Question:  Why are the males killing the babies?

     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.