Nocturnal Hypothermia and Oxygen Consumption in Manakins

-Red-capped Manakins (Pipra mentalis) and Golden-collared Manakins (Manacus vitellinus), two species of small, frugivorous, tropical passerines, showed facultative hypothermia and reduced oxygen consumption at night. The lowest body temperature measured in M. vitellinus (30.5”C, when the ambient temperature was 19.5”C) represents a reduction of more than 40% in the difference between body temperature and ambient temperature. Under these circumstances, a reduction of more than 40% in the rate of oxygen consumption would also occur during the period of regulated hypothermia. Even moderate hypothermia at ambient temperatures typical of the lowland tropics substantially reduces the energy expenditure of a resting bird. Hypothermia (lowered body temperature) has been regarded as an adaptive response by an organism to varying combinations of environmental and internal conditions that cause energy shortages either on a daily or a seasonal basis. Such factors include (1) small size because of the high mass-specific metabolism associated with it and the inability of small organisms to carry large energy reserves either as fat or in the gut, (2) insectivory or nectarivory and associated extreme fluctuations in the availability of food and/or rapid passage of food through the gut, (3) low ambient temperature (T& and (4) types of inclement weather that prohibit foraging and/or increase heat loss (see Bartholomew 1972, Hainsworth and Wolf 1978). Degrees of hypothermia have been documented in at least eight avian orders. In the Caprimulgiformes and in the Apodiformes (Trochilidae), body temperature (TJ may drop below 20°C and sometimes approach ambient temperature (even T,‘s as low as 7°C). More moderate nightly drops in T, to between 20 and 30°C have been reported in the Coliiformes, Columbiformes, Apodiformes (Apodidae), and Passeriformes (Nectarinidae). Diurnal cycles in normothermic T, are well known, however, and it is not yet possible to distinguish clearly between these cycles and the varying degrees of nocturnal hypothermia (T,,‘s as low as 30°C) reported in Cuculiformes, Falconiformes, Strigiformes, and the passerine families Nectariniidae, Hirundinidae, Paridae, Ploceidae, and Fringtlhdae. (See Dawson and Hudson [1970], Calder and King [1974], Chaplin [1976], and Biebach [1977].) In lowland tropical environments, ambient air temperatures rarely drop more than 15-20°C below the T, of normothermic birds. Since the rate of heat production in endotherms is proportional to the difference between T, and T,, a decrease of 8-10°C in regulated T, would decrease this difference by half and would yield an energy savings of about 50% during the period of hypothermia. Such fluctuations in T, have been reported in the Smooth-billed Ani (Crotophaga anz] in Panama (Warren 1960) and several Brazilian hummingbirds (Morrison 1962). We document here similar low nocturnal T,‘s in two species of small, tropical passerines, the Red-capped Manakin (Pipra mentalis) and the Golden-collared Manakin (Manacus vitellinus). These birds are common in the understory of second-growth forests in Panama and are almost entirely frugivorous. Because food passes through their gut very rapidly (in about 18 min), their feeding bouts are brief but frequent throughout the day (Worthington, in press). Thus, manakins are vulnerable to daily and seasonal food shortages (Foster 1977; Worthington, in press). Vleck and Vleck (1979) suggested that manakins may reduce their resting metabolic rate to ameliorate the effects of such energy shortages. Our study was undertaken to examine this possibility, and our data demonstrating hypothermia and associated low metabolic rates support their suggestion. MATERIALS AND METHODS Birds were mist-netted on Barro Colorado Island, Panama (9”N, SO’W). Three individuals (1 s and 1 P P. mentalis, 1 P M. vitellinus) were maintained in captivity for at least one week before we measured their rates of oxygen consumption (VoJ. On some occasions an individual was fasted from noon until