The golden-mantled ground squirrel (Callospermophilus lateralis) is a medium-sized (150-350 g) squirrel that is abundant throughout the mountainous and forest areas of western North America. The animals may be active for as little as four months of the year, spending the rest of the time (often October to May) hibernating. In the late summer & early autumn the squirrels increase their body lipid stores to provide metabolic fuel for winter hibernation and also change the composition of these lipids to increase the content of polyunsaturated fatty acids that keeps the lipid stores fluid and able to be catabolized at cold body temperatures. During hibernation their core body temperatures drop to near ambient, their metabolic rate significantly decreases to less than 5% of euthermic resting rate, and heart rate and breathing drop to 10% of normal. To induce and maintain the hibernating state, various metabolic and cellular adjustments are in place to ensure that energy consumption is accordingly reprioritized, to facilitate a heavy reliance on lipid fuel reserves, and to readjust metabolism for function under extreme hypothermia.