A FEW weeks ago, the news was out that female chimpanzees are able to hunt and kill their own prey and, in one population, they're using spears to get the meat, in spite of the fact that males are more usually known for their hunting skills.
Now reports have come in from another study site that female chimps are capable of ferocious and deadly attacks on members of their own species.
Male chimpanzees are well known for violent and sometimes lethal behaviour, but Simon Townsend, of St Andrews University, and his co-researchers report three instances where gangs of female chimpanzees brutally attacked mothers and killed their babies with a bite through the front of the skull at a site in Budongo forest in Uganda.
It appears that the attacks, reported last week in Current Biology, were perpetrated by females from the resident community against immigrant and peripheral females and their newborns.
Infant-killing in apes and monkeys is not that unusual. I witnessed the grisly deed myself while studying the behaviour of samango monkeys in South Africa some years ago.
However, such infanticidal behaviour is usually perpetrated by adult males, as it was with the samangos. The instances I saw happened when a foreign adult male had kicked out the single resident adult male from a troop of females. By then killing the suckling infants in the troop he ensured the speedy return of the mothers to fertility, meaning that he could sire his own youngsters. In contrast, the male chimpanzees at Budongo, far from instigating the aggression, actually tried to defend the infants.
Infant-killing by female chimpanzees has previously been described only by Jane Goodall in the 1970s at Gombe Stream National Park in Tanzania, when the mother-and-daughter duo, Passion and Pom, killed and ate at least three infants.
Goodall was horrified and the behaviour was thought to be pathological. Professor Anne Pusey, of the Jane Goodall Institute Centre for Primate Studies at the University of Minnesota, has worked at Gombe for many years and has made observations which may help to explain such shocking activity. She noticed that females tend to be aggressive towards others who are trying to immigrate, and puts this down to competition over food, which can limit the number of babies a female chimp can raise.
New females and their offspring coming into the area are a threat. "Killing the infants is a way of removing a feeding competitor when they are very vulnerable and young," says Prof Pusey.
This idea appears to be borne out by the observations at Budongo, and it seems that infanticide could well be a natural response to particular social and ecological conditions. The number of adult females in the chimpanzee community has doubled over the last five years due to immigration by 13 females, many with dependent offspring, but the community has not been able to increase its home range.
Mr Townsend thinks the resulting pressure has led to the infanticidal behaviour. "Females are experiencing competition for mates and for food and they're responding in a violent manner towards this," he says.
Professor Martin Muller, of Boston University, writing a commentary in Current Biology, points out it would be difficult for a lone female to overcome a mother and seize her infant, as illustrated by
a mother and seize her infant, as illustrated by failed infanticide attempts by females at Gombe.
Budongo females spend a lot of time together in "mothering parties" and so have the capacity to attack in concert, while in other populations females are more likely to travel alone. This could help to explain why more of this violent female behaviour has not been seen in other areas, he suggests.
Prof Muller also raises the worrying possibility that forest disturbance through human activity could force the movement of females between communities, leading to this aggressive behaviour.
After female chimps have given birth, they try to stay with and travel with adult males, says Prof Pusey, possibly for protection against aggression from females. In fact, some biologists have suggested that the risk of infanticide can at least partly account for why male and female apes and monkeys stay together in groups at all.
Protection n of infants has even been proposed as an underlying reason why we humans form pair-bonds, so that fathers can be around to help look after their offspring. Usually, the risk of harm to youngsters comes from other adult males, but it seems that because of the particular ecological circumstances in which chimpanzees live, chimp babies can be vulnerable to female, as well as male, aggression.
According to Mr Townsend, the behaviour at Budongo suggests that female chimpanzees employ aggression strategically, and that we can reject the stereotype of violent and demonic males in contrast to quiet peaceful females. "If their resources are under threat, females can become just as violently aggressive as males," he says.
As Prof Muller puts it: "The myth of the passive female can be laid to rest, alongside her victims."
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