Bats marry the night

We know what Bruce Wayne picked as a “creature of the night”: a bat.

But why are bats so strongly nocturnal? Why don’t we see bats out flying around in the daytime (besides a few out on remote islands)? After all, most people can quickly think of one line of birds that is largely nocturnal.

If a bird had flown through Bruce Wayne’s window, we might have had a very different character in stately Wayne Manor.

Voigt and Lewanzik test an hypothesis that bats fly at night because they can’t take the heat during the day. Bats face particular problems dealing with high temperatures. First, flying is hard work! The energetic cost of flying is about 10 times more than resting levels. Second, bats’ dark wings soak up a lot of light energy and have no sweat glands for cooling.

Testing this idea is not difficult – in theory. Fly bats by day. Fly bats by night. Measure their metabolic costs in both conditions and compare. Testing this idea is much trickier in practice. Bats are small and they need a lot of room to fly. I had a tough time understanding the methods they used to determine the metabolic costs of the bats. They involve a lot of chemical isotopes. Also math.

They authors used short-tailed fruit bats (Carollia perspicillata). This species live in the tropics near the equator, and so is one that would not be expected to face great extremes between night and day temperatures.

At the end, the authors found that the bats’ metabolism went up 15% when they flew in the day compared to when they flew at night. Daytime body temperature went up a couple of degrees, too, which is noteworthy considering that we usually think of mammals as regulating their core body temperatures very closely.

The authors do note that all of their results don’t mean that dealing with heat is the one and only reason that bats are nocturnal. Several factors could be working in combination to keep bats married to the night.

The authors mention that bats might be able to reduce their metabolic costs of daylight flight if only they didn’t have such dark wings. If they had light coloured wings, so much more energy would be reflected off the wings that the energetic costs might be manageable. Mammals can have light fur and skin, but the authors suggest that this might make them easier targets for predators to see.

Plus, if bats were light coloured, we might have had this:

Barely registers on the “striking terror into the heart” scale.





Reference

Voigt C, Lewanzik D. 2011. Trapped in the darkness of the night: thermal and energetic constraints of daylight flight in bats. Proceedings of the Royal Society B: Biological Sciences: In press. DOI: 10.1098/rspb.2010.229

Bat photo by guppiecat on Flickr; used under a Creative Commons license.What if Bruce Wayne had not become the Batman? from here.