  Most bacteria swim through fluids by rotating helical flagella which can take
one of twelve distinct polymorphic shapes. The most common helical waveform is
the "normal" form, used during forward swimming runs. To shed light on the
prevalence of the normal form in locomotion, we gather all available
experimental measurements of the various polymorphic forms and compute their
intrinsic hydrodynamic efficiencies. The normal helical form is found to be the
most hydrodynamically efficient of the twelve polymorphic forms by a
significant margin - a conclusion valid for both the peritrichous and polar
flagellar families, and robust to a change in the effective flagellum diameter
or length. The hydrodynamic optimality of the normal polymorph suggests that,
although energetic costs of locomotion are small for bacteria, fluid mechanical
forces may have played a significant role in the evolution of the flagellum.
