Studies with active normothermic thirteen‐line ground squirrels,Spermophilus tridecimlineatus, demonstrated an annual cycle in the core‐skin temperature gradient which persisted in thermoneutral ambient temperature conditions (30°C). The thermal gradient was at a maximum during the October through January interval and at a minimum for the February‐March interval. This pattern suggests that the fat stores of the animal affect its heat conservation capabilities and possibly the manner of temperature regulation.