Turtles (<Emphasis Type="Italic">Chelodina longicollis</Emphasis>) regulate muscle metabolic enzyme activity in response to seasonal variation in body temperature

Fluctuations in the thermal environment may elicit different responses in animals: migration to climatically different areas, regulation of body temperature, modification of biochemical reaction rates, or assuming a state of dormancy. Many ectothermic reptiles are active over a range of body temperatures that vary seasonally. Here we test the hypothesis that metabolic enzyme activity acclimatises seasonally in freshwater turtles (Chelodina longicollis) in addition to, or instead of, behavioural regulation of body temperatures. We measured body temperatures in free-ranging turtles (n=3) by radiotelemetry, and we assayed phosphofructokinase (PFK), lactate dehydrogenase (LDH), citrate synthase (CS) and cytochrome c oxidase (CCO) activities in early autumn (March, n=10 turtles), late autumn (May, n=7) and mid-winter (July, n=7) over a range of assay temperatures (10 °C, 15 °C, 20 °C, 25 °C). Body temperatures were either not different from, or higher than expected from a theoretical null-distribution of a randomly moving animal. Field body temperatures at any season were lower, however, than expected from animals that maximised their sun exposure. Turtles maintained constant PFK, LDH and CCO activities in different months, despite body temperature differences of nearly 13.0 °C between March (average daily body temperature=24.4 °C) and July (average=11.4 °C). CS activity did not vary between March and May (average daily body temperature=20.2 °C), but it decreased in July. Thus C. longicollis use a combination of behavioural thermoregulation and biochemical acclimatisation in response to seasonally changing thermal conditions. Ectothermic reptiles were often thought not to acclimatise biochemically, and our results show that behavioural attainment of a preferred body temperature is not mandatory for activity or physiological performance in turtles.Abbreviations CS citrate synthase - CCO cytochrome c oxidase - LDH lactate dehydrogenase - PFK phosphofructokinase Communicated by I.D. Hume