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Integrative and Comparative Biology Advance Access originally published online on January 6, 2006
Integrative and Comparative Biology 2006 46(1):5-17; doi:10.1093/icb/icj003
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© The Society for Integrative and Comparative Biology 2006. All rights reserved. For permissions, please email: journals.permissions{at}oxfordjournals.org.

Are mountain passes higher in the tropics? Janzen's hypothesis revisited

Cameron K. Ghalambor1,*, Raymond B. Huey{dagger}, Paul R. Martin{dagger}, Joshua J. Tewksbury{dagger} and George Wang{dagger}
*Department of Biology and Graduate Degree Program in Ecology, Colorado State University Fort Collins, Colorado 80523
{dagger}Department of Biology, University of Washington Seattle, Washington 98195

Correspondence: 1E-mail: cameron1{at}lamar.colostate.edu

Synopsis In 1967 Daniel Janzen published an influential paper titled "Why Mountain Passes Are Higher in the Tropics." Janzen derived a simple climatic-physiological model predicting that tropical mountain passes would be more effective barriers to organismal dispersal than would temperate-zone passes of equivalent altitude. This prediction derived from a recognition that the annual variation in ambient temperature at any site is relatively low in the tropics. Such low variation within sites not only reduces the seasonal overlap in thermal regimes between low- and high-altitude sites, but should also select for organisms with narrow physiological tolerances to temperature. As a result, Janzen predicted that tropical lowland organisms are more likely to encounter a mountain pass as a physiological barrier to dispersal (hence "higher"), which should in turn favor smaller distributions and an increase in species turnover along altitudinal gradients. This synthetic hypothesis has long been at the center of discussions of latitudinal patterns of physiological adaptation and of species diversity. Here we review some of the key assumptions and predictions of Janzen's hypothesis. We find general support for many assumptions and predictions, but call attention to several issues that somewhat ameliorate the generality of Janzen's classic hypothesis.


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C. A. Deutsch, J. J. Tewksbury, R. B. Huey, K. S. Sheldon, C. K. Ghalambor, D. C. Haak, and P. R. Martin
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