figure 1.

Experimental flight cage (1.40 × 1.00 × 0.80 m) viewed from the top with potted control and experimental plant. Target presentation on experimental plant (e.g. complete dragonfly).

Figure 2.

Picture series gained from a high-speed infrared video recording (EMS) of M. microtis scanning and acquiring target (complete dragonfly): white arrows indicate the subsequent direction of the individual's movement. Small white cross marks the back of the bat's head. (a) Beginning of the scanning behaviour with the bat moving upwards, to the right-hand side of the leaf. (b) Movement from the right-hand side (distance to prey ca 10 cm) of the leaf downwards. (c) Movement towards the centre of the leaf with the dragonfly (distance ca 14 cm). (d) The bat flies closer towards the leaf while moving slightly upwards (from a distance of ca 8 cm to a distance of ca 5 cm to prey). Bat hovering on the spot close to prey. (e) With the head directed towards the prey, the bat briefly flies backwards (approx. 5 cm). (f) Bat changes flight direction again and moves forward with its head turned slightly upwards (distance approx. 9 cm). (g) Final approach. (h) End of scanning by touching the experimental leaf and landing on the prey. Taking prey off the leaf. (i) Take off with the dragonfly.

Figure 3.

Number of landings on the seven presented targets versus prey rejections of seven M. microtis individuals.