Meeting Abstract
P3.127 Jan. 6 Rapid turnover of newly ingested sugars in the pool of oxidized substrates in hovering hummingbirds WELCH, KC*; SUAREZ, RK; Univ. of California, Santa Barbara; Univ. of California, Santa Barbara k_welch@lifesci.ucsb.edu
Hovering hummingbirds exhibit some of the highest mass-specific rates of aerobic metabolism among vertebrates. Much like nectar-feeding insects, hummingbirds obtain most of their dietary calories from the simple sugars in floral nectar during hovering flight. Previous studies on small hummingbirds revealed extremely high intestinal capacities for both active and passive absorption of glucose as well as high enzymatic capacities for muscle glucose oxidation. These traits led us to hypothesize that hovering hummingbirds directly fuel their flight muscles with dietary sugar and that newly ingested sugars appear and disappear rapidly within the pool of oxidized substrates. Respiratory quotients (RQ, equal to rates of CO2 production divided by rates of O2 consumption) estimated by flow-through respirometry indicate that fasted hummingbirds rapidly switch from oxidizing lipids (RQ = 0.7) to oxidizing carbohydrates (RQ = 1.0) within minutes of feeding. By combining this indirect respirometric approach with a stable carbon isotope analysis of respired breath, we show that broad-tailed (Selasphorus platycercus), rufous (S. rufus), and Anna�s hummingbirds (Calypte anna) incorporate newly ingested sugar into the pool of oxidized substrates at a rate of approximately 10% per minute. Additionally, the isotopic signature of specific sugars disappears quickly from the pool of oxidized substrates when those sugars are replaced with sugars characterized by a different isotopic signature. Within 45 minutes of access to a sugar solution, virtually all of hovering metabolism is supported by these newly ingested sugars. This capacity, found also in nectarivorous insects, is unique among vertebrates and suggests convergence in physiological and biochemical traits of hovering pollinators.
