We have a gruesome lecture topic today! But interesting because it's got a controversial history and it's a pretty strange set of behaviors which we never would have made sense of without thinking of them in terms of modern evolutionary theory. The scientist who has played the biggest role in infanticide, and also wrote the cp article, is Sara Hrdy.

Infanticide among langurs

We're talking about the Asian leaf monkey, from the genus presbytis. They form polygynous bands ( one male groups.) In langurs, males disperse and female remain. Males, instead of going off alone, often form all-male groups which wander around getting into trouble. One thing they do is look for opportunities to overthrow a male with a harem and take over his group of females. Then one of the band will become alpha and kick all the others out again. When this happens, he usually kills all the babies who haven't weaned yet.

This is the usual scenario- a male has a group of females and a group of males will drive him out. Sometimes this is drawn out over a period of time; it's a while before they drive him off and a new male is established. At that time, any unweaned infants in the group will usually be killed by the new male. This was first documented in the 60's by Sugiyama.

Taxonomic distribution

Infanticide occurs in lots of different kind of birds including common ones like sparrows, and swallows. It also happens in rodents like mice and ground squirrels. You also see it in lions. It's a similar sort of situation- females live in matrilineal groups and only the males disperse. A set of several males (usually related) live with the group and enjoy mating privileges until they're driven out and then the new males kill all the babies.

There are plenty of infanticidal primates- lemur catta, red howlers, red colobus, silver leaf monkeys. There are several cercopithecine examples- red-tails, blue monkeys among them. A couple of different savannah baboons, and also chimps and gorillas among the apes.

The two main hypotheses

Population density hypothesis

This was the early explanation. It says that infanticide is a pathological behavior. It isn't part of the normal makeup of the species but is because of abnormally high population densities. Like the way when you crowd lab rats together they kill each other. This made sense for the langurs since one of the groups that was studied was being crowded into little areas by deforestation, and in other study areas people were feeding them and this usually makes levels of aggression rise.

Sexual selection hypothesis

There was a lot of initial resistance to this idea (it was primarily Sara Hrdy's) but there is some really convincing evidence for it.

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 Data: Circumstances and victims of infanticide

In most cases of infanticide, you just assume it happened, but you don't know for sure- a new male comes in and begins chasing the mom with baby- they disappear and when you come back the next day, the baby is gone, so you assume infanticide. This data, however, is from a database in which they used cases where they actually knew that infanticide had occurred.

Evaluating the hypotheses: predictions and tests

Predictions of high population density hypothesis
Infanticide occurs as high population densities
Infanticide will not necessarily benefit the killer

Does infanticide actually occur at higher population densities? 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

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.

Prediction 3- reproductive benefits derived by infanticidal males

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 back come 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.