GHALAMBOR, Cameron/ K; HUEY, Raymond/ B; MARTIN, Paul/ R; Colorado State University; University of Washington; University of Washington: Why mountain passes are higher in the tropics- revisited

Organismal biologists have long been interested in how the abiotic environment shapes the physiology, ecology, and evolution of organisms. Yet, relatively few attempts have been made to develop general hypotheses that make testable predictions as to how global variation in climate shapes observed patterns of diversity in physiological tolerance, population demography, geographic distribution, and species diversity. One remarkable exception is Daniel Janzen�s seminal paper, �Why Mountain Passes are Higher in the Tropics�. Janzen reasoned that the reduced seasonality of tropical environments has two important consequences. First, he convincingly showed the thermal regimes of high versus low-altitude sites in the temperate zones have a much higher annual overlap than do those of tropical sites. Second, tropical organisms should be specialized for narrower thermal regimes, although data was unavailable to test this assumption. He then proposed that elevation gradients in the tropics pose a stronger barrier to dispersal because organisms moving up or down an elevational gradient would likely experience temperature regimes outside the range normally experienced. Hence, tropical mountains are physiologically �higher� than their temperate counterparts. Here, we critically evaluate the assumptions and predictions made by Janzen�s synthetic hypothesis. We find evidence that both supports and contradicts Janzen�s hypothesis, but note that critical tests remain to be conducted. We suggest that not all organisms are expected to conform to Janzen�s hypothesis and propose a framework for conducting future tests.