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I would think that having a bunch of different male partners constantly hanging around would be the surest way to increase infanticide because the males want their own offspring to survive at the expense of other males’ offspring.
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Infanticide is extremely common among some groups of monkeys and apes (including humans). It is used for a variety of reasons, including a child born too soon to another child, a way to get ride of female children in a society that prefers males, a way to get rid of handicapped children, and a way to cause females to go into estrus again.
Infanticide as a way to increase one’s reproductive success was first observed by Sarah Hrdy (she’s an amazing anthropologist, check out her work) in 1977 in her study of Hanuman langurs. This has also been observed in oragutans, chimpanzees, gorillas, and other species. In 1999, Borries et al. collected DNA samples from langurs who kills infants, the infants, and the the subsequent infants birthed by the mothers of the killed infants. They found that all of the infants killed belonged to some other primate than the killer and that in most cases, the new infant produced was the offspring of the killer.
As Frans de Waal (he’s a must read) puts it, females avoid infanticide when each male has reason to consider the child to be his. Infanticide has not been observed in bonobos due to the “sluttiness” of the females.
It is interesting to note that female chimps face a high danger of domestic violence, particularly if they are known to sleep around . The female chimps who face the most beatings are those that are most promiscuous. You would think that being beaten up would convince them not to sleep around, but they keep doing it. In order to get around this, they will often go to other groups and sleep with the males there. More than 50% of the offspring produced by a female in a tribe will have been fathered by someone outside of it.
Female bonobos escape this violence because of two factors. One, the constant sex with other females forges relationships between them that causes them to unite when threatened with male hostility. (Meredith Small, another amazing anthropologist wrote a very interesting article about this subject and applied it to human relationships). But the whole reason that the females were able to do this was due to the differences in the geographical environment the two groups ended up in. In the bonobos, food was more plentiful so the females could hang around with each other a lot and form friendships. In tribes where food was harder to come back, the females had to go out farther from each other to forge and strong relationships between them were never formed.
This can be paralleled with several human societies. For example, I have read multiple accounts of the hostility displayed by the Yamamoto towards each other and of the males towards the females. In most of the accounts about these people, the details of their perilous economic system is mentioned.
If you are looking for a fun, informative reading on animal sexuality in general, check out “Dr Tatiana’s Sex Advice To All Creation” by Olivia Judson. She talks about how, in most species, the more a female mates, the greater fertility she has and the more offspring are produced.
