Note: This is, so far, only applicable by mating /after/ the 100 Year Mark. The ape-like humans will populate the direct surroundings of the player after having mated after this time.
The Homo Erectus, once slain, cannot be eaten. In addition to this, the entire map is with nearly no consumable food, leaving the player in an extreme world of survival. And, combined with the decreasing amounts of food, Homo Erectus will actively hunt and eat any animal it sees, despite the power or size of the prey, ultimately making it nearly impossible for carnivores needing large amounts of food to survive, having to juggle this along with the Prime Mates and numerous Homo Erectus. However, this is not true with all carnivores: for example an ERC-003 or other robotic dog could survive 1000 years in the Yoyogi Park Under.
Secondary Note: Oddly, if you mate just before the year reaches 100, (i.e., while the year 99 is touching the year 100, but has not changed yet) it seems to prevent the population of Homo Erectus in the territory you mated in and one or two territories nearby, however they will fully take over the other territories until you mate, upon which they will appear in every territory.