Elderly and an avowed pacifist fond of rambling philosophical discussions of the afterlife, Mandemus appeared to bear no grudges toward his former human captors, and wanted everyone to simply live in peace. The nuclear war that destroyed much of the planet (including the city which the apes narrowly escaped) proved the futility of weapons and fighting to Mandemus. With such an attitude, the ape leader Caesar appointed him keeper of Ape City's small armory, "and of Caesar's conscience", reasoning that Mandemus would talk him out of anything impulsive or unwise because of his wisdom and restraint. Besides his other duties, Mandemus also became a teacher in Ape City's first school. One of his students was the genius orangutan Virgil, who also became a teacher, and an advisor of Caesar. When Aldo decided to take over the Ape settlement in Caesar's absence, his first move was to break into the armory and seize the weapons from Mandemus. After Caesar had defeated his enemies, Mandemus asked him to destroy the weapons so they could not be used again in such a way, a request refused by Caesar.
- While Mandemus's appearance in Battle was brief, the Marvel Comics original Apes story Quest for the Planet of the Apes provided a longer backstory to the relationship between Mandemus and Caesar, wherein the already aged Mandemus saved Caesar's life during an earlier confrontation by jumping in front of a bullet.
- According to Ty Templeton, author of Revolution on the Planet of the Apes, Mandemus was the first ape to acquire the power of speech, due to being in Caesar's presence in Armando's circus.
- Battle for the Planet of the Apes
- Battle for the Planet of the Apes (novel)
- Battle for the Planet of the Apes (Power Records) (Power Records)
- Planet of the Apes Magazine: Quest for the Planet of the Apes
- Planet of the Apes Magazine: Battle for the Planet of the Apes