How Does Rain Mess With Bat Flight---Thermodynamics or Aerodynamics?

By Valerie Ross
May 5, 2011 2:29 AMJul 18, 2023 7:43 PM


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What's the News: Bats have to use twice as much energy to fly when they're wet as when they're dry, a new study in Biology Letters found, which may help explain why many bats refrain from flying in heavy rain. How the Heck:

  • The researchers captured ten Sowell's short-tailed bats in Costa Rica.

  • Each of the bats flew around a large outdoor cage in three different circumstances: dry, wet (the researchers dampened their fur and wings with tap water) on an otherwise dry day, and wet on a fairly rainy day.

  • By measuring the bat's metabolism, the researchers found that wet bats expended twice as much energy during a short flight as dry bats did (twenty and ten times their resting rate, respectively).

  • The wet bats didn't weigh more than the dry ones, ruling out the idea that the damp bats simply had to work harder to carry extra water weight. Nor did already wet bats burn more energy flying on a rainy day than a dry one, meaning the problem isn't that raindrops mess with the bats' flight mechanics.

  • The researchers suggest two other explanations: Being wet might cool the bats down (the same way sweat cools humans down), meaning that they have to boost their metabolisms to stay warm. Or, wet fur might make the bats less aerodynamic, meaning it takes more power to fly.

What's the Context:

  • Another theory about why bats tend not to fly in the rain posits that rain gets in the way of echolocation, the means by which bats find their food. This study doesn't disprove that theory; bats could have more than one reason for staying out of the rain.

  • Of animal groups that fly, only bats are furry; birds, for instance, have feathers to protect them from the rain.

Reference: Christian C. Voigt, Karin Schneeberger, Silke L. Voigt-Heucke and Daniel Lewanzik. "Rain Increases the Energy Cost of Bat Flight." Biology Letters online before print, May 4, 2011. DOI: 10.1098/rsbl.2011.0313

Image: Flickr / Angell Williams

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