The orangutan symbolizes Pi's mother in the story.
Orangutan babies tend to spend a long time with the mother, an average of 8 to 9 years which is much longer than most other ape species. Orangutans are also known to be good mothers. This is described in the story by Pi. Therefore the orangutan symbolizing Pi's mother makes sense.
In some cultures, the orangutan symbolizes the 'old man' of the forest probably because of its slow movements and solitary nature. This could tie into the story in that the mother would have been the wise and old man character in Pi's life. She always knew how to help him especially in the conflicts which Pi had with his father - the mother is the balance.