During the hibernation season, hibernating mammals show a sequence of torpor bouts that are interrupted by periodic arousals and brief normothermic periods. The functional significance of periodic arousals is still uncertain. We hypothesized that the imbalances in water economy may play a role in the timing of periodic arousals in hibernating species. We applied furosemide, a diuretic drug, to assess whether hibernating European ground squirrels respond to elevated urine production by shortening their torpor bouts. Urine production in the treated squirrels increased and led to more frequent arousals, presumably to restore water balance by recovery of lost water from blood and tissues. The length of the subsequent normothermic phase was not affected by the diuretic treatment. Body mass change correlated primarily with the amount of voided urine. Although our study did not identify the underlying mechanism, our results support the view that water economy, and water loss may play a role in the timing of periodic arousals.
The low body temperature state of hibernating mammals is interrupted regularly by short euthermic phases. The frequency of these euthermic phases changes according to the phase of hibernation. This typical pattern, common among ground squirrels (genus Spermophilus F. Cuvier, 1825), can be regulated by internal factors and affected by outside temperature. To evaluate the effects of internal annual timing and ambient temperature, we monitored nest temperatures of overwintering European ground squirrels (Spermophilus citellus (L., 1766)) under three temperature conditions during late hibernation (9, 5, 0 8C). Our results showed that in spite of constant thermal conditions, an annual timing effect changed torpor-bout length (TBL) at 9 and 5 8C but not at 0 8C. Ambient temperature had an independent influence on TBL and euthermic phases. The loss in body mass was higher at higher ambient temperatures and was affected by TBL and euthermic phases. Thus, the hibernation pattern of European ground squirrels is regulated by both an annual timing pattern and the ambient temperature in the hibernacula.Résumé : La condition de température corporelle basse chez les mammifères en hibernation est interrompue à répétition par de courts épisodes d'euthermie. La fréquence de ces épisodes d'euthermie change en fonction de la phase de l'hibernation. Ce patron typique, fréquent chez les spermophiles (le genre Spermophilus F. Cuvier, 1825), peut être sous le contrôle de facteurs internes et être affecté aussi par la température externe. Afin d'évaluer les effets du rythme annuel interne et de la température ambiante, nous avons enregistré la température de nids de sousliks d'Europe (Spermophilus citellus (L., 1766)) pendant l'hiver dans trois conditions de température vers la fin de l'hibernation (9, 5 et 0 8C). Nos résultats montrent que, malgré les conditions de température constante, il se produit un effet du rythme annuel interne qui change la durée des épisodes de torpeur (TBL) à 9 et à 5 8C, mais pas à 0 8C. La température ambiante a une influence indépendante sur la TBL et les phases d'euthermie. La perte de masse corporelle est plus prononcée aux températures ambiantes plus élevées et elle est affectée par la TBL et les phases d'euthermie. Ainsi le patron d'hibernation du souslik d'Europe est sous le contrôle à la fois d'un rythme annuel et de la température ambiante dans l'hibernacle.[Traduit par la Rédaction]
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