The optimal length for mammalian eyelashes is one-third of the eye's width, which helps to retain moisture and keep out dust.

Credit: Paul Domenick/Getty

David Hu and his colleagues at the Georgia Institute of Technology in Atlanta measured the eyelash lengths of 22 species of mammals and found this ratio. They tested mock-ups of a mammalian eye in a wind tunnel to see how airflow changed depending on eyelash length. Lashes that were shorter than one-third of eye width were not optimal at blocking air from blowing onto the ocular surface. However, longer lashes directed more airflow towards the eye, making it susceptible to drying out. Understanding how eyelashes function could lead to devices that protect optical sensors, the authors say.

J. R. Soc. Interface http://doi.org/2gk (2015)