Urban bioacoustics: it’s not just noise PAIGE S. WARREN* †, MADHUSUDAN KATTI ‡, MICHAEL ERMANN § & ANTHONY BRAZEL ** *Biology Department, Virginia Polytechnic Institute and State University yCenter for Environmental Studies, Arizona State University zDepartment of Biology, California State University at Fresno xSchool of Architecture & Design, Virginia Polytechnic Institute and State University **Department of Geography, Arizona State University (Received 3 March 2004; initial acceptance 18 May 2005; final acceptance 19 July 2005; published online 10 February 2006; MS. number: ARV-28R) The acoustic environment has a major influence in shaping animal communication systems. Humans, particularly in cities, profoundly alter the acoustic structure of their environment. Recent articles have identified effects of noise on animal communication and behaviour. These studies, however, serve to highlight the surprising dearth of research on the behavioural responses of animals to altered acoustic environments. We argue that noise is not the only aspect of urban bioacoustics that researchers should explore. In addition to elevated noise levels, urban areas are characterized by a spatial heterogeneity in noise levels, predictable diurnal variation in noise levels and the existence of many vertical reflective surfaces. All of these characteristics have parallels in natural environments. We suggest that cities are a fruitful area for future research on the evolution of animal communication systems, with more general implications for conservation in human-altered environments. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Research on acoustic communication in animals has provided some of the clearest demonstrations of the ways in which organisms adapt to their environments. Classic work by Morton (1975) and Wiley & Richards (1978, 1982) provided a theoretical framework for predict- ing the effects of habitat structure on acoustic signal struc- ture and calling behaviour, and this has been supported by subsequent empirical work (e.g. Hunter & Krebs 1979; Morton 1987; Ryan et al. 1990). Most work to date has focused on natural environments such as forests, meadows and ponds. Increasing urbanization worldwide is creating novel environments that animals must either adapt to or abandon. Several recent studies provide the first examples of bird songs responding adaptively to noisy urban environments (Brumm & Todt 2002; Slabbe- koorn & Peet 2003). Little work has been done, however, to characterize the overall acoustic environment of hu- man-dominated ecosystems such as cities. Although noise is the most noticeable element of the urban acoustic environment, it is not the only factor that distinguishes urban areas. Despite the heterogeneity of urban landscapes, human modifications of the environ- ment in cities have some common characteristics likely to affect communication, such as elevated noise levels, abundant large flat surfaces and altered sound channels. These characteristics have parallels in natural systems, such as streams, canyons and windy environments. The long history of research on habitat acoustics in animal behaviour research provides a basis for making predictions about the effects that novel acoustic environments should have on signal design, signal detection and the timing of signalling. We provide here both a review of existing literature and a conceptual overview of topics that are ripe for further research. Perhaps because noise pollution is an issue for human residents of urban areas, much existing behav- ioural research focuses on effects of noise. Thus, we devote the first portion of our review to addressing the effects of noise on animal communication systems, identifying critical areas in need of further research. We draw upon this literature as well as our own research in Phoenix, Arizona, U.S.A., to characterize the spatial distribution of Correspondence and present address: P. S. Warren, Holdsworth Hall, Department of Natural Resources Conservation, University of Massa- chusetts, Amherst, MA 01003, U.S.A. (email: pswarren@forwild. umass.edu). 491 0003–3472/05/$30.00/0 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2006, 71, 491–502 doi:10.1016/j.anbehav.2005.07.014