Abstract
Hummingbirds and nectar bats coevolved with the plants they visit to feed on floral nectars rich in sugars. The extremely high metabolic costs imposed by small size and hovering flight in combination with reliance upon sugars as their main source of dietary calories resulted in convergent evolution of a suite of structural and functional traits. These allow high rates of aerobic energy metabolism in the flight muscles, fueled almost entirely by the oxidation of dietary sugars, during flight. High intestinal sucrase activities enable high rates of sucrose hydrolysis. Intestinal absorption of glucose and fructose occurs mainly through a paracellular pathway. In the fasted state, energy metabolism during flight relies on the oxidation of fat synthesized from previously-ingested sugar. During repeated bouts of hover-feeding, the enhanced digestive capacities, in combination with high capacities for sugar transport and oxidation in the flight muscles, allow the operation of the “sugar oxidation cascade”, the pathway by which dietary sugars are directly oxidized by flight muscles during exercise. It is suggested that the potentially harmful effects of nectar diets are prevented by locomotory exercise, just as in human hunter-gatherers who consume large quantities of honey.
Highlights
Hummingbirds and nectar bats became nectarivorous animals in a process that involved coevolution with the flowering plants offering them nectar [1]
A species that is unable to rely to the same extent on the direct oxidation of dietary sugar to fuel exercise and that suffers from the adverse effects of excessive sugar ingestion
Vmax values serve as relative measures of mitochondrial content [41] and show that nectar bat and hummingbird flight muscles have much higher mitochondrial oxidative capacities than shrew and rat leg muscles
Summary
Hummingbirds and nectar bats became nectarivorous animals in a process that involved coevolution with the flowering plants offering them nectar [1]. As their diets, foraging and feeding modes evolved, so did the suite of morphological, physiological and biochemical traits that made them adapted for “aerial refueling” [2,3,4]. Hummingbirds rely mainly on the sugars in floral nectar to fuel their high metabolic rates [5]. Some nectar bat species can hover while feeding [6], behaving as “hummingbirds of the night”. A species that is unable to rely to the same extent on the direct oxidation of dietary sugar to fuel exercise and that suffers from the adverse effects of excessive sugar ingestion
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