Etymologically estivation is the summer version of hibernation, deriving as they do from Latin aestivus (relating to summer) and hibernus (relating to winter). I had thought that estivate had taken on the meaning of becoming torpid during a dry season but AHD4 claims that estivate can mean either to pass the summer in a dormant or torpid state or to spend the summer, as at a special place. Hibernate has taken on the general meaning of being in a torpid or dormant state irrespective of the season.