Torpor patterns of hibernating eastern chipmunks Tamias striatus vary in response to the size and fatty acid composition of food hoards

Abstract

Summary Many endotherms employ torpor during periods of resource scarcity, but this state of substantially reduced body temperature and metabolism appears to impose significant physiological costs. Accordingly, individuals can be expected to vary the expression of torpor according to the size of their energy reserves. Although dietary polyunsaturated fatty acids (PUFAs) are important for maintaining the fluidity of membrane phospholipids and depot fats at low body temperatures, they are also prone to autoxidation, which can result in significant somatic damage. Dietary PUFA may thus influence the depth and duration of torpor during hibernation. We evaluated the hypothesis that both an increase in the size of the burrow food hoard and an elevation of its PUFA composition can cause chipmunks to reduce their use of torpor both by reducing the time spent torpid and by maintaining higher body temperature during torpor. We provided individual chipmunks with equicaloric natural‐PUFA and high‐PUFA supplements 10 days prior to autumn immergence. We measured seven parameters that characterize the depth and duration of torpor used by hibernating chipmunks using temperature‐sensitive data loggers mounted on neck collars. We compared torpor patterns for the natural‐PUFA, high‐PUFA and control groups at a study site in southern Quebec, Canada. We also compared control animals from Quebec with unsupplemented controls from a more southerly site in Pennsylvania, USA characterized by higher food availability and less severe winters. Chipmunks provided with natural‐PUFA supplements spent less than half as much time in torpor as control animals at the same study site, and when in torpor they exhibited skin temperatures almost twice as high as controls. Chipmunks provided with high‐PUFA supplements significantly reduced the depth and duration of torpor bouts compared with animals provided with natural‐PUFA supplements. The torpor patterns of unsupplemented chipmunks at the southern site approximated those of natural‐PUFA chipmunks at the main study site. Our results provide clear evidence that chipmunks adjust the depth and duration of torpor expression according to both the size and the composition of their energy reserves. Furthermore, both the extent and the nutritional form of environmental energy availability are important determinants of the cost and benefits of torpor expression by free‐ranging endotherms.