  The early universe may have contained internally thermalized dark sectors
that were decoupled from the Standard Model. In such scenarios, the relic dark
thermal bath, composed of the lightest particle in the dark sector, can give
rise to an epoch of early matter domination prior to Big Bang Nucleosynthesis,
which has a potentially observable impact on the smallest dark matter
structures. This lightest dark particle can easily and generically have
number-changing self-interactions that give rise to "cannibal" behavior. We
consider cosmologies where an initially sub-dominant cannibal species comes to
temporarily drive the expansion of the universe, and we provide a simple map
between the particle properties of the cannibal species and the key features of
the enhanced dark matter perturbation growth in such cosmologies. We further
demonstrate that cannibal self-interactions can determine the small-scale
cutoff in the matter power spectrum even when the cannibal self-interactions
freeze out prior to cannibal domination.
