I looked up WordNet (version 3.0) and found these:

‘estivate’/’aestivate’ verb sleep during summer‘hibernate’ verb 1. sleep during winter 2. be in an inactive or dormant state

They say about (animals) ‘hole up’ at different (opposing) seasons (summer and winter).

Not quite ‘antonym’ (= word with opposing meaning) vs synonym (= word with contextually same meaning) as we would usually understand.

;-)