Facultative torpor in free-ranging black-tailed prairie dogs (Cynomys ludovicianus)

Although facultative torpor has been observed in laboratory populations of black-tailed prairie dogs (Cynomys ludovicianus), it is widely believed tha t these animals remain normothermic throughout winter in the field. We moni tored body temperatures (T-b) of 5 black-tailed prairie dogs in the field f or about 100 days during winter and spring 1998-1999. All animals entered t orpor during this period. Major torpor bouts averaged 140.5 h +/- 2.4 SD (r ange = 77.6-214.4) with a minimum T-b of 19.0 +/- 3.6 degreesC. Minor torpo r bouts averaged 46.2 +/- 20.9 h (9.5-98.5), with minimum T-b reaching 30.3 +/- 2.3 degreesC. There was no distinct relationship between ambient air t emperature (T-a) and entry into torpor. All torpor bouts occurred following a sudden reduction in T-a, but not all sudden reductions in T-a induced to rpor. Precipitation was not associated with entry into or arousal from torp or. Our study demonstrates that facultative torpor occurs in free-ranging b lack-tailed prairie dogs in the field.