Evidence for a circadian rhythm in the oxygen consumption of resting angulate tortoise (Chersina angulata))

Abstract

Some ectotherms show daily fluctuations in their metabolic rate (MR) within their experienced temperature range. The rhythms may disappear or persist in constant conditions in the laboratory. The resting oxygen consumption (RVO2) of resting Chersina angulata was measured using open flow through respirometry over 24 hours to detect the presence of daily fluctuations in MR. The tortoises were acclimated in an environmentally controlled room to an ambient temperature of 26 ± 1 °C and a 14L:10D light regime prior to the resting VO2 measurements. The VO2 was measured when tortoises were resting at a constant temperature of 26 ± 1 °C and at three different light regimes, 14L:10D, DD and LL. Daily rhythms in the MR were detected at all three light regimes. However, the rhythm was more pronounced at 14L:10D than at DD and LL. Higher values of resting MR were registered in the early hours of the day, followed by a decrease as the day progressed, while the lowest values were recorded in the late hours of the day. Under constant conditions, the rhythmic pattern persisted, but with smaller amplitudes. Consequently, the rhythm is considered to be circadian and endogenous.