Stoking the brightest fires of life among vertebrates

Abstract

Hummingbirds and nectarivorous bats in flight display some of the highest rates of aerobic metabolism among vertebrates. Analysis of the pathway of oxygen, i.e., the oxygen transport cascade, reveals the concerted upregulation of capacities for O2 flux from the external environment, through the respiratory and cardiovascular systems, into muscle mitochondria. Pathways for aerobic energy metabolism are highly conserved, but enzymatic capacities for carbohydrate and fatty acid oxidation, as well as for aerobic ATP synthesis, are also upregulated in concert. Despite evidence indicating sufficient capacities for fatty acid oxidation to support hovering, repeated bouts of hover-feeding in hummingbirds and nectar bats involve the oxidation of carbohydrate. Recent studies reveal that recently ingested sugar directly fuels flight, giving rise to the concept of the sucrose oxidation cascade. The ecological and bioenergetic advantages conferred by sugar oxidation during foraging are discussed.