Murder in the Troop
http://www.pbs.org/wnet/nature/episodes/murder-in-the-troop/introduction/2053/
Amid the pristine wilderness of Africa’s Zambezi region unfolds a sinister tale of political
upheaval, sexual intrigue, and Murder in the Troop.
Along the banks of the mighty Zambezi River in the high plains of northern Zimbabwe, a troop
of chacma baboons tend to their daily routines in accordance with the rules of social order that
make for a stable and cohesive community.
Alliances can mean the difference between life and death, as each day brings a myriad of
challenges. Months of searing heat and drought grip the land, and ambushes by rival troops and
predators such as leopards, crocodiles, and pythons pose imminent threats.
A rare set of twins born to the dominant male and female face an especially difficult trial. In an
environment where only a quarter of infants survive their first year, the odds are stacked against
them. But their “royal parentage” could carry some advantages for their survival.
In the Zambezi, privilege can be lost as quickly as it is gained. A large male from a neighboring
troop enters their ranks, determined to unseat the dominant male. In a swift and decisive battle,
the king is deposed and the entire stability of the troop is thrown into turmoil. Under the rule of
the new alpha male, the twins are no longer safe — the intruder has cast his eye on their mother.
But the infants are not yet weaned and until they are, their mother will remain sexually
unavailable. For the tyrant, the equation is simple and brutal, but just how far will he go?
The rains are late and the dry season stretches into its eighth month. Competition for food brings
the troop’s inherent rivalries to the surface, and bouts of infighting break out. The alliances that
were once the binding thread of the group begin to come undone.
Meanwhile, only one of the twins has survived, and he must grapple with his brother’s murder,
his father’s overthrow, and a mother whose affections have been commandeered by the one
responsible for the heinous deeds.
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Inside Chacma Baboon Society
They revel in the good times and comfort in the bad, offering one another companionship and
support, strength and protection. After all, what are friends for?
That is precisely the question that a devoted sector of scientists and behaviorists are puzzling to
answer. But the friendships they are concerned with are not those that develop among humans,
but rather those of the chacma baboons that inhabit southern Africa’s Zambezi region.
As NATURE’S Murder in the Troop shows, chacmas live in large troops, governed by strict
hierarchical order, in which alliances and family ties are the foundation. Spending a large part of
their day in close proximity to one another, they engage in a host of communal bonding rituals,
from grooming to foraging to collectively caring for their young. These activities serve to
strengthen their social network as they build trust within the troop and buffer against outside
threats such as predators or rivals. It would seem there could be no safer environment in which to
raise a family.
During grooming, the pulling of hair releases endorphins.
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But there is a dark side to chacma society. Desiring to father their own offspring, highly
aggressive alpha males often resort to infanticide — killing baboon babies that are still
dependent on their mothers’ milk. A nursing mother will not come into heat, but without a baby
to feed, she will stop lactating and once again become sexually available.
“Infanticide is common in chacma baboons,” says Dr. Ryne Palombit, Associate Professor at
Rutgers University’s Center for Human Evolutionary Studies. He has been studying chacma
baboon troops in southern Africa for over a decade. “It is usually perpetrated by a male who has
recently immigrated into the group and has attained alpha status. That is not to say that all males
who accomplish these two things always attack and kill infants.”
But the attacks occur often enough to shake the nerves of every new mother in a troop. In some
years, Dr. Palombit has seen troops suffer an infanticide rate of up to 75 percent of all infant
deaths. So, what is a mother to do in order to avoid the risk of her baby being killed? According
to Dr. Palombit, those with young offspring often seek out friendships with males. “In chacmas,
just about every lactating female has at least one male friend,” offers Palombit. “I think
friendships are likely to reduce the risk of infanticide, and we’re trying to identify how that
happens.”
A female will develop a bond with a male solely for the protection he may offer, especially when
it comes to defending her baby from an attack. “We have data suggesting that although many
males may rush over to an infant when the alpha male attacks it,” says Palombit, “it is usually
only the male friend of the infant’s mother who will get involved directly and actively. For
example, he’ll actually fight the attacking male or scoop up the infant and run off with it.” In
some instances, a male consort may take on the role of foster parent after a mother’s death.
Friendships typically involve no sexual behavior, as lactating females almost never copulate. But
the relationship can benefit the male in other ways. “The female invests heavily in the
relationship by keeping in close proximity to him and grooming him,” says Palombit.
Furthermore, a male may gain an advantage in joining a troop if he forms an association with a
female on the inside.
As their hierarchical law dictates, chacmas typically consort with those only within their own
rank. But females will sometimes seek friendships with males of higher rank, as they are usually
more assertive in their behavior and therefore may offer more security. But Palombit is careful to
note that despite social status, some baboons are simply better protectors than others.
While friendships with males may be useful in guarding against infant death, it is important to
remember that in the wild there are no guarantees. Only a quarter of all chacma babies survive
their first year. And when the worst happens, bereaved mothers turn to their female friends for
comfort, often using the act of grooming as a tactile form of “grief therapy.” When the fur is
stroked and pulled during grooming, endorphins are released, producing a natural high that
relaxes the animal.
A 2006 paper published in Proceedings of the Royal Society of Biological Sciences underscores
the important physical benefits of companionship in chacma society, especially during times of
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loss. In a 14-year study of chacma baboons in Botswana’s Okavango Delta, researchers regularly
measured the levels of glucocorticoids, a group of stress hormones, in 21 females. In humans,
these levels rise after the death of a close friend or relative. The baboons who had suffered losses
were found to have elevated levels of the hormones as well. The study further indicated that after
the death of a family member, females compensated for their loss by “reaching out” more than
usual to other females. They increased their grooming rate and broadened their circle of
grooming partners — even “breaking rank” to form alliances with those beneath them.
Researchers believe that the contact serves as a coping mechanism for the baboons, as their
glucocorticoid levels dropped significantly just after grooming.
Not only is this study the first to provide direct evidence that some animals mourn the loss of
individuals, along with the work of Dr. Palombit and his colleagues, it illustrates how social
bonds reduce stress and keep the troops cohesive.
Whether they involve fiercely defending each other or offering gentle and nurturing support, one
thing is certain — friendships are vital to chacma society. And though we have not yet pieced
together all the intricacies of their relationships, these baboons are giving us a pretty good
indication of what friends really are for.
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