REVIEW Complex echo classification by echo-locating bats: a review Yossi Yovel • Matthias O. Franz • Peter Stilz • Hans-Ulrich Schnitzler Received: 2 February 2010 / Revised: 11 August 2010 / Accepted: 28 August 2010 Ó Springer-Verlag 2010 Abstract Echo-locating bats constantly emit ultrasonic pulses and analyze the returning echoes to detect, localize, and classify objects in their surroundings. Echo classification is essential for bats’ everyday life; for instance, it enables bats to use acoustical landmarks for navigation and to rec- ognize food sources from other objects. Most of the research of echo based object classification in echo-locating bats was done in the context of simple artificial objects. These objects might represent prey, flower, or fruit and are characterized by simple echoes with a single up to several reflectors. Bats, however, must also be able to use echoes that return from complex structures such as plants or other types of back- ground. Such echoes are characterized by superpositions of many reflections that can only be described using a stochastic statistical approach. Scientists have only lately started to address the issue of complex echo classification by echo- locating bats. Some behavioral evidence showing that bats can classify complex echoes has been accumulated and several hypotheses have been suggested as to how they do so. Here, we present a first review of this data. We raise some hypotheses regarding possible interpretations of the data and point out necessary future directions that should be pursued. Keywords Echolocation Á Classification Á Bat Á Statistics Á Behavior Introduction Using echolocation, bats routinely detect, localize, and classify targets in their surroundings (Griffin 1958; Schnitzler et al. 2003). Among these three fundamental tasks, the question how bats use echolocation to classify targets is the least studied and the least understood. The objects a bat has to classify can be grossly divided into two categories: Foraging bats have to recognize their food, e.g. insects, fruit, etc. while orienting bats have to classify objects in the environment, e.g. water surfaces, meadows, walls, vegetation, etc. The mostly small food targets often differ from the mostly large extended targets in the environment in their complexity. Small food items are mainly comprised of a few reflectors and thus reflect rather simple echoes, whereas extended targets like vegetation produce complex echoes that are usually superpositions of many reflections from the many reflectors a plant contains (i.e. its leaves and branches). Most previous work on clas- sification was done in the context of simple objects. These studies mainly focused on testing and explaining the abili- ties of bats to discriminate finely controlled spectral and temporal differences. Such studies helped to understand the limits of the system and to reveal its mode of operation, but they generally relied on unnatural deterministic echoes. Complex echoes from natural textures such as plants were either ignored or regarded as disturbing clutter. The recent work that will be described in this review, however, dem- onstrates how these echoes could contribute to a meaningful acoustic image of the bat’s surroundings. The importance of complex echoes to bats is very high. Vegetation can be an Y. Yovel (&) Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel e-mail: yossiyovel@hotmail.com M. O. Franz Faculty of Informatics, University of Applied Sciences, Constance, Germany Y. Yovel Á P. Stilz Á H.-U. Schnitzler Animal Physiology, Institute of Neurobiology, University of Tuebingen, Auf der Morgenstelle 28, 72076 Tuebingen, Germany 123 J Comp Physiol A DOI 10.1007/s00359-010-0584-7