I'd use 'eat' - what they were designed to do was (to) eat.
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English Patrick..what they were designed to do – eating.Hello. I believe eating is correct and fitting.
English PatrickCan anyone else explain the grammar that applies here?
English PatrickBy the way, are you a native English speaker (I notice your profile says Czech)?I live in the Czech republic, but I am a native speaker of (British) English - and have been so for over sixty years.