Hovering hummingbirds have some of the highest metabolic rates ever recorded. Fuelled almost directly by the nectar they consume, hummingbirds easily maintain metabolic rates 10 times greater than the most highly trained human athletes. But hummingbirds aren't the only creatures that push the metabolic limits. A few varieties of bats also hover while sipping nectar. Knowing that the metabolisms of hummingbirds and nectarivorous bats have evolved to perform similar feats, Ken Welch, Gerardo Herrera and Raul Suarez wondered whether the bat's metabolism was more human-like, fuelling hovering flight with onboard stores, or hummingbird-like, fuelled with newly ingested sugar (p. 310). Travelling to Colima, Mexico, to work with Herrera, Welch and Suarez prepared to discover which fuel the bats had selected.

Trapping the Pallas' long-tongued bats at night with mist nets in a local banana plantation, the team gathered 12 of the tiny mammals ready to measure their metabolic rates. But before the team could put the hoverers through their paces, they had to find a way to distinguish whether the bats were burning fuel laid down earlier, or sugars consumed from nectar minutes before.

Welch explains that it is possible to distinguish whether animals are burning carbohydrate or fat by measuring the ratio of the carbon dioxide exhaled to the oxygen consumed. According to Welch, when the ratio is approximately 0.7, animals are burning fat. However, if the ratio rises to 1.0, the animal has switched from using fat as a fuel, to sugars or carbohydrates. Tempting the tiny mammals to hover with their heads in a bat-sized respirometry mask, the team measured the mammals' oxygen consumption and carbon dioxide production rates. Initially, the mammals burned fat, but the hovering bat's metabolism soon switched to burning carbohydrate almost exclusively.

But were the bats burning carbohydrates from their body stores, or sugars from nectar they had just ingested? Carbohydrates and sugars all contain carbon, and carbon is naturally found in 3 different forms in the environment(C12, C13 and C14). Welch explains that the ratio of C13/C12 incorporated by plants into their structures depends on the plant's physiology; the C13/C12 signatures of sugar cane and sugar beet are completely different. The teams' strategy was to feed the bats on a sugar beet-derived diet for several weeks before feeding them sugar cane sucrose while hovering. By measuring the C13/C12 signature in their exhaled carbon dioxide, the team could distinguish whether the bats were burning recently consumed nectar or carbohydrates from sugar beet-derived body stores.

Storing the bat's breath in sealed vials, the team returned to their Santa Barbara lab to analyse the gas's composition and found that the hovering bats were burning the sugar cane sucrose they had consumed less than an hour earlier. Welch explains that the finest human athletes can only derive 30% of their energy from recently consumed sugars, but almost 80% of the bat's energy was coming from cane sugar consumed during the previous hour. Amazingly the bat's metabolism was more similar to a hummingbird's than a mammal's.

Welch, K. C., Jr, Herrera M., L. G. and Suarez, R. K.(
). Dietary sugar as a direct fuel for flight in the nectarivorous bat Glossophaga soricina.
J. Exp. Biol.