  This paper deals with the asymptotic behavior of the first Bloch eigenvalue
in a heterogeneous medium with a high contrast $\ep Y$-periodic conductivity.
When the conductivity is bounded in $L^1$ and the constant of the
Poincar\'e-Wirtinger weighted by the conductivity is very small with respect to
$\ep^{-2}$, the first Bloch eigenvalue converges as $\ep\to 0$ to a limit which
preserves the second-order expansion with respect to the Bloch parameter. In
dimension two the expansion of the limit can be improved until the fourth-order
under the same hypotheses. On the contrary, in dimension three a fibers
reinforced medium combined with a $L^1$-unbounded conductivity leads us to a
discontinuity of the limit first Bloch eigenvalue as the Bloch parameter tends
to zero but remains not orthogonal to the direction of the fibers. Therefore,
the high contrast conductivity of the microstructure induces an anomalous
effect, since for a given low-contrast conductivity the first Bloch eigenvalue
is known to be analytic with respect to the Bloch parameter around zero.
