Suspecting that the hormone's seasonal fluxes might be related to hibernation, the researchers decided to give it a closer look.
Next, Kondo's team examined whether HPc makes its way to the brain, the presumed control site for hibernation. Though blood concentrations of the hormone were highest during the summer, the researchers couldn't find HPc in the fluid bathing the chipmunks' brains during that season.
The study is "a nice first step" toward understanding hibernation's molecular mechanism, says Sandy Martin of the University of Colorado School of Medicine in Aurora.
Marmots must wait longer for the snow to melt after they've emerged, while ground squirrels and chipmunks need to store more food for their longer hibernations.
Inouye and his colleagues found that least chipmunks come out of hibernation 12 days later and golden-mantled ground squirrels, 27 days later than they did in 1974.