Body Temperatures of Incubating versus Non-Incubating Roadrunners

Abstract

Core temperatures of free-living Roadrunners (Geococcyx californianus) were monitored with temperature-sensitive radio transmitters. Breeding males were found to perform all of the nocturnal incubation and to maintain normothermic body temperatures at night. Body temperatures of females and non-breeding males dropped at night to as low as 330C, depending on the ambient temperature. Facultative, short-term hypothermia appears to be a regular energysaving feature of this species that is conspicuously not employed by the nocturnal incubator. Most birds exhibit a circadian pattern of body temperature, involving a reduction of 1 to 30C during the nighttime hours (Dawson 1954, Aschoffand Pohl 1970). Lower nocturnal body temperatures appear to be due to reduced motor activity and other cyclic phenomena of sleep, since nocturnally active birds show a reversal of the pattern, i.e., lower diurnal body temperatures. Only a handful of species exhibit true hypothermia, defined operationally as a reduction of body temperature below 360C that may or may not involve torpor (Dawson and Hudson 1970). Facultative hypothermia is generally argued to be an energy-saving adaptation in response to periodic food shortages and/or excessively low ambient temperatures (Calder and King 1974). Species that exhibit hypothermia include nectar-feeding hummingbirds, aerial insectivores such as caprimulgids, swallows and swifts, small passerines overwintering in cold climates, and a few tropical species such as anis and colies (Dawson and Hudson 1970, Calder and King 1974). It has been argued that short-term hypothermia should occur only in small animals due to the theoretically prohibitive effects of thermal inertia and high rewarming cost for large animals (Calder and King 1974), yet vultures are known to lower body temperatures to 340C. Hypothermia is a complex phenomenon for which the costs and benefits are still poorly understood. The constraints that hypothermia places on breeding birds have not been explored. In particular, incubation and hypothermia may not be compatible. Incubating hummingbirds have generally been found to maintain normothermic body temperatures at night except during unusual cold (Calder 1971, Vleck 1982). Egg temperatures fall well below normal incubation temperatures during these lapses into hypothermia. It is not clear whether non-incubators under similar conditions would show the same or different responses. If incubators typically maintain high body temperatures under conditions when non-incubators show hypothermia, then incubation may be metabolically expensive. Furthermore, the female of a facultatively hypothermic species may be energetically constrained during incubation if she has laid a large clutch of eggs and must recuperate depleted reserves via hypothermia. Thus facultative hypothermia may have important implications for both clutch size and the sex that performs the incubation. The Roadrunner (Geococcyx californianus) is a large (350 g) cuckoo that has been reported to lower its body temperature under stressful conditions and to warm up by sunning (Ohmart and Lasiewski 1971). Hypothermic birds have been found to conserve a significant amount of energy compared to normothermic birds at the same ambient temperature. The Roadrunner is also among the few species in which the male performs most of the incubation, especially at night (Calder 1967, Ohmart 1973, Skutch 1976). In order to obtain a better understanding of the breeding energetics of this unusual bird, I monitored the body temperatures of free-living Roadrunners using radio-telemetry techniques while simultaneously following breeding activities. I here report significant differences in body temperature for Roadrunners as a function of sex and