A fascinating question scientists have long entertained is why do most bats primarily choose to fly at night? And why have they evolved so heavily towards nocturnal activity? The strongest hypothesis about this related to predator avoidance. But no one knew exactly why. 

Two researchers, Christian C. Voigt and Daniel Lewanzik, created an ingenious experiment that was recently published in the Royal Society. In a laboratory in the jungles of Costa Rica, they injected the bats with honey water that had been enriched with C¹³ and then put the bats in a flight chamber and measured the stable isotopes of carbon exhaled or respired by the bats. (To perform the stable isotope measurements, they brought along a Picarro analyzer for carbon isotope measurements in ambient air.)

This was a very clever way to measure the metabolism of the bats. Voigt and Lewanzik also measured the core body temperature of the bats. They performed these experiments both in the daytime and after darkness. They found that bats have a 15% higher metabolic rates and higher core body temperatures when they fly during daylight, implying that flight during daylight hours is less efficient. This, they postulated, could be explained by the fact that bat wing membranes in such a way that they do a poor job of dissipating heat. This is in addition to predation risk (their predators can see bats more easily in the daytime). Spoiler alert - here's their conclusion to their research.

"In conclusion, diurnal foraging activities of modernbats are probably constrained by sunlight-related thermaland energetic parameters, and influenced by predationrisk. Resource patches profitable to bats at night possiblyturn unprofitable during daytime because of elevatedforaging costs. Thus, bats foraging at daytime may notonly be more exposed to visually oriented aerial predatorsbut also experience suboptimal foraging conditions.These multiple constraints may have forced bats intothe nocturnal niche while this species-rich taxon evolvedand spread across ecosystems worldwide"

We've talked briefly with Christian about his work and can't wait to see what his next research efforts are. The concept of measuring respiration or metabolism from continuous flows or ambient sources in remote settings is completely novel and opens up a whole new realm of science as researchers can now get world class data almost anywhere on Earth. We'll keep you posted on Christian and his team and can't wait to see what he does next.