Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) live in the prairies and grasslands across North America. Faced with deep cold over the winter months, the animals retreat into their burrows and enter hibernation. Hibernation is characterized by seasonal heterothermy in which body temperature tracks changes in ambient temperature although a minimum body temperature (about 0-5°C) is maintained if ambient temperature falls below 0°C. A regulated suppression of metabolic rate to less than 5% of euthermic resting rate allows the squirrels to conserve up to 90% of the energy that would otherwise have been needed maintain euthermia throughout the winter season and includes a strong suppression of all vital functions (e.g. breathing, heart rate) as well as many energy-consuming cellular activities such as protein synthesis. Hibernation is supported by a period of high intensity eating during the late summer that builds up huge reserves of body lipids that fuel the nonfeeding winter months. Animals cycle through long periods (days or weeks) of torpor that are interspersed with short periods (<1 day) of arousal back to euthermia. The heat generation needed to rewarm their bodies during arousal comes mainly from a specialized tissue called brown fat.