Metabolic and Thermal Responses to Heat and Cold in Streamertail Hummingbirds (Trochilus polytmus and Trochilus scitulus, Trochilidae)

Abstract

Metabolic rate and body temperature were measured simultaneously in Trochilus polytmus and Trochilus scitulus, endemic Jamaican hummingbirds, at ambient temperatures from 00 to 41?C. Between 220 and 29?C both species showed a metabolic plateau. At higher temperatures they became hyperthermic and showed a steady reduction of metabolic rate. HUMMINGBIRDS HAVE the highest metabolic rates of all homoiothermic animals (Lasiewski 1967, Pearson 1950). The energy requirements and the thermoregulatory system of these diminutive birds are especially interesting to biologists. Our present knowledge of metabolism in Trochilidae is based mainly on studies of North and Central American species (Bartholomew 1957, Lasiewski 1963, 1967, Pearson 1950, 1953, 1954, Withers 1976, Wolf and Hainsworth 1972). In these investigations the minimal measured metabolic rate was generally interpreted as standard metabolic rate and these values were compared with standard metabolism values at thermoneutrality for larger bird species. In all hummingbird experiments, the minimum (standard) metabolic rates were evaluated without sufficient information about their relationship to thermoneutrality. Therefore, in our experiments we recorded simultaneously ambient temperature, body temperature, oxygen consumption, carbon dioxide production, and the onset of panting in freely moving Trochilus polytmus and Trochilus scitulus. Our data reveal that thermoregulation of these endemic Jamaican hummingbirds cannot be explained readily by classical concepts of thermoregulation as applied to laraer species of birds and mammals (Heldmaier 1975, Scholander et a1. 1950, Wunder et al. 1977). METHODS AND MATERIALS Sixteen streamertail hummingbirds were trapped in September 1976 at Green Hills and Hectors River in Jamaica, and kept at our institute in Germany in indoor aviaries until the beginning of the experiments in June 1977. The birds were fed with artificial nectar consisting of a mixture of amino acids, glucose, fructose, and vitamins. Trochilus polytmus (average weight 6.3 g, n=55) and T. scitulus (average weight 4.9 g, n 5) were kept in plastic boxes (2.8 1), each provided with a perch and a feeder which allowed food intake and free movement. They were then exposed to a range of ambient temperatures from 00 to 41?C. All tests were performed between 8 a.m. and 6 p.m. at a light intensity of 1200 LUX. Body temperature (Tb) was measured using a copper-constantan thermocouple (0 0.1 mm) fixed axillarily under the wing. Ambient temperature (Ta) was measured by a thermocouple attached to the inside wall of the box. Oxygen consumption and carbon dioxide production of the hummingbirds was measured in an open-flow system by a 2-channel paramagnetic 02 analyzer (Oxytest, Hartmann and Braun, accuracy ? 0.1 % 02), and a 2channel infra-red CO2 analyzer (Uras 2T, Hartmann and Braun, accuracy ?0.1C% C02). Effluent air was dried and adjusted to a flow rate of 50 1 per hour. All four variables (body temperature, ambient temperature, oxygen consumption, carbon dioxide production) were recorded continuously. Measurements were made only after the birds had been sitting quietly for at least five minutes. Metabolic values were corrected to standard temperature and pressure.