  Excitable pulses are among the most widespread dynamical patterns that occur
in many different systems, ranging from biological cells to chemical reactions
and ecological populations. Traditionally, the mutual annihilation of two
colliding pulses is regarded as their prototypical signature. Here we show that
colliding excitable pulses may exhibit soliton-like crossover and pulse
nucleation if the system obeys a mass conservation constraint. In contrast to
previous observations in systems without mass conservation, these alternative
collision scenarios are robustly observed over a wide range of parameters. We
demonstrate our findings using a model of intracellular actin waves since, on
time scales of wave propagations over the cell scale, cells obey the
conservation of actin monomers. The results provide a key concept to understand
the ubiquitous occurrence of actin waves in cells, suggesting why they are so
common, and why their dynamics is robust and long-lived.
