1. Baby blue whales drink 200 liters of their mother's milk.

Question

2. Baby blue whales drink 200 liters of their mothers' milk.

(Which one is grammatical? Do we have to use "mother's" or "mothers'"?)

Answer 1

Does a whale mother have more than one baby at a time?

Do baby whales have only one mother?

Think about what makes sense -- one baby whale per mother. Therefore ...

A baby blue whale ... its mother's milk.
or
Baby blue whales ... their mothers' milk.