Student or Learner
'It's animals that are needlessly active that will not survive, but animals that are most efficient and use their waking time to do vital functions, and are otherwise asleep that will survive.'
Why there is singular?
(Not a teacher)
If it is indeed the 'it's' in 'it's animals' that you are asking 'why is it singular?', it's because 'it' here is a dummy pronoun, as euncu brought up. Dummy pronouns are used because English syntax requires a subject, but they don't refer to anything. Dummy pronouns aren't ever plural, because they don't refer to anything and are thus singular by default.
The construction 'it are' doesn't exist in English.
Thank you, I understand with "it's", but why there "that" not "those"?