Thermoregulatory performance of fledgling American coots (Fulica americana) in air and water

Abstract

The precision and metabolic costs of temperature regulation in air and water were studied in hand-reared fledgling American coots (Fulica americana). In birds acclimated to both 23 and 8 °C, cloacal temperature was independent of air temperature over an air temperature range of −10 to 33 °C. Minimum resting metabolic rate and thermal conductance averaged 1.2–1.6 times higher in water than in air, and neither variable was significantly affected by cold acclimation. Despite differences in minimal thermal conductance, the lower critical temperature was similar in air and water (19–21 °C). Trunk skin temperature averaged 3.4–4.3 °C below cloacal temperature in both media, and did not differ between dorsal and ventral aspects of the bird. Peripheral cooling was most pronounced in submerged appendages. Coots displayed a high resistance to immersion hypothermia; cloacal temperature remained stable at 40–41 °C during 4 h exposure to 5 °C water. Exposure to a −10 °C atmosphere of helium–oxygen (helox) similarly failed to induce even mild hypothermia in all but one of the birds tested. The unusual tolerance of F. americana to helox and immersion in cold water could not be attributed to an enhanced thermogenic ability. Maximum resting metabolic rate of birds exposed to cold was only 3.0–3.5 times the thermoneutral rate. Aquatic homeothermy in this species may instead reflect superior insulation and exceptional buoyancy provided by a dense, water-repellent plumage.