No matter how hard a terrestrial animal pushes itself, its exertions are negligible in comparison with those of a hovering hummingbird. Sustaining metabolic rates (per gram of body mass) that are approximately 10 times those of elite human athletes, hummingbirds directly fuel their flight muscles with sugar from their nectar diet. William Karasov, from the University of Wisconsin-Madison, USA, and Enrique Caviedes-Vidal, from Universidad Nacional de San Luis, Argentina, explain that birds have smaller guts than mammals of the same size, to reduce their cargo costs. However, it also turns out that their intestines are leakier – water-soluble nutrients pass through the junctions between gut cells (in a process known as paracellular absorption) to compensate for the smaller surface area – in addition to using molecules embedded in the intestine surface to transport nutrients into the blood. Could the same hold true for other fliers? ‘The only other living vertebrates that actively fly are bats’, says Karasov, so would their intestines be as leaky as those of hummingbirds?
Karasov recalls that Nelly Rodriguez-Peña arrived in his lab at just the right time to test the theory. He says, ‘She did a wonderful job of identifying a practical, catchable population of bats’, adding that Eddy Price taught her how to feed sugar solutions to small animals and then collect the minute blood samples that would be needed to determine how leaky the bats’ intestines were. ‘Then she took the method and equipment and set up a makeshift field lab [in Mexico] to do the work’, says Karasov.
Having gently trapped nectar-sipping Saussure's long-nosed bats in nets at dusk as they left their cave roost on Don Panchito Island, Rodriguez-Peña then worked through the night. First she fed the animals with a solution made from several sugars, including l-rhamnose (which can only enter the bat's bloodstream by paracellular absorption because there are no protein transporters to carry the sugar across the intestine wall) and d-(+)-cellobiose (a large sugar that also enters the bloodstream by paracellular absorption, but at a slower rate). Then she patiently collected minute blood samples (28 μl) from the animals over a 2 h period. After releasing the bats, Rodriguez-Peña relocated to Cesar Flores-Ortiz's lab at the Universidad Nacional Autonoma de México with the blood samples to find out how much of each sugar was in the bats' blood.
Using high-performance liquid chromatography to analyse the blood samples, Rodriguez-Peña saw the rhamnose levels increased substantially; the intestines were leaky and the animals were using paracellular routes to absorb sugar into the blood. And when she measured the amount of cellobiose in the blood samples, she found that the molecule also crossed the intestine, but at much lower levels. ‘It [the junction between adjacent intestine wall cells] is almost like a filter, and if molecules are too big they cannot get through; that is what makes the junction tight against large foreign molecules’, explains Karasov. In fact, the junctions were so leaky that almost as much rhamnose appeared in the bats’ blood as if it had been injected directly.
So hovering nectar-feeding bats use the same leaky intestine mechanism as hummingbirds to absorb as much sugar as possible into the blood to meet their immense metabolic demands. Karasov says, ‘I don't know if this is a phenomenon of small bats and small birds, or all bats and birds, so we need to study some really big bats next’.