Sugar Metabolism in Hummingbirds and Nectar Bats

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.

The silence of prey: Hummingbirds do not respond to potential ambush predators on flowers

“…2019). As nectar production in Heliconiaceae is very limited, these birds need to visit a high number of flowers to meet their physiological needs (Suarez & Welch 2017). Hence, the risk of being preyed upon in the presence of the potential but infrequent predator is compensated by the benefit of the energetic reward (but see Lima 1998).…”

“…2018). Hummingbirds are nectarivorous and spend most of their active period foraging on flowers (Suarez & Welch 2017). Therefore, hummingbirds represent one of the most important vertebrate pollinator groups and also a good model to investigate the ecological outcomes of the interaction between vertebrate pollinators and ambush predators.…”

Induced pluripotent stem cells as a tool for comparative physiology: lessons from the thirteen-lined ground squirrel

“…In work propelled by the fast-developing stem cell field, iPSCs have been successfully generated from non-mammalian vertebrate and invertebrate species (Rossello et al, 2013), offering a great opportunity for comparative biologists to cultivate cells and even tissues from rare organisms or from those which cannot be bred in the laboratory. For example, hummingbirds have attracted comparative biologists' interests because of their exceptional visual (Gaede et al, 2017) and metabolic (Suarez and Welch, 2017;Welch and Suarez, 2007) adaptations that facilitate their rapid flight. If hummingbird iPSCs were available, brain and retina organoids or other suitable cell types could be derived, and nutrients in the culture media could be manipulated and monitored, paving the way to the in vitro discovery of the neuronal and metabolic adaptive mechanisms that underlie the extraordinary performance of the whole animal.…”

Neonicotinoid pesticides exert metabolic effects on avian pollinators

“…This family of small, energetically extreme animals regularly experience relatively large variations in body weight throughout the day as they feed, urinate, and expend energy. Hummingbirds can lose up to 10% of their mass overnight 36 . Therefore, we applied respirometry techniques and predicted that exposure to imidacloprid would induce a dose-dependent decrease in energy expenditure and subsequently, feeding and flying behaviour 32,33,37 .…”