  Many swimming and flying animals are observed to cruise in a narrow range of
Strouhal numbers, where the Strouhal number ${St = 2fA/U}$ is a dimensionless
parameter that relates stroke frequency $f$, amplitude $A$, and forward speed
$U$. Dolphins, sharks, bony fish, birds, bats, and insects typically cruise in
the range $0.2 < St < 0.4$, which coincides with the Strouhal number range for
maximum efficiency as found by experiments on heaving and pitching airfoils. It
has therefore been postulated that natural selection has tuned animals to use
this range of Strouhal numbers because it confers high efficiency, but the
reason why this is so is still unclear. Here, by using simple scaling
arguments, we argue that the Strouhal number for peak efficiency is largely
determined by fluid drag on the fins and wings.
