title: Vertebrate range sizes indicate that mountains may be 'higher' in the tropics.
abstract: In 1967, Daniel Janzen proposed the influential, but largely untested hypothesis, that tropical mountain passes are physiologically higher than temperate mountains. I test his key prediction, the one upon which all the others rely: namely, that elevational range sizes of organisms get larger on mountains at increasing latitudes. My analyses use 170 montane gradients spanning 36.5 degrees S to 48.2 degrees N latitude compiled from over 80 years of research and 16,500 species of rodents, bats, birds, lizards, snakes, salamanders, and frogs. In support of Janzen's prediction, I find that elevational range size increases with increasing latitude for all vertebrate groups except rodents. I document additional lines of evidence for temperature variability as a plausible mechanism for trends in vertebrate range size, including strong effects of thermoregulation and daily temperature variability, and a weak effect of precipitation.
authors.affiliation: Department of Ecology & Evolutionary Biology and CU Museum of Natural History, University of Colorado, Boulder, CO 80309, USA. christy.mccain@colorado.edu
authors.forename: Christy M
authors.initials: CM
authors.lastname: McCain
mesh_terms: D000531:Altitude; D000818:Animals; D001001:Anura / Q000145:classification; D044822:Biodiversity; D001717:Birds / Q000145:classification; D002685:Chiroptera / Q000145:classification; D003710:Demography; D017753:Ecosystem; D005843:Geography / Q000706:statistics & numerical data; D008116:Lizards / Q000145:classification; D011156:Population Density; D011157:Population Dynamics; D012377:Rodentia / Q000145:classification; D012911:Snakes / Q000145:classification; D013696:Temperature; D014329:Tropical Climate; D014562:Caudata / Q000145:classification; D014714:Vertebrates / Q000145:classification