Patterns of ultradian rhythms of activity and metabolic rate in relation to average daily energy expenditure in root voles

Abstract

Microtine species are known to have an ultradian pattern of activity. Earlier study suggested that period of activity oscillations decreased after magnification of the average energy expenditure. Moreover, the lack of daily variation in nonshivering thermogenesis capacity suggests that the level of metabolic rate (MR) depends mainly on animal activity. Thus, (1) one could expect that ultradian oscillations of MR and activity were similarly dependent on the average level of energy expenditure. Moreover, I expected that animals at lower ambient temperature (T_a) should minimize their energy expenditure and the time of exposure to low T_a outside the nest. Thus (2) their mean activity near a feeder does not increase together with average level of energy expenditure. I tested these hypotheses using root voles Microtus oeconomus Pallas, 1776 acclimated to T_a of 20°C (12L∶12D). The MR and activity both inside (A_n) and outside the nest (A_f), were measured continuously at T_a of 10 and 20°C. Average oxygen consumption differed between T_as. The period of MR and A_f rhythm was shorter at lower T_a, but the period of A_n did not differ significantly. The level of energy expenditure did not affect mean A_f. Close correlation between MR and A_f length period suggests that oscillations of MR during the day are affected by activity related to feeding, rather than low cost A_n.