Editing Hummingbird (section)

=== Metabolism ===

Hummingbirds have the highest metabolism of all vertebrate animals – a necessity to support the rapid beating of their wings during hovering and fast forward flight.<ref name=Hargrove/><ref>{{Cite journal |last1=Altshuler |first1=D.L. |last2=Dudley |first2=R. |year=2002 |title=The ecological and evolutionary interface of hummingbird flight physiology |journal=The Journal of Experimental Biology |volume=205 |issue=Pt 16 |pages=2325–336 |doi=10.1242/jeb.205.16.2325 |pmid=12124359|bibcode=2002JExpB.205.2325A |url=https://journals.biologists.com/jeb/article/205/16/2325/9117/The-ecological-and-evolutionary-interface-of|url-access=subscription }}</ref> During flight and hovering, oxygen consumption per gram of muscle tissue in a hummingbird is about 10 times higher than that measured in elite human athletes.<ref name="suarez"/> Hummingbirds achieve this extraordinary capacity for oxygen consumption by an exceptional density and proximity of capillaries and [[mitochondrion|mitochondria]] in their flight muscles.<ref name="suarez91">{{cite journal |author1=Suarez R.K. |author2=Lighton J.R. |author3=Brown G.S. |author4=Mathieu-Costello O. |title=Mitochondrial respiration in hummingbird flight muscles |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=88 |issue=11 |pages=4870–3 |date=June 1991 |pmid=2052568 |pmc=51768 |doi=10.1073/pnas.88.11.4870|bibcode=1991PNAS...88.4870S |doi-access=free }}</ref>

Hummingbirds are rare among vertebrates in their ability to rapidly make use of ingested sugars to fuel energetically expensive hovering flight, powering up to 100% of their metabolic needs with the sugars they drink.<ref>{{Cite journal |last1=Welch |first1=K.C. Jr. |last2=Chen |first2=C.C. |year=2014 |title=Sugar flux through the flight muscles of hovering vertebrate nectarivores: A review |journal=Journal of Comparative Physiology B |volume=184 |issue=8 |pages=945–959 |doi=10.1007/s00360-014-0843-y |pmid=25031038 |s2cid=11109453}}</ref> Hummingbird flight muscles have extremely high capacities for [[Redox|oxidizing]] [[carbohydrate]]s and [[fatty acid]]s via [[hexokinase]], [[carnitine palmitoyltransferase]], and [[citrate synthase]] [[enzyme]]s at rates that are the highest known for vertebrate [[skeletal muscle]].<ref name="fuel">{{cite journal | last1=Suarez |first1=R.K. |last2=Lighton |first2=J.R. |last3=Moyes |first3=C.D.|last4=Brown|first4=G.S.|last5=Gass|first5=C.L.|last6=Hochachka |first6=P.W. |display-authors=3 | title = Fuel selection in rufous hummingbirds: ecological implications of metabolic biochemistry | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 87 | issue = 23 | pages = 9207–10 | date = 1 December 1990 | pmid = 2251266 | pmc = 55133 | doi = 10.1073/pnas.87.23.9207|bibcode=1990PNAS...87.9207S |doi-access=free }}</ref> To sustain rapid wingbeats during flight and hovering, hummingbirds expend the [[human equivalent]] of 150,000 [[calorie]]s per day,<ref name="bartlett">{{cite web |last1=Barlett |first1=Paige |title=Fueling the hummingbird's extreme biology |url=https://www.hopkinsmedicine.org/research/advancements-in-research/fundamentals/in-depth/fueling-the-hummingbirds-extreme-biology |publisher=Johns Hopkins Medicine |access-date=27 March 2023 |date=2018 |archive-date=22 March 2023 |archive-url=https://web.archive.org/web/20230322213148/https://www.hopkinsmedicine.org/research/advancements-in-research/fundamentals/in-depth/fueling-the-hummingbirds-extreme-biology |url-status=dead }}</ref> an amount estimated to be 10 times the energy consumption by a [[marathon]] runner in competition.<ref name="campbell">{{cite web |last1=Campbell|first1=Don |title=Hummingbird metabolism unique in burning glucose and fructose equally |url=https://utsc.utoronto.ca/news-events/archived/hummingbird-metabolism-unique-burning-glucose-and-fructose-equally |publisher=University of Toronto - Scarborough |access-date=27 March 2023 |date=3 December 2013}}</ref>

Hummingbirds can use newly ingested sugars to fuel hovering flight within 30–45 minutes of consumption.<ref name="chen">{{Cite journal |last1=Chen|first1=Chris Chin Wah |last2=Welch|first2=Kenneth Collins |year=2014 |title=Hummingbirds can fuel expensive hovering flight completely with either exogenous glucose or fructose |journal=Functional Ecology |volume=28 |issue=3 |pages=589–600 |doi=10.1111/1365-2435.12202 |doi-access=free|bibcode=2014FuEco..28..589C }}</ref><ref>{{Cite journal |last1=Welch |first1=K.C. Jr. |last2=Suarez |first2=R.K. |year=2007 |title=Oxidation rate and turnover of ingested sugar in hovering Anna's (''Calypte anna'') and rufous (''Selasphorus rufus'') hummingbirds |journal=Journal of Experimental Biology |volume=210 |issue=Pt 12 |pages=2154–162 |doi=10.1242/jeb.005363 |pmid=17562889 |doi-access=free|bibcode=2007JExpB.210.2154W }}</ref> These data suggest that hummingbirds are able to oxidize sugar in flight muscles at rates rapid enough to satisfy their extreme metabolic demands {{ndash}} as indicated by a 2017 review showing that hummingbirds have in their flight muscles a mechanism for "direct oxidation" of sugars into maximal [[Adenosine triphosphate|ATP]] yield to support a high metabolic rate for hovering, foraging at altitude, and migrating.<ref name="Suarez">{{Cite journal |last1=Suarez |first1=Raul |last2=Welch |first2=Kenneth |date=12 July 2017 |title=Sugar metabolism in hummingbirds and nectar bats |journal=Nutrients |volume=9 |issue=7 |page=743 |doi=10.3390/nu9070743 |issn=2072-6643 |pmc=5537857 |pmid=28704953 |doi-access=free}}</ref> This adaptation occurred through the [[natural selection|evolutionary]] loss of a key [[gene]], [[fructose-bisphosphatase 2]] (''FBP2''), coinciding with the onset of hovering by hummingbirds estimated by fossil evidence to be some 35 million years ago.<ref name="callier">{{cite web |first=Viviane |last=Callier |title=Evolution Turns These Knobs to Make a Hummingbird Hyperquick and a Cavefish Sluggishly Slow|url=https://www.scientificamerican.com/article/evolution-turns-these-knobs-to-make-a-hummingbird-hyperquick-and-a-cavefish-sluggishly-slow/|publisher=Scientific American |date=24 February 2023 |access-date=27 February 2023}}</ref><ref name="osipova">{{cite journal|display-authors=3 |last1=Osipova |first1=Ekaterina |last2=Barsacchi |first2=Rico |last3=Brown |first3=Tom |last4=Sadanandan |first4=Keren |last5=Gaede |first5=Andrea H. |last6=Monte |first6=Amanda |last7=Jarrells |first7=Julia |last8=Moebius |first8=Claudia |last9=Pippel |first9=Martin |last10=Altshuler |first10=Douglas L. |last11=Winkler |first11=Sylke |last12=Bickle |first12=Marc |last13=Baldwin |first13=Maude W. |last14=Hiller |first14=Michael |title=Loss of a gluconeogenic muscle enzyme contributed to adaptive metabolic traits in hummingbirds |journal=Science|volume=379 |issue=6628 |date=2023-01-13 |issn=0036-8075 |doi=10.1126/science.abn7050 |pages=185–190|pmid=36634192 |bibcode=2023Sci...379..185O |s2cid=255749672 |url=https://www.science.org/doi/10.1126/science.abn7050|url-access=subscription }}</ref> Without ''FBP2'', [[glycolysis]] and mitochondrial respiration in flight muscles are enhanced, enabling hummingbirds to metabolize sugar more efficiently for energy.<ref name=callier/><ref name=osipova/>

By relying on newly ingested sugars to fuel flight, hummingbirds reserve their limited fat stores to sustain their overnight [[fasting]] during torpor or to power migratory flights.<ref name="chen"/> Studies of hummingbird metabolism address how a [[bird migration|migrating]] ruby-throated hummingbird can cross {{Convert|800|km|mi|abbr=on}} of the [[Gulf of Mexico]] on a nonstop flight.<ref name="Hargrove"/> This hummingbird, like other long-distance migrating birds, stores fat as a fuel reserve, augmenting its weight by as much as 100%, then enabling metabolic fuel for flying over open water.<ref name="Hargrove"/><ref name="Skutch, 1973">{{Cite book |last1=Skutch |first1=Alexander F. |url=https://archive.org/details/lifeofhummingbir00skut |title=The Life of the Hummingbird |last2=Singer |first2=Arthur B. |publisher=Crown Publishers |year=1973 |isbn=978-0-517-50572-4 |location=New York |url-access=registration |name-list-style=amp}}</ref> The amount of fat (1–2 g) used by a migrating hummingbird to cross the Gulf of Mexico in a single flight is similar to that used by a human climbing about {{convert|50|ft|m}}.<ref name="Hargrove"/>

The [[heart rate]] of hummingbirds can reach as high as 1,260 beats per minute, a rate measured in a [[blue-throated hummingbird]] with a [[respiratory rate|breathing rate]] of 250 breaths per minute at rest.<ref name="Hargrove"/><ref>{{Cite journal |last=Lasiewski |first=Robert C. |year=1964 |title=Body temperatures, heart and breathing rate, and evaporative water loss in hummingbirds |journal=Physiological Zoology |volume=37 |issue=2 |pages=212–223 |doi=10.1086/physzool.37.2.30152332 |s2cid=87037075}}</ref>