Heterothermic responses in the speckled mousebird (Colius striatus)

We investigated normothermic thermoregulation and heterothermic responses to restricted food in the speckled mousebird Colius striatus, in the context of the widely accepted distinction between normothermia, rest-phase hypothermia, and torpor. Normothermic thermoregulation differed from typical endothermic patterns in that rest-phase body temperature (Tb) was not maintained with respect to a constant setpoint. Instead, Tb decreased during the course of the rest-phase, with the highest cooling rates observed at moderate ambient temperatures (Ta). Restricted food was associated with significant reductions in rest-phase Tb and metabolic rate. The lowest Tb recorded in a bird which was able to arouse spontaneously, was 18.2 degrees C. However, we were unable to clearly discern between normothermic, hypothermic and torpor Tb ranges. Furthermore, heterothermic responses did not accord with the patterns typically observed in birds and mammals. Metabolic suppression normally associated with entry into torpor and the defence of a torpor Tb setpoint was largely absent. The mousebirds significantly reduced their energy expenditure when heterothermic at moderate TaS only. We suggest that the observed patterns of thermoregulation in C. striatus, as well those previously reported in Colius colius, are associated with plesiomorphic clustering behaviour in the Coliiformes, and the tandem evolution of behavioural and metabolic thermoregulation.