  Collisionless plasma shocks are a common feature of many space and
astrophysical systems and are sources of high-energy particles and non-thermal
emission, channeling as much as 20\% of the shock's energy into non-thermal
particles. The generation and acceleration of these non-thermal particles have
been extensively studied, however, how these particles feed back on the shock
hydrodynamics has not been fully treated. This work presents the results of
self-consistent hybrid particle-in-cell simulations that show the effect of
self-generated non-thermal particle populations on the nature of collisionless,
quasi-parallel shocks. They contribute to a significant heat flux density
upstream of the shock. Non-thermal particles downstream of the shock leak into
the upstream region, taking energy away from the shock. This increases the
compression ratio, slows the shock down, and flattens the non-thermal
population's spectral index for lower Mach number shocks. We incorporate this
into a revised theory for the Rankine-Hugoniot jump conditions that include
this effect and it shows excellent agreement with simulations. The results have
the potential to explain discrepancies between predictions and observations in
a wide range of systems, such as inaccuracies of predictions of arrival times
of coronal mass ejections and the conflicting radio and x-ray observations of
intracluster shocks.
  These effects will likely need to be included in fluid modeling to accurately
predict shock evolution.
