Ecology and physiology of free ranging black-tailed and Utah prairie dogs

Abstract

The ability of animals to enter torpor depends upon numerous physiological and environmental factors. Thus, torpor may be influenced by varying climatic conditions over a broad geographic range of a species. Because of their varied geographic distribution and distinct torpor patterns, prairie dogs present a model system for the study of torpor. Utah prairie dogs (Cynomys parvidens), which are believed to hibernate continuously during winter, can be contrasted with black-tailed prairie dogs (Cynomys ludovicianus), which enter torpor only during periods of environmental or physiological stress. The objective of my research was to evaluate differences in over-winter body temperature patterns used by these closely related species, and to examine the influence that diet quality and habitat conditions have on these patterns. I monitored body temperature for > 6 months in adult (>1 y) black-tailed and Utah prairie dogs from colonies located along elevational gradients in northern Colorado and southern Utah, and also examined their seasonal changes in body mass, lipid composition of stored fat, and diet and dietary nutrient quality. My results indicate that body temperature patterns of both black-tailed and Utah prairie dogs differ across elevations, with lower elevation populations entering more shallow and infrequent torpor than prairie dogs at higher elevations. Body temperature patterns of black-tailed prairie dogs showed strong circadian rhythmicity, but torpor patterns of Utah prairie dogs did not display these same circadian patterns. I observed a single population of black-tailed prairie dogs with torpor patterns that strongly resembled those of free-ranging hibernators. This population experienced multiple and sequential bouts of torpor that increased in length and depth as winter progressed. Prairie dogs monitored from an adjacent colony remained euthermic for the majority of winter, and it appears that subtle differences in environment between these colonies may have been responsible for the distinct over-winter body temperature patterns observed in each colony. My results also show that body masses of prairie dogs were variable across seasons and elevations, with prairie dogs from higher elevation sites generally having greater body mass in all seasons sampled. Seasonal changes in lipid composition of stored fat indicated that black-tailed prairie dogs do not have a clear pattern of essential fatty acid metabolism during winter, whereas Utah prairie dogs appeared to preferentially metabolize lipid in a manner that promotes deep and continuous torpor. Both black-tailed and Utah prairie dogs exercise strong dietary selection, and appear to prefer species higher in lipid and nitrogen content, relative to other plant species available on colonies. Vegetation on prairie dog colonies at higher elevations was generally higher in linoleic acid and nitrogen content than vegetation on colonies at lower elevations. Collectively, my results underscore the considerable variation in both physiological and environmental factors that exist within the ranges of black-tailed and Utah prairie dogs. Because torpor can result in significant energy savings, it is an essential component of the life history of prairie dogs that allows them to persist in habitats where food resources fluctuate seasonally and where environmental conditions are often unfavorable.