The complete and correct hierarchy of taxonomic groups from largest to smallest are:
1.1) Archaea 1.2) Bactera 1.3) Eukarya
2.1) Animalia 2.2) Plantae 2.3) Fungi 2.4) Protista 2.5) Archaea 2.6) Bacteria
3) Phylum - has 35 phyla
Oxytocin causes contractions during pregnancy, it also is the reason why mothers feel "motherly" towards their children after childbirth as there are high levels on oxytocin in the mother's blood.
Well, I cannot see your list of suggestions but I am assuming one of them would be muscles.
Muscles use a lot of energy and require a large number of mitochondria.