What is the correct form of possessive case for the noun 'mice'?

Question

Mice is plural form of mouse. It is an animate noun. I wonder what possessive does it have in the plural? Should we write:
1)

mice's behavior

2)

mice' behavior

or how?

Answer 1

I know it's confusing, but we just use 's.

Irregular Plurals

Some nouns have irregular plural forms without s (man → men). To show possession, we usually add 's to the plural form of these nouns:

singular noun         plural noun
  my child's dog        my children's dog
  the man's work        the men's work
  the mouse's cage      the mice's cage
  a person's clothes    people's clothes

(English Club)

The issue of whether to use 's or just ' only occurs when the word ends in an s (e.g., the collars of many dogs, the dogs' collars).

I don't think there is often a problem when the possessive is an irregular plural like above.