  We study implications of the large-N species solution to the hierarchy
problem, proposed by G. Dvali, for reheating of the universe after inflation.
Dvali's proposal contains additional N~10^{32} Z_2-conserved quantum fields
beyond the Standard Model particles with mass ~1 TeV, which weaken gravity by a
factor of 1/N, and thus explain the hierarchy between the Plank scale and the
electroweak scale. We show that, in this scenario, the decay rates of inflaton
fields through gravitational decay channels are enhanced by a factor of N, and
thus they decay into N species of the quantum fields very efficiently, in the
limit that quantum gravity effects are unimportant for the gravitational decay
rate. In order not to over-reheat the universe, inflaton mass, vacuum
expectation value of inflaton, or non-minimal gravitational coupling should be
tightly fine-tuned. Our conclusion holds even when the gravitational decay is
prohibited by some symmetry of the theory; the universe may still be
over-reheated via annihilation of inflatons, if the number density of inflaton
quanta is greater than the critical value.
