The orang-utan is the mother-figure on the boat because she
acted as the protector to the defenseless. When the hyena brutally
mamed and killed the zebra early on, it was Orange Juice who
reacted. Although Pi was angry at the hyena for his cruelty, he
does not react out of fear for his own safety. Orange Juice,
despite fear and seasickness, confronts the hyena because her
natural inclination is to protect. Pi even states that his memories
of her at the zoo where as a mother to her two babies.
Unfortunately her bravery caused her demise. Pi\'s alternative
version of the story supports the same theory.