MCKECHNIE, A.E.; LOVEGROVE, B.G.: Thermoregulation in mousebirds (Coliiformes): the role of clustering behavior.
The mousebirds (Coliiformes) are a phylogenetically ancient order endemic to the Afrotropics. From an energetic perspective, the mousebirds are of interest in that they exhibit well-developed communal roosting (clustering) as well as a pronounced capacity for torpor. We investigated thermoregulation in two species (Colius colius and C. striatus) under laboratory, semi-natural and natural conditions. Under laboratory conditions, the rest-phase body temperature (Tb) of single birds was partly dependent on ambient temperature (Ta). Neither species defended a constant Tb setpoint during the rest-phase, as typically occurs in endotherms. Rather, Tb decreased during the course of the rest-phase. In C. colius, the rate of Tb cooling was strongly dependent on Ta, and several single birds became pathologically hypothermic, even at Tas well above those normally experienced by free-ranging individuals. Clustering behavior in C. colius led to energy savings of up to 50%, and facilitated the defence of an approximately constant rest-phase Tb. Clustering behavior in C. striatus moderated hypothermic responses to restricted feeding under semi-natural conditions. In free-ranging C. colius, clustering behavior facilitated the defence of a relatively constant Tb even at low Ta, although on one occasion a group of ca. 10 individuals exhibited a synchronized shallow hypothermic response. We argue that in the Coliiformes, thermoregulation and sociality may have evolved in tandem. Moreover, the mousebirds seem to represent an exception to the observation that avian torpor is generally more pronounced in free-ranging birds than in captivity.
