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

By Valerie Ross | May 4, 2011 5:29 pm

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

  • Angus

    I did a three-week tropical ecology class in Costa Rica a few years ago, part oc which involved mist netting for bats. I remember when I held one in my hands that it felt surprisingly warm; the little dudes do NOT have much in the way of insulation.

    The other thing that’s not mentioned is the difference between bat and bird wings—a bird’s wing is mostly feathers, not flesh. A bat’s is entirely skin, all of which has blood vessels running through it and is very thin. I imagine it acts like an African elephant’s ear in terms of shedding heat, and that the effects of evaporative cooling on a surface area that comprises roughly 3/4 of the bat’s total skin would be huge. That’s my theory.

  • Celia Su

    That’s quite fascinating.

  • Brian Too

    OK OK, but what I really want to know is this:

    Does wet bat smell like wet dog? If not, is it better or worse? Discuss amongst yourselves…

  • Mark Spencer

    I’m not really interested bout bats but there is no harm of addition knowledge. :) Very Informative post.

  • eyesoars

    Or, wet fur might make the bats less aerodynamic, meaning it takes more power to fly.

    There are real-world gliders and aircraft that suffer similarly: for instance, the PIK-20 and Nimbus 2 gliders are notorious for performing poorly when their wings (Wortmann FX-67-K-170 airfoils) are wet — water on the wings destroys their laminar airflow characteristics, substantially diminishing the gliders’ L/D ratio. The same is true of some airliners, though to lesser degree.

    It strikes me as likely in the case of bats that the very fine fur/vellum is adapted for boundary-layer control: wetting it down would almost certainly destroy those properties. (E.g., high-performance aircraft can be improved by adding thin tape (0.005″) to create turbulence at appropriate places, or by roughening or waxing the wing in appropriate regions.)

  • Dan Davis

    Additionally, most bats are insectivores. When it is raining, there is significantly decreased insect activity making food more scarce. I would be interested in a study with more variables and see which factor accounts for the most variance for the bats not flying while it is raining.

  • Wildlife Control

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