Whilst it is not an official game mode, realism is a popular mode within the community, due to the focus it puts on more realistic gameplay behavior. Behaviour like killing on sight are generally heavily frowned upon in this mode even when such behavior can be common with animals in real life.
In this game mode, all players must follow a guideline of behavior set out in their species' profile. Often, this includes things such as how a creature interacts with it's environment or other creatures, how a creature behaves under various conditions and (for carnivores) what other animals are food and which are threats.
Most realism servers tend to be strict about following the species profiles, and thus tend to encourage new players to read them (often by having groups that players need to join to play on these servers).