letters to nature 742 NATURE | VOL 414 | 13 DECEMBER 2001 | www.nature.com demonstration that ¯ow in a low-viscosity crustal channel that is coupled to surface denudation provides an internally consistent explanation not only for ductile extrusion of the GHS but for many other salient features of the Himalayan±Tibetan system. The critical factors are the presence of low-viscosity material in the middle to lower crust, a variation in crustal thickness between plateau and foreland, and surface denudation that is focused on the plateau ¯ank. The range of model styles, and by implication the tectonics of natural orogens, is sensitive to variations in denudation rate and upper-crust strength. M Received 20 April; accepted 2 November 2001. 1. Royden, L. H. Coupling and decoupling of crust and mantle in convergent orogens: implications for strain partitioning in the crust. J. Geophys. Res. 101, 17679±17705 (1996). 2. Clark, M. K. & Royden, L. H. Topographic ooze: Building the eastern margin of Tibet by lower crustal ¯ow. Geology 28, 703±706 (2000). 3. Shen, F., Royden, L. H. & Burch®el, B. C. Large-scale crustal deformation of the Tibetan Plateau. J. Geophys. Res. 106, 6793±6816 (2001). 4. Grujic, D. et al. Ductile extrusion of the Higher Himalayan Crystalline in Bhutan: evidence from quartz microfabrics. Tectonophysics 260, 21±43 (1996). 5. Wu, C. et al. Yadong cross structure and the South Tibetan Detachment in the east central Himalaya (898-908E). Tectonics 17, 28±45 (1998). 6. Vannay, J.-C. & Grasemann, B. Himalayan inverted metamorphism and syn-convergence extension as a consequence of a general shear extrusion. Geol. Mag. 138, 253±276 (2001). 7. Grujic, D., Hollister, L. S. & Parrish, R. R. Himalayan metamorphic sequence as an orogenic channel; insight from Bhutan. Earth Planet. Sci Lett. (in the press). 8. Burch®el, B. C. et al. The South Tibetan detachment system, Himalayan orogen: Extension contemporaneous with and parallel to shortening in a collisional mountain belt. Spec. Pap. Geol. Soc. Am. 269, (1992). 9. Nelson, K. D. et al. Partially molten middle crust beneath southern Tibet: a synthesis of Project INDEPTH results. Science 274, 1684±1688 (1996). 10. Hodges, K. V. Tectonics of the Himalaya and southern Tibet from two perspectives. Geol. Soc. Am. Bull. 112, 324±350 (2000). 11. Willett, S. D., Beaumont, C. & Fullsack, P. Mechanical model for the tectonics of doubly vergent compressional orogens. Geology 21, 371±374 (1993). 12. Beaumont, C., Jamieson, R. A., Nguyen, M. H. & Lee, B. in Slave - Northern Cordillera Lithospheric Evolution (SNORCLE) and Cordilleran Tectonics Workshop (eds Cook, F. & Erdmer, P.) 112±170 (Lithoprobe Report 79, Lithoprobe Secretariat, University of British Colombia, Vancouver, 2001). 13. Jamieson, R. A., Beaumont, C., Nguyen, M. H. & Lee, B. Interaction of metamorphism, deformation, and exhumation in large convergent orogens. J. Metamorph. Geol. 20, 1±16 (2002). 14. Gleason, G. C. & Tullis, J. A ¯ow law for dislocation creep of quartz aggregates determined with the molten salt cell. Tectonophysics 247, 1±23 (1995). 15. Hauck, M. L., Nelson, K. D., Brown, L. D., Zhao, W. & Ross, A. R. Crustal structure of the Himalyan orogen at ,908 east longitude from Project INDEPTH deep re¯ection pro®les. Tectonics 17, 481±500 (1998). 16. Owens, T. J. & Zandt, G. Implications of crustal property variations for models of Tibetan plateau evolution. Nature 387, 37±43 (1997). 17. Zeitler, P. K. et al. Crustal reworking at Nanga Parbat,: Pakistan: Metamorphic consequences of thermal-mechanical coupling facilitated by erosion. Tectonics 20, 712±718 (2001). 18. DeCelles, P. G. et al. Stratigraphy, structure, and tectonic evolution of the Himalayan fold-thrust belt in western Nepal. Tectonics 20, 487±509 (2001). 19. Lee, J. et al. Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints. Tectonics 19, 872±895 (2000). 20. White, N. M. et al. Constraints on the structural evolution, exhumation, and erosion of the High Himalayan Slab, NW India, from foreland basin deposits. Earth Planet. Sci. Lett. (in the press). 21. France-Lanord, C., Derry, L. & Michard, A. in Himalayan Tectonics (eds Treloar, P. J. & Searle, M. P.) 603±621 (Spec. Publ. 74, Geological Society, London, 1993). 22. Harrison, T. M. et al. A Late Miocene-Pliocene origin for the central Himalayan inverted meta- morphism. Earth Planet. Sci. Lett. 146, E1±E7 (1997). 23. Parrish, R. R. & Hodges, K. V. Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya. Geol. Soc. Am. Bull. 108, 904±911 (1996). 24. DeCelles, P. G., Gehrels, G. E., Quade, J., LaReau, B. & Spurlin, M. Tectonic implications of U-Pb zircon ages of the Himalayan orogenic belt in Nepal. Science 288, 497±499 (2000). 25. Inger, S. & Harris, N. B. W. Tectonothermal evolution of the High Himalayan Crystalline Sequence, Langtang Valley, northern Nepal. J. Metamorph. Geol. 10, 439±452 (1992). 26. Macfarlane, A. M. An evaluation of the inverted metamorphic gradient at Langtang National Park, central Nepal Himalaya. J. Metamorph. Geol. 13, 595±612 (1995). 27. Fraser, G., Worley, B. & Sandiford, M. High-precision geothermobarometry across the High Himalayan metamorphic sequence, Langtang Valley, Nepal. J. Metamorph. Geol. 18, 665±682 (2000). 28. Searle, M. P. et al. Shisha Pangma leucogranite, South Tibetan Himalaya: Field relations, geochemistry, age, origin, and emplacement. J. Geol. 105, 307±326 (1997). 29. Hodges, K. V., Parrish, R. R. & Searle, M. P. Tectonicevolution of the centralAnnapurna range, Nepalese Himalaya. Tectonics 15, 1264±1291 (1996). 30. Mackwell, S. J., Zimmerman, M. E. & Kohlstedt, D. L. High-temperature deformation of dry diabase with application to tectonics on Venus. J. Geophys. Res. 103, 975±984 (1998). Supplementary Information accompanies the paper on Nature's website (http://www.nature.com). Acknowledgements This research was funded by Lithoprobe Supporting Geoscience and NSERC Research grants to C.B. and R.A.J., and the Inco Fellowship of the Canadian Institute for Advanced Research to C.B. All the models were run using the ®nite element thermal-mechanical program developed by P. Fullsack. The work bene®ted from discussions with J. Braun, L. Brown, L. Derry,P. Fullsack, D. Grujic, D. Nelson, S. Medvedev, O. Vanderhaeghe and K. Whipple. Comments by L. Royden substantially improved the manuscript. Correspondence and requests for materials should be addressed to C.B. (e-mail: Chris.Beaumont@Dal.Ca). ................................................................. Effect of acoustic clutter on prey detection by bats Raphae È l Arlettaz*², Gareth Jones³ & Paul A. Racey§ * Division of Conservation Biology, Zoological Institute, University of Bern, Baltzerstrasse 6, CH-3012 Bern, Switzerland ² Institute of Ecology, University of Lausanne, CH-1015 Lausanne, Switzerland ³ School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK § Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK .............................................................................................................................................. Bats that capture animal prey from substrates often emit char- acteristic echolocation calls that are short-duration, frequency- modulated (FM) and broadband 1 . Such calls seem to be suited to locating prey in uncluttered habitats, including ¯ying prey, but may be less effective for ®nding prey among cluttered back- grounds because echoes re¯ecting from the substrate mask the acoustic signature of prey 2±4 . Perhaps these call designs serve primarily for spatial orientation 5±7 . Furthermore, it has been unclear whether the acoustic image conveyed by FM echoes enables ®ne texture discrimination 3,8,9 , or whether gleaning bats that forage in echo-cluttering environments must locate prey by using other cues, such as prey-generated sounds 5±7,10±13 . Here we show that two species of insectivorous gleaning bats perform badly when compelled to detect silent and immobile prey in clutter, but are very ef®cient at capturing noisy prey items among highly cluttered backgrounds, and both dead or live prey in uncluttered habitats. These ®ndings suggest that the short, broadband FM echolocation calls associated with gleaning bats are not adapted to detecting prey in clutter. Two major and non exclusive 14±17 foraging tactics can be distin- guished among insectivorous bats: aerial hawking (that is, the capture of airborne prey) and substrate gleaning. About one third of all microchiropteran bat species capture prey from substrates 15 . Unlike aerial-hawking bats that include longer-duration, and almost constant-frequency components in their echolocation calls, gleaning species emit calls that are often of low intensity and which often sweep from high to low frequencies (frequency-modu- lated (FM) calls) in a few milliseconds 1,4 . A major outstanding problem in echolocation biology is the extent to which these calls are used for distinguishing prey items from substrates, particularly when the substrate is complex and generates a lot of echo clutter 3 (that is, echoes from objects other than the target of interest). Some bat species emit calls at a high repetition rate (`feeding buzzes') to localize aerial prey, but switch off echolocation immediately before taking prey from surfaces: the bats may then listen instead for prey- generated sounds 7,11 . Most experiments on gleaning bats have investigated prey detection on simple surfaces, where background echoes may not mask prey echoes. In such situations, bats may still use echolocation to detect prey 18 . Bats may also use echolocation to detect prey positioned close to ¯at surfaces 19 , and may even detect ¯ying insects in grass by monitoring the insect's movement over successive echoes 20 . © 2001 Macmillan Magazines Ltd