Board Thread:Questions and Answers/@comment-29602226-20160831060724/@comment-30272238-20161020193626

That spirit energy in humans can be replenished by normal food and that monster girls have alternative diets listed besides male spirit energy would suggest spirit energy is produce by literally every living thing except monster girls and it is entirely possible for a monster girl to survive off normal food if she really wanted.

Male spirit energy just happens to be concentrated and more tasty. Since male spirit energy is apparently the most concentrated form, monster girls who don't have a male should be weaker than ones who do.