Intensity control during target approach

their movements, coupling between action and perception is probably essential to ensure high accuracy in maintaining a specific trajectory that is followed by the animal (Heisenberg and Wolf, 1988). Bats are good model systems for studying the interaction between sensory and motor systems because they use echoes from calls generated by their motor systems as sensory information. Bats cannot perceive echo information about their environment without sending motor commands to emit a call. By recording the echolocation calls emitted by the bat, the investigator can monitor precisely when and how bats update their sensory information. Bat echolocation is highly adaptable and is strongly dependent on the surroundings of the bat as well as the task that it faces (Neuweiler, 1990; Fenton, 1995; Schnitzler and Kalko, 2001). In this paper, the extent to which echolocation behaviour is stereotypical or flexible is assessed in a bat responding to sensory information from a target. Daubenton’s bat (Myotis daubentonii) hunts in its natural habitat for small insects by trawling and aerial hawking (Jones and Rayner, 1988; Kalko and Schnitzler, 1989; Vaughan, 1997). Because of the agility of its natural prey, Myotis daubentonii must have a short response time, but to catch its prey also requires good accuracy. Here we have studied the adaptation of signal intensity in echolocation behaviour during prey capture. Technical difficulties mean that the reduction of emitted signal intensity in bats during target approach (intensity compensation) has only been studied in three species: Pteronotus parnellii, Noctilio leporinus and Eptesicus fuscus (Kobler et al., 1985; Hartley et al., 1989; Hartley, 1992b). None of these studies tested the flexibility of the bat to adjust absolute emitted signal intensity, according to target size. We assessed the flexibility of intensity compensation in Myotis daubentonii by offering the animals three target types (small sphere, big sphere or mealworm) that differed in dimensions and thus target strength. Our goal was to determine whether intensity compensation is independent of target type, or is tightly coupled to sensory input. Our hypothesis was that the intensity emitted by the bat becomes higher at small (weakly reflecting) targets, and lower at big (strongly reflecting) targets. We also tried to determine whether emitted intensity of echolocation is under closed-loop or open-loop (with no, or little, sensory feedback) control. In greater horseshoe bats Rhinolophus ferrumequinum, accuracy in emitted frequency to 2865 The Journal of Experimental Biology 205, 2865–2874 (2002) Printed in Great Britain © The Company of Biologists Limited JEB4288