  We study the critical behaviour of a model with non-dissipative couplings
aimed at describing the collective behaviour of natural swarms, using the
dynamical renormalization group. At one loop, we find a crossover between a
conservative yet unstable fixed point, characterized by a dynamical critical
exponent $z=d/2$, and a dissipative stable fixed point with $z=2$, a result we
confirm through numerical simulations. The crossover is regulated by a
conservation length scale that is larger the smaller the effective friction, so
that in finite-size biological systems with low dissipation, dynamics is ruled
by the conservative fixed point. In three dimensions this mechanism gives
$z=3/2$, a value significantly closer to the experimental result $z\approx 1$
than the value $z\approx 2$ found in fully dissipative models, either at or off
equilibrium. This result indicates that non-dissipative dynamical couplings are
necessary to develop a theory of natural swarms fully consistent with
experiments
