Does behavioural hypothermia promote post-exercise recovery in cold-submerged frogs?

Abstract

At the low temperatures of the overwintering environment of the frog Rana temporaria, small changes in ambient temperature have large effects on metabolism and behaviour, especially since Q10 values are often greatly elevated in the cold. How the overwintering aquatic frog copes with variable thermal environments in terms of its overall activity metabolism and recovery from pursuit by predators is poorly understood, as is the role of behavioural thermoregulation in furthering recovery from intense activity. Exhaustive exercise was chosen as the method of evaluating activity capacity (defined by time to exhaustion, total distance swum and number of leg contractions before exhaustion) and was determined at 1.5 and 7 degreesC. Other cohorts of frogs were examined at both temperatures to determine the metabolic (acid-base, lactate, glucose, ATP and creatine phosphate) and respiratory responses to exercise in cold-submerged frogs. Finally, temperature preference before and after exercise was determined in a thermal gradient to define the importance of behavioural thermoregulation on the recovery rates of relevant metabolic and respiratory processes. Activity capacity was significantly reduced in frogs exercised at 1.5 versus 7 degreesC, although similar levels of tissue acid-base metabolites and lactate were reached. Blood pH, plasma PCO2 and lactate levels recovered more rapidly at 1.5 degreesC than at 7 degreesC; however, intracellular pH and the recovery of tissue metabolite levels were independent of temperature. Resting aerobic metabolic rates were strongly affected by temperature (Q10=3.82); however, rates determined immediately after exercise showed a reduced temperature sensitivity (Q10=1.67) and, therefore, a reduced factorial aerobic scope. Excess oxygen consumption recovered to resting values after 5–6.25 h, and 67 % recovery times tended to be slightly faster at the lower temperatures. Exercise in the cold, therefore, provided an immediately higher factorial scope, which could be involved in the faster rate of recovery of blood lactate levels in the colder frogs. In addition, exercise significantly lowered the preferred temperature of the frogs from 6.7 to 3.6 degreesC for nearly 7 h, after which they returned to their normal, unstressed preferred temperatures. Thus, a transient behavioural hypothermia in the skin-breathing, overwintering frog may be an important strategy for minimising post-exercise stress and maintaining aerobic metabolism during recovery from intense activity.