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
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
The two main hypotheses
Population density hypothesis
- Infanticide is due to high population densities, and is an
aberrant and dysfunctional behavior.
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
- Infanticide is a male reproductive tactic:
- Loss of suckling infant leads to the onset of estrous in the
- Males gain a reproductive advantage through earlier conception by
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
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
- 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
Evaluating the hypotheses: predictions and tests
- Predictions of high population density
- 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
Mothers will become sexually active earlier than if their infants had
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
Prediction 2- the effect of infanticide on interbirth intervals
- In four cases where data exists, infanticide shortened the
interbirth interval by 66%.
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
- 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
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
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.
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
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.