Respiratory Properties of Blood and Myoglobin in Hummingbirds

Abstract

Blood respiratory properties were studied in several species of Brazilian hummingbirds. Three species, Melanotrochilus fuscus, Eupetomena macroura, and Amazilia versicolor, were studied with respect to O₂-Hb equilibrium curves and effect of pH and temperature. O₂ equilibrium of purified hemolyzates and myoglobin solutions from pectoral muscle were studied for Melanotrochilus fuscus. Hematocrit, hemoglobin content, and myoglobin concentration were studied for 11 species. Hematocrits averaged 56.3%; corresponding O₂ capacity was 22.1 vol%, and MCHC 27.0%. Average blood 02 affinity for the three species was 44.0 mmHg at pH 7.40 and 39 C. The Bohr coefficient ø was -0.39. The Hill plots were nonlinear and had increasing n-values at higher 02 saturations. The influence of temperature on the O₂-Hb equilibrium of whole blood was moderate, expressed by ΔH values of 7.0-8.0 Kcal mol⁻¹ The data are discussed with focus on the rapid changes in O₂ uptake and body temperature in hummingbirds associated with entry and arousal from torpor and the high O₂ uptakes of hovering flight. Hummingbird blood shows a very high unloading efficiency for 02. The increase in n-value at higher O₂ saturation is important because it prevents arterial desaturation during flight and nocturnal hypothermia.