The relation between the torpor cycle and heat exchange in the California pocket mouse Perognathus californicus.

Abstract

AbstractThe changes in body temperature (TB) associated with the torpor cycle of P. californicus are described by the equation \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{{\rm dT}_{\rm B} }}{{{\rm dt}}} = \frac{{{\rm heat production - heat loss}}}{{\rm K}} $\end{document} where t is time, and K is the heat capacity of body tissue. This equation can be solved after substituting appropriate expressions for maximum and minimum aerobic heat production and heat loss to give theoretical maximum rates of entry into and arousal from torpor.The measured time course of body temperature and oxygen consumption during entry into torpor compare favorably with theoretical curves calculated under conditions of minimum heat production and maximum heat loss. Thus P. californicus appears able to “switch off its thermostat” so that oxygen consumption during entry into torpor falls almost to the minimum level for a given body temperature. Heat loss during entry into torpor appears to be facilitated by an increase in thermal conductance.During arousal from torpor, body temperature increases faster than can be accounted for assuming maximum heat production and minimum heat loss. This could be explained by anaerobic heat production and by a decreased thermal conductance resulting from the posterior vasoconstriction typical of arousing hibernators.Torpor periods of short duration are feasible for P. californicus, for it can enter torpor and arouse immediately thereafter at an ambient temperature of 15° with an expenditure of energy only 55% of that required to maintain a high body temperature over the same period of time. Arousal from torpor at an ambient temperature of 15° requires about 75% of the total energy expended during a ten hour torpor cycle; entry into torpor and torpor itself account for only 9 and 16% of the total energy expenditure, respectively.The quantitative relations between heat production and heat loss presented in this paper suggest further investigations of the effects of body size on heat production and loss, and of physiological phenomena which alter heat exchange.