Obligate hibernators express circannual patterns of body mass and hibernation, which persist under constant laboratory conditions. I hypothesized that in the 13-lined ground squirrel (Ictidomys tridecemlineatus) thermogenic brown adipose tissue (BAT) and lipid storing white adipose tissue (WAT) volume would follow a circannual pattern. I housed animals at either 25°C (thermoneutral) or 5°C with 12h L:12h D photoperiods for an entire year. I determined volume and water-fat ratio of WAT and BAT using magnetic resonance imaging (MRI). BAT volume follows a circannual pattern in both conditions, increasing prior to winter, decreasing in late winter with no change in water-fat ratio. Both body mass and WAT volume of cold-housed animals declined throughout the winter and recovered after hibernation. By contrast, thermoneutral housing produced no circannual pattern in body mass even though WAT volume declined in late winter. Warm-housed animals never entered torpor indicating that they might not be obligate hibernators
