Metabolic depression during dormancy in the African tortoiseKinixys spekii

Abstract

Two hypotheses may explain the lack of metabolic depression at low temperature in hibernating reptiles from warm but not cool climates. (1) Physiological ability; reptiles from cool climates have resting metabolic rates elevated at low temperatures and so have greater potential for metabolic depression. (2) Energy benefit; hibernation is generally short in warm climates, so the energy saving possible at low temperature is small. These hypotheses can be separated in a warm-climate reptile that hibernates for a long period, such as the African tortoise Kinixys spekii, which is dormant for 7–8 months a year, in winter and in the following hot dry season. Dormant tortoises had a reduced rate of oxygen consumption [Formula: see text] between 10 and 25 °C compared with resting [Formula: see text] measured at the same temperatures in the active season, and at 10–15 °C compared with the resting [Formula: see text] of controls kept active in winter. These data are consistent with hypothesis 2 but not hypothesis 1. The metabolic rate during dormancy was, on average, 59% of resting [Formula: see text], with minimum rates of 47–48% at 10 and 25 °C, similar to the minimum rates of about 50% of resting [Formula: see text] in other hibernating reptiles. Aestivating ectotherms achieve greater metabolic depression, to about 25% of resting [Formula: see text], possibly as a result of reduced gas exchange.