40 degrees or more.

Temperatures naturally change as you move up or down a mountain, the researchers explained. But as one moves southward on the globe from the poles toward the equator, seasonal temperature swings begin to stabilize. "Tolerance to extreme temperatures is much narrower for tropical montane species relative to temperate species," Cadena explained.

The researchers also looked at pairs of species that had recently diverged, and found that tropical species stayed more constant in their temperature preferences as one species split into two. "The thermal niches of tropical species tend to be more evolutionarily conserved over long periods of time relative to temperate zone species," Cadena added.

"The inability of many tropical species to adapt to climate conditions at other elevations forces them to stay in the particular zonethat they're in, and is a key ingredient that drives the formation of new species" Kozak said. Given the inevitable climatic differences between mountain peaks and valleys, this inability to budge may prevent tropical montane species from dispersing to other elevations. Over time, the isolation leads to populations diverging to form new species, the authors explained.

"In contrast, in temperate mountains, seasonal temperature swings mean that at some point during the year the species at the bottom of a mountain experience the same cold temperatures found at the top of the mountain. So mountain peaks and valleys don't create the same barriers to dispersal which would normally cause the formation of new species," Kozak said.

The same narrow temperature tolerance that seems to boost biodiversity in tropical mountains may also