Mammalian hibernators experience dramatic reductions in body temperature, metabolic rate, respiratory rate, and heart rate during hibernation. These changes are precisely controlled and reversible with only internally driven mechanisms, suggesting specific biochemical regulation. We present a model that integrates our observations of differential liver gene expression during preparation for, and maintenance of, the hibernating state, with the known phylogenetic interspersion of hibernating species in several major mammalian lineages. This model predicts a major role for the differential expression of existing mammalian genes in the biochemical regulation of hibernation.Hibernation is an adaptive strategy that is used by species in several mammalian orders to conserve energy in cold or inhospitable environments. During hibernation, these mammals dramatically lower their metabolic, heart, and respiratory rates as well as their body temperature in a precisely controlled manner. This strategy helps some, particularly small, mammals to survive cold winter conditions in temperate climates when food and water are scarce yet the demand for metabolic heat generation is high. Hibernating species are frequently closely related to nonhibernating species and are found in five orders of eutherian mammals as well as in a few species of marsupials and monotremes (ref. 1 and Fig. 1). The adaptive significance of hibernation, coupled with the interspersed phylogenetic distribution of hibernating species among mammals, makes hibernation an ideal model system to test the hypothesis that the molecular basis of important adaptive events during evolution involves mechanisms that lead to the differential regulation of existing genes and not the invention of new genes (reviewed in ref.3).The evolutionary origin of hibernation is unknown; either of two hypotheses can explain the interspersed nature of the distribution of extant hibernating species throughout Mammalia. One postulates that the common ancestor of all mammals was a hibernator, but the ability to hibernate was lost independently along multiple lineages. The alternative proposes that the common ancestor of all mammals was not a hibernator, and the ability to hibernate has been gained independently along multiple lineages (Fig. 1). With either scenario, we predict that a small number of regulatory changes determine the phenotype of hibernation, rather than the de novo creation or loss of numerous hibernation-specific biochemical functions, encoded by a collection of hibernation-specific genes. Thus, hibernation must be the result of a reprogramming of existing mammalian biochemical capabilities through the differential expression of existing genes.There is little evidence in the literature for differential gene expression during hibernation, although there are a few reports that demonstrate increases or decreases (4) in the amount of specific proteins relative to the active state. Mitochondrial uncoupling protein (thermogenin) is an example of a protein that increases i...
1973. Physiology of thermoregulation in the pika, Ochotona princeps. Can. J . Zool. 51: 11-16.The mean body temperature of pika measured by radiotelemetry under field and laboratory conditions was 40.1°C (range = 37.942.7) over an ambient temperature range of -9.3 to 24°C. The maximum daily fluctuation in any one individual was less than 2.6"C and no seasonal difference in the level of body temperature maintained was observed. Hypcrthermia and death occurred after a 2-h exposure to ambient temperatures higher than 28"C, inclusive.The basal metahl~c ratc was 1.53 cc 0 2 / g h and the thermal conductance was between 0.096 and 0.050 cc Ol/g h 'C, 143% and 101-53F1, sespecrively, of their predicted weight-specific values. The relatively high body temperature of pika 15 attributed to its high basal metabolic rate and good insulation. The low thermal conductance, which ind~cates a reduction of energy consumption at ambient tenlperaturc~ below the lower critical temperature (21°C), favors the overwinter survival of this species when only limited food storage is available. MACARTHUR,R.A., et L. C. H. WANG. 1973. Physiology of thermoregulation in the pika, Ochotona princeps. Can. J. Zool. 51 : 11-16. La tempkrature moyenne de l'ochotone, mesurk par radiotklkmktrie en nature et en laboratoire, est de 40.1°C (ktendue = 37.9 a 42.7) ii des tempkratures ambiantes allant de -9.3 a DOC. La !ariation maximale enregistree chez un individu, dans la m8me journk, est de moins de 2.6'C et le nlveau de tempdrature du corps est le meme en toutes saisons. Une exposition de 2 h ii des temperatures de 28°C ou plus cause l'hyperthermie et la mort. Le m6tabolisme de base est de 1.53 cc 0 2 / g h et la conductibilitk thermique varie entre 0.096 et 0.050 cc Oz/g h "C, reprhentant respectivement 143% et 101-53% des valeurs prkvues d'aprks le poids del'animal. La temperature relativement dlevk de l'ochotone est due ii son mktabolisme basal 8evk et son isolation efficace. La faible conductibilitk thermique, indice d'une reduction de la consommation d'dnergie une temp6rature ambiante infkrieure A la tempkrature critique minirnale (21°C), permet la survie de I'animal durant l'hiver, alors que les ressources alimentaires disponibles sont limitkes.praduit par le journal]
Using radiotelemetry for body-temperature sensing in field animals, torpor season in the Richardson's ground squirrel (Spermophilus richardsonii) was found to commence from mid-July in adults and mid-September in juveniles and to terminate for both groups in mid-March. The duration of torpor averaged 3 and 4.3 days in July and August and gradually lengthened to 19.1 days in January, after which it shortened to 14.2 and 6.0 days in February and March. The duration of intertorpor homeothermy ranged from 300 to 1500 min. The metabolic costs associated with entry into torpor, during torpor, arousal from torpor, and intertorpor were quantified in the laboratory at ambient temperatures comparable with those observed in burrow. Applying these metabolic data to the time budget for torpor in field animals, the estimated seasonal energetic costs for entry into torpor averaged 12.8% of total, deep torpor 16.6%, arousal from torpor 19.0%, and intertorpor 51.6%. The monthly energy savings by utilization of torpor ranged from 38.8% in July and 51.2% in March to between 81.6 and 96.0% between August and February. For the whole torpor season, which extended 8 months, the energy savings amounted to 87.8%.
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