About the cover

A population of the hibernating mammal Marmota flaviventris — the yellow-bellied marmot — has provided a unique data set that illustrates the effect of climate change on the annual events of animal life. Data from the past 33 years of a long-term study show that the animals, living in a subalpine habitat in the Upper East River Valley, Colorado, now emerge earlier from hibernation than they used to. This gives them a longer growing season so that they are now heavier when they start to hibernate again. At the same time, the fitness of large individuals has increased, leading to a rapid increase in population size. On the cover, a yellow-bellied marmot photographed in the Rocky Mountains. Photo credit: Mary Plage/Photolibrary.