The Collaborative International Dictionary of English v.0.48:

AEstivate \[AE]s"ti*vate\, v. i. [L. aestivare, aestivatum.]
   1. To spend the summer.
      [1913 Webster]

   2. (Zool.) To pass the summer in a state of torpor.
      [1913 Webster] [Spelt also estivate.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

aestivate
    v 1: sleep during summer; "certain animals estivate" [syn:
         estivate, aestivate] [ant: hibernate, hole up]