A general law for animal locomotion?

Anders HedenstromHedenström

Trends in Ecology and Evolution 2004, 19:217-219

The propulsion system of animals that fly or swim are quite different from
each other in their morphology and function, yet the propulsive efficiency
could be maximized by a surprising similarity in the fine tuning of flapping
frequency, amplitude and forward speed, according to a new study by Taylor
et al. This conclusion was based on an analysis of the Strouhal number,
which is a dynamic similarity index relevant to propulsion that relies on
vortex shedding for thrust generation. Such fine-tuning of the propulsive
system suggests possible consequences for physiological and ecological
adaptations related to, for example muscle operating frequency and optimal
speed of muscle contraction.

Evolutionary biology is the search for general patterns and constraints that
govern adaptation of size, structure and behaviour, whereas biomechanics is
a science where biology meets physics and where biologists profit from
research on, for example fluid dynamics, and where evolutionary adaptations
illustrated by nature can provide hints and solutions for engineers who want
to design nature-inspired devices. Animal locomotion is a field where the
optimality approach to evolutionary biology has its strongest applications,
because physical laws provide both the opportunities and constraints for
organismal functional design. The properties governing animal locomotion can
often be characterized by an appropriate 'dynamic similarity index', where
similar numerical values of the index indicate that forces acting on animals
of different scale are comparable. A dynamic similarity index applicable to
oscillating propulsion is the Strouhal number (St), defined as St=fh/U,

where f is the flapping frequency, h is the peak-peak tip amplitude of
oscillating appendages (fins and wings) and U is the forward speed [1]. A
high St means that the wings move up and down a lot at a high frequency but
a low forward speed, whereas a low St represents a more shallow wingbeat at
low frequency and fast forward speed. The range of St for cruising swimmers,
for example, is confined to the range 0.2http://gateways.bmn.com/magazine/article?pii=S0169534704000369

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