Heterothermy in mousebirds: Evidence of avian proto-torpor?

Patterns of normothermy and heterothermy in two mousebird species differed in several respects from typical endothermic patterns. Primarily, heterothe rmic responses in Colius striatus lacked the entry and maintenance phases c haracteristic of typical torpor bouts. These observations suggest that mous ebirds may exhibit "proto-torpor", a form of torpor intermediate between hy pothesized ancestral wide-amplitude T-b, cycling and modern heterothermy. C lustering behaviour also formed an obligatory component of thermoregulation , and was necessary for the defence of a constant T-b, during the rest-phas e. The evolution of typical avian torpor appears to have been arrested in t he mousebirds by the development of sociality and clustering.