A 33-year-long study shows that ground-dwelling squirrels are leaving hibernation earlier -- and eating more -- as the planet warms.
Marmots, which are ground-dwelling squirrels, are growing larger due to longer summers.
The marmot population in the Rocky Mountains is also increasing due to the changes.
While these squirrels are benefiting from the climate shift now, the longer term effects remain uncertain.
Longer summers are causing marmots -- which are large, ground-dwelling squirrels -- to become heftier, heartier and more plentiful, according to a 33-year study published in the latest issue of Nature.
The study is the first to show that a shift in seasonal timing can cause an animal to change its body mass and population size. In this case, marmots living at around two miles elevation in the Colorado Rocky Mountains are, for now, benefiting from the shift.
That may not remain true, however, if the climate continues to change.
"If climatic predictions come to pass, we will ultimately have less summer rainfall, and late summer droughts are a real problem for marmots," co-author Daniel Blumstein, a professor and chair of UCLA's Department of Ecology and Evolutionary Biology, told Discovery News.
There is also the potential problem of having too many marmots in the ecosystem. But, as Blumstein reminded, "before we start worrying about being knee-deep in marmots, populations of predators and prey are often linked, and we've had a concomitant increase in the predators (foxes and coyotes) in some of our colonies."
For the study, Blumstein and his colleagues analyzed data on body mass, survival and reproduction of yellow-bellied marmots living in the Rocky Mountains. Every year, the scientists trapped marmots at each colony multiple times during the summer and individually marked them using numbered ear tags. They recorded the sex, mass and reproductive condition of each captured animal.
The researchers found that marmots are waking up from hibernation around 21 days earlier now. With more time to eat, the ground-dwelling squirrels are getting fatter. Adult females, for example, are about a full pound heavier now than they were in the earlier years of the study.
The researchers also calculate that the marmot population has increased by .56 marmots per year between 1976 and 2001 to 14.2 marmots per year between 2001 and 2008. Plus pups were weaned 12 days earlier during post 2000 study years than during pre-2000 years.
The findings, says Imperial College London biologist, Arpat Ozgul, show how a shift in an animal body size can trigger a shift in the animal's overall population size.
Co-author Kenneth Armitage of the University of Kansas points out it is unlikely other hibernators in the Rocky Mountains, such as smaller ground-dwelling squirrels and chipmunks, will respond to the longer summers in the way that marmots have done.
These other hibernators "are much more closely tied to seed eating and they store seeds for use during hibernation," Armitage explained.
For years, Susanne Griffin has been studying marmots living in Olympic National Park in the state of Washington.
Griffin told Discovery News: "I have long suspected that climate change and a resulting longer growing season should allow marmots to grow larger and breed earlier in the season."
While marmots are benefiting from the climate change now, "predictions of their future population dynamics are tricky," she said. She points out that leaving hibernation early could subject the animals to more predation.
In the future, the scientists hope more long-term studies can show how climate change may be affecting other animals.