Infanticide (the killing of infants of the same species but of no relation) can be observed across the animal kingdom from lions to birds, but primates have been observed to practice this phenomenon more than any other order. It has been observed in several species ranging from chimps to macaques, but it appears that a large proportion of infanticides have one thing in common; they are followed by a mating of the aggressor with the widowed mother. Some of the observations make use of game theory, and explain these actions in terms of risks and rewards. It is my hypothesis that this leads to a greater fitness and reproductive success for males who commit infanticide over those who don’t.
Infanticide among Primates
Starting with chimpanzees, which have been observed on multiple occasions to kill infants, though most of these killings are directed at other groups of chimps . This has been observed on multiple occasions, and would often include cannibalizing the infant . It has also been observed that females who have had their child killed will occasionally join the group that killed it, and allow to be copulated with . Though admittedly from the article, they were not absolutely confident in whether every female that had an infant killed was from an outside group . This infanticide seems to be more of a result of warfare and conflict, rather than of outright greater fitness to individual chimps, however it does occasionally lead to a new female in the group . This both lowers the reproductive success of their rivals, which allows them to exploit their rivals lesser numbers and increase territory. One thing that sets chimps apart from the other primates that are going to be mentioned is that their infanticide is directed outside the group, rather than from within it, which is contrary to the other examples that we will see.
Mountain Gorillas typically exhibit infanticide during power struggles in their group dynamics. There have been two types of changes in social strategies that have resulted in infanticide at some point, but each seemingly with their own pros and cons for the mothers and infants at other points in time . The first strategy is a single male group, which will consist of a single silverback and multiple females . Typically the females and infants will be safe from infanticide as long as the male is the leader of their harem. Once the dominant male is either dead or exiled by another male, the new silverback will typically kill any and all infant of the previous males line. This insures that the females will start ovulating so that he can start to reproduce with them quicker . The Second social strategy is for the females to live in multi-male groups, where there is still a dominant male, but the infanticide rate is often lower than that of the single male groups . This gets a bit tricky though, due to one male groups males remaining dominant longer than the multi male groups...