general $\gamma _{e}$ depends on microscopic details of the model, but there is no reason to exclude a priori that it could change sign and indeed does for the case of long thin hard rods with excluded volume interactions [@Baskaran2008]. The linear analysis here is of limited utility because of the existence of a homogeneous instability but is shown here to indicate that the build up of polarization due to the momentum conserving nature of the interactions has a dramatic consequence on the dynamics of the system. This may indeed be the mechanism responsible for the smectic order observed within a single polar cluster in simulations of self-propelled rods [@Yang2010; @Sam2012]. Finally, we stress that the nonlinear homogeneous terms proportional to $\gamma $ and responsible for the renormalization of $\gamma
_{e}$ always vanish in an equilibrium state because the nematic symmetry of such a state by definition forbids a nonzero uniform value of the mean polarization.

Discussion {#sec:discussion}
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We have considered in this paper the hydrodynamics of active overdamped fluids that can order in nematic states. These are collections of active particles that interact via apolar (nematic) aligning interactions, such as steric repulsion or medium-mediated hydrodynamic couplings. One can 