The Long and Short of It

Optimal moisture retention is not
always the goal (vippeextensions.com)

Last year Dr. David Hu of the Georgia Institute of Technology made an eye-opening discovery [bold added]:

no matter what species of mammal he examined (and he studied 22 of them), the length of its [eye]lashes was on average a third of the width of its eye.

A good scientist doesn't stop with the observation. He asks why:

he and his team built a small wind tunnel, and also models of eyes that could have their ersatz lashes swapped for ones of different lengths. Using these, they studied airflow around and through the lashes, how that affected evaporation from the model eye's cornea, and also how many particles (in the form of dust-sized drops of coloured water) settled on the cornea's surface. They then used fluid mechanics to try to work out what was going on.

Nature has, it turns out, arrived at the optimum eyelash length to keep the cornea moist and dust-free. By reducing air flow over the cornea, eyelashes create a boundary layer of slow-moving air. That stops dust getting through, and also promotes water retention, since moisture is not blown away. Up to a point, the boundary layer grows thicker as the lashes grow longer. But long lashes also act as a funnel, channelling moving air into the eye and disrupting the protective layer. The thickest boundary layer comes when there is a one-to-three ratio between lash length and eye width.

If lashes are longer or shorter than one-third the eye-width, the eyes dry out....math found in nature that's not as meaningful as the golden mean or Fibonacci sequence, but certainly less mysterious.