Once upon a time in a valley in Complexania, there were Werewolves and Vampires. They could live from the magic field in the valley, as long as they did not grow too large. The valley is also magically rolled up to a torus surface, so have no fear kids, the creatures cannot escape. Their size is genetically given, but when they reproduce, the target size might mutate by plus/minus 10 percent. If one of this creatures could gather enough magic (which is easier when they are small), an offspring was created in a free field beside it. So far it is clear that being smaller is advantageous because you can save more energy and reproduce faster. However, a werewolf is also able to kill a smaller vampire and steal its energy. Vice versa, large vampires are able to kill and consume werewolves which are smaller than them. This triggered an arms race of larger and larger creatures in the valley. At one time they grew so large that they had to constantly feed on their foes, since the magic field alone was not able to support their hunger for energy any more. So they grew and fought each other, numbers went up and down on both sides, until one species was left. Or both died.
Do you want to know who won the battle? Find out for yourself and use the simulation below:
In case you cannot see the simulation, your browser does not support applets. I made a video of the simulation for this case:
Small vampires and werewolves, which can feed sufficiently from the magic field, are characterized by light red and gray color, respectively. The red and black squares indicate larger vampires and werewolves.
Have fun and happy Halloween!