  We investigate the cosmological effects of neutrino lumps in Growing Neutrino
Quintessence. The strongly non-linear effects are resolved by means of
numerical N-body simulations which include relativistic particles, non-linear
scalar field equations and backreaction effects. For the investigated models
with a constant coupling between the scalar field and the neutrinos the
backreaction effects are so strong that a realistic cosmology is hard to
realize. This points towards the necessity of a field dependent coupling in
Growing Neutrino Quintessence. In this case realistic models of dynamical Dark
Energy exist which are testable by the observation or non-observation of large
neutrino lumps.
