  The evolution towards equipartition in the $\beta$-FPU chain is studied
considering as initial condition the highest frequency mode. Above an
analytically derived energy threshold, this zone-boundary mode is shown to be
modulationally unstable and to give rise to a striking localization process.
The spontaneously created excitations have strong similarity with moving exact
breathers solutions. But they have a finite lifetime and their dynamics is
chaotic. These chaotic breathers are able to collect very efficiently the
energy in the chain. Therefore their size grows in time and they can transport
a very large quantity of energy. These features can be explained analyzing the
dynamics of perturbed exact breathers of the FPU chain. In particular, a close
connection between the Lyapunov spectrum of the chaotic breathers and the
Floquet spectrum of the exact ones has been found. The emergence of chaotic
breathers is convincingly explained by the absorption of high frequency phonons
whereas a breather's metastability is for the first time identified. The
lifetime of the chaotic breather is related to the time necessary for the
system to reach equipartition. The equipartition time turns out to be dependent
on the system energy density $\epsilon$ only. Moreover, such time diverges as
$\epsilon^{-2}$ in the limit $\epsilon \to 0$ and vanishes as $\epsilon^{-1/4}$
for $\epsilon \to \infty$.
