A little over a decade ago, Sarah Hrdy,
a Harvard graduate student, decided to study the Hanuman langur monkeys.
Their populations had become dense near towns, as they were constantly
being fed by the villagers. Hrdy set out to determine whether or
not overcrowding caused infanticide, in particular males killing babies.
She soon decided that overcrowding was
not the reason for infanticide. The males seemed very tolerant towards
the annoying play of the babies. The problem turned out to be males
from outside the troop.
The troops of monkeys consist
of usually one male and many females. Eventually, an outsider will
challenge the male and chase him away. The new male then proceeds
to kill all the old male's babies and mate with all the females of the
group.
Hrdy explained this behavior
as an adaptative one. She said it was "an evolutionary strategy for
fathering as many offspring as possible", since the females can't conceive
if they are currently nursing a baby. Therefore, the male kills off
the babies so that the females can become impregnated with his babies as
soon as possible. He does not have time to waste waiting for the
other male's babies to grow up, because he will be chased away by another
male within about two years. So, he must kill and mate quickly to
ensure that his babies are weaned before the next male comes around.
In the time since this study
of Hrdy's, this behavior has been seen in other species. Mice, bears,
foxes, dwarf mongooses, and others are noted in the article.
A little over ten years
ago, Stephen Emlen, a Cornell researcher,, reported the behavior in the
female gender of a bird called the Jacana. Having killed a female
for DNA testing, he observed a new female kill the old female's chicks
almost immediatly and then convince the male to mate with her.
This behavior has also been
seen among the lions. Behavioral ecologists Craig Packer and his
wife Anne Pusey, from the University of Minnesota, have observed the lions
of the Serengeti. Usually within 6 months after a new male takes
over the pride, the old males babies are all dead.
In only the last couple
of years, the behavior has been seen in the lemurs of Madagascar.
Bucknell University researcher Michael Periera has studied them and explains
the behavior as adaptive even though the killing doesn't "hasten" the new
male's "own fatherhood". The new male must wait for breeding season,
but by killing the babies right away, he makes sure the females are in
top form to conceive his own offspring.
Some other researchers have
criticized Hrdy's ideas and those of other researchers agreeing with her.
They say that most of the data is not actually seen, but inferred simply
because of the absence of babies. In addition, they say that not
enough studies have been done to see if the new male actually does produce
offspring after he has killed the previous male's babies.
Moreover, Hrdy's
ideas have been applied to humans by other researchers. But most
claim that it just doesn't make sense for humans, including Hrdy.
Even though the "preschool American stepchild is 60 times more likely than
a biological child to be the victim of infanticide", Hrdy and others agree
that this abuse is more likely to be due to the competition over the mother.
Another criticism is of
the supposed genetic transfer of the "infanticide gene". Hrdy states
that infanticide is not genetic, but an innate choice that the animal makes.
Darwinian selection operates on the animal to "choose" whatever course
of action that will allow its genes to be passed on. In the case
of the male taking over a group, he chooses to kill the old male's babies,
which allows his own genes to be passed on. Hrdy states that infanticide
is just "one extreme in a spectrum of parental care".
In conclusion, Hrdy sees
the past 25 years of questioning and criticism of her theory of infanticide
as a necessary process. She states that it makes it clear "how long
it takes to test and flesh out the shocking ideas of sociobiology".
Theories are important and have a place in science. They help us
to understand the real world by the process of exploring and questioning.
Kari Thompson
Carl Zimmer, First Kill the Babies, Discover, v. 17, Sept. 1996, p.
72-26.