Over the past 20 years, a group of scientists from Canada, the U.S., Scotland and France have been following a population of Columbian ground squirrels in the Alberta. These animals hibernate for eight to nine months per year and emerge in late spring to feed and reproduce in the summer.
But the researchers saw adult female squirrels emerging later every year. By 2011, the squirrels were emerging about 10 days later, the scientists wrote in a paper published in the journal Nature on Wednesday.
"When you're only active for three or four months, 10 days is a fairly substantial chunk of time," University of Alberta ecologist and lead author Jeffrey Lane said in a phone interview Wednesday.
Earlier emergence also coincided with a decline in the survival rate of adult females. At the start of the study, about 87 percent of the female ground squirrels active in the summer would return the following summer. In the most recent year, only 67 percent are reemerging after going into the ground. The ground squirrel population has shifted from a growing one to one that's basically maintaining its numbers.
(The scientists follow only females for two main reasons: They're more relevant for studying population dynamics, since they produce the babies, and the males tend to disperse after they grow up, making them harder to track.)
The scientists don't know if waking up later means the ground squirrels stay awake longer and consequently foraging later as vegetation dries out, or if they're heading into the ground at the same time as usual and spending less time aboveground building up fat reserves for the winter. Neither scenario is optimal for the creature's survival.
Lane says researchers are also still not sure exactly how ground squirrels decide when to come up out of the earth. The burrows they hibernate in, called hibernacula, cut off most cues to the world above.
"They're kind of in little sensory deprivation chambers," he said.
During hibernation, the ground squirrels can't sense the light changing as the seasons shift. They probably can't sense temperature changes either -- scientists have taken temperature readings and found that the hibernaculum doesn't seem to register large swings of hot and cold that would wake the ground squirrels up.
What they do think is that the ground squirrels are able to somehow sense when the snow above melts.
Over the past 20 years, the snow in the ground squirrel's habitat has been melting later -- a delay of nearly three days per year over the past 20 years -- thanks to the increasing likelihood of late-season snowstorms. During the first decade of the study, the researchers saw only one year with a substantial snowfall happening after the middle of April. But seven of the most recent 10 years saw snowstorms past mid-April.
That increase in winter snowfall jives with climate models showing a trend toward moister winters and more heavy snow and rainfalls, according to the authors.
Lane and his colleagues think their study is especially significant, given that most previous work on how climate change affects the seasonal nature of animal life cycles has focused on birds and temperature shifts.
"The results presented here highlight the extent to which aspects of climate change other than warming temperature may affect natural populations," they wrote.
SOURCE: Lane et al. "Delayed phenology and reduced fitness associated with climate change in a wild hibernator." Nature online ahead of print Aug. 8, 2012.