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← Infanticide in higher mammals

VrijVlinder's Avatar Jump to comment 9 by VrijVlinder

@Comment 6 by raytoman: Apes however are not solitary and the tribe typically collaborates to protect and rear all their young.

One must not assume all primates are non infanticidal .

Infanticide Sexual selection hypothesis

Infanticide is a male reproductive tactic: Loss of suckling infant leads to the onset of estrous in the mother Males gain a reproductive advantage through earlier conception by females.

Some examples from chimps- when a new female with an infant comes into the group, usually the infant will be killed by the males in the group. As a result, soon the female is in estrous again, which she wouldn't have been for years- and so one of the males in group can have a child by her.

The majority (67%) of all infanticides occur in one-male groups Most (21 or 91%) were committed by strange males 17 (74%) were committed by immigrant males 4 (17.4%) were committed by extra-group males Only 2 (< 10%) were committed by a male within the social group, but in both cases it was a male who had just increased his dominance rank. This is significant because only a higher-ranking male can benefit from a female coming into estrous sooner. 13 cases (=57%) occurred after takeovers by males All 23 of the infant victims were still unweaned

Predictions of high population density hypothesis

Infanticide occurs as high population densities Infanticide will not necessarily benefit the killer

When you plot data matching infanticide occurrences and the population densities, the data points are pretty scattered. However, when you separate them out by one-male groups and multi-male groups, you see that infanticide is a lot more common in one-male groups than in multi male groups. This is consistent with the sexual selection hypothesis because it's the males coming into the groups who are doing the killing when they take over.

Predictions of sexual selection hypothesis

Infanticidal males will not typically be the fathers of the offspring killed Mothers will become sexually active earlier than if their infants had lived Infanticidal males benefit reproductively by killing offspring

Prediction 1- relatedness of infanticidal males an infant victims

In 22 of 23 cases, the infanticidal male was not the probable father because: he was not in the group at the time the infant victim was born; he was sexually immature; or he was of low rank and probably did not father in the infant.

There are no verified cases of a father killing his offspring.

Prediction 2- the effect of infanticide on interbirth intervals

Note that there are two reasons why a male would want to have the females in estrous quickly- one is just to have more kids in his lifetime. The second thing to consider is that he is going to get overthrown sometime too, so he needs to get kids started early so they're weaned by the time some new male comes in so the new guy doesn't kill them.In four cases where data exists, infanticide shortened the interbirth interval by 66%.

Prediction 3- reproductive benefits derived by infanticidal males

In 8 of 21 cases the infanticidal male mated with the mother after killing her infant Males may have sired subsequent offspring in 7 of 14 births following committing infanticide.

Response of females to infanticidal attacks

What do the females have to say about all this? This guy comes and kills their kid and then wants to mate with them! Why would they put up with it?!

The answer is, they're kind of stuck between a rock and a hard place, being in one male groups. If a female doesn't mate with the new guy, then she decreases her own reproductive success. Females who hold a grudge and don't mate are thus selected against, so females who forgive an forget end up having more kids.

Also, if you mate with the guy who killed your baby, then you know that your future sons will have good genes for getting mates for themselves when they grow up and take over a group.

Data from gorillas: When a usurper comes in and kills a female's baby, even though the silverback was trying to save it, females are more likely to leave the old guy and go with the new guy! This new guy has shown how tough he is, and the guy who tried to protect her obviously wasn't able to do it, so she may as well go with the tougher guy and get his tougher genes for her future kids.

Females do have some ways of responding to this threat to their reproductive success. Although refusing to mate won't really work, there are things they can do.

Female coalitions One is to form a coalition of females against the infanticidal male. In some cases, this method is effective and together they can protect their babies from the males- see description in CP. This happens in langurs, redtails, and blue monkeys. Note that they are all matrilineal species, so females are living with relatives.

Help from the males A-- male defense in patrilineal societies In these female-dispersing species, you don't tend to see coalitions between females- but if there are multiple males in the group they will form coalitions to try and protect the babies against potentially infanticidal males. For instance, a new male who has just joined and couldn't have fathered any of the offspring, or else a male who has just risen up in the hierarchy and hadn't mated before so wasn't anyone's father. (Although if he's related to other males who have mated, then he wouldn't be as likely to commit infanticide.)

B-- male-female coalitions: baboons When a new male joins a group, he wants to (well really his ancestors have been selected to) kill the babies, but a female and the guy who was likely to have fathered her baby will join together to protect the infant. Sometimes they're effective and sometimes they're not, but it seems when the male tries to help, they are more likely to be successful in protecting the infant. This could be why there's less infanticide in multi male groups.

Post-conception estrus and promiscuity This is not necessarily a conscious deception- it's just a behavioral trait that has been selected for.

Sometimes when a male takes over a group and begins attacking, pregnant females will extend estrous or even come back into estrous even though there's no way they could possibly conceive- he copulates with them when when she later has an infant he figures it's his and so doesn't kill it. This has been documented in colobines, including langurs and red colobus. Females will extend estrous longer into their pregnancy, and they will copulate a lot more, especially with the new male.

Outstanding problems

This doesn't fit into the picture very well, but it's about chimps. All infanticide we've spoken of so far was committed by males, and this is the rule in primates and other animals. There are some exceptions, and one was documented by Jane Goodall.

A female named Passion began killing and eating several of the babies in her community. Together with her daughter Pom, over a period of many years they attacked and killed infants in their group. Usually when males kill a baby, they don't eat it, but these females seemed to be after meat; they'd chase and consume the infant. They were actually seen to eat 3, chase 3 others, and there were 8 others who disappeared under mysterious circumstances. In this period, there were almost no infants weaned successfully.

So this is kind of a question mark because it's only been these two individuals documented- and the daughter probably learned it from the mom- so maybe we can label this one pathological and say that it's not a part of normal chimp behavior.

But infanticide is.

Mon, 02 Jul 2012 02:03:44 UTC | #948422