  Active fluid transport is a hallmark of many biological transport networks.
While animal circulatory systems generally rely on a single heart to drive
flows, other organisms employ decentralized local pumps to distribute fluids
and nutrients. Here, we study the decentralized pumping mechanism in the slime
mold Physarum polycephalum which is locally triggered by active release,
uptake, and transport of a chemical solute within the organism's vascular
network to drive global oscillations.Based on a conceptual network model
combining active elasticity and fluid transport we identify a set of
contractile modes specific to each network and show that modes corresponding to
large-scale oscillations are preferentially and robustly excited both in model
simulations and in experimental data obtained from living Physarum plasmodia.
These dominant modes are computed explicitly and shown to drive large-scale
flows within the organism. Furthermore, Physarum must transport nutrients over
long distances. As each mode corresponds to pure shuttle flow, long-range,
directed transport must rely on a non-linear coupling beyond harmonic dynamics.
Using simulations, we demonstrate that the network's transport capability is
optimized when two dominant modes are excited at a phase shift of $\pi/2$,
resulting in contractile excitations similar to those observed in real
Physarum. Our results provide a conceptual framework for understanding active
decentralized transport in Physarum and other contractile biological networks,
such as brain vasculature, as well as decentralized transportation networks
more generally.
