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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Online content for Murder in the Troop was originally posted April 2006.

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.

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 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.

Baboon Bonding
http://www.int.iol.co.za/index.php?set_id=1&click_id=31&art_id=qw1068798062574b221
Find out why bonding makes baboons better mothers.

Baboons from Cradle to Grave
http://www.princeton.edu/~baboon/cradle_to_grave.html
Learn about the complete life cycle of baboons on this site sponsored by the Amboseli Baboon Research Project.

Baboon Society
http://www.princeton.edu/~baboon/social_life.html
Take a glimpse into the social lives of baboons.

Chacma Baboon Communication
http://members.tripod.com/uakari/papio_ursinus.html
Find out more about the chacma’s range, ecology, and various means of communication on this Web site.

Chacma Baboon Info
http://animaldiversity.ummz.umich.edu/site/accounts/information/Papio_ursinus.html
All you every wanted to know about chacma baboons can be found on this Web site from the University of Michigan’s Museum of Zoology.

Chacma Friendships and Infanticide
http://www.rci.rutgers.edu/~palombit/
Discover more about Dr. Ryne Palombit’s research on the chacma baboon.

Chacma Overview
http://www.blueplanetbiomes.org/chacma_baboon.htm
Read a brief overview of the chacma baboon.

The Leakey Foundation
http://www.leakeyfoundation.org
Learn what the Leakey Foundation is doing to increase scientific knowledge and public understanding of human origins, evolution, behavior, and survival.

Rafting the Zambezi
http://away.com/great_adventures/zambezi-rafting.html
Read about the thrills offered by a rafting trip down the mighty Zambezi River.

South Africa’s Chacma Baboons
http://www.sunsetbeach.co.za/cape_point/chacma_baboons.htm
Learn about life among troops of chacmas living on South Africa’s Cape of Good Hope.

Wildlife Safari Information
http://www.wildlifesafari.info
Plan your African safari with the help of this Web site loaded with tips, planning tools, and resources.

Books

Cheney, Dorothy L., and Robert M. Seyfarth. HOW MONKEYS SEE THE WORLD: INSIDE THE MIND OF ANOTHER SPECIES. Chicago: University of Chicago Press,1990.

Duffy, Rosaleen. KILLING FOR CONSERVATION: WILDLIFE POLICY IN ZIMBABWE. London: The International African Institute, 2000.

Dugard, Martin. INTO AFRICA: THE EPIC ADVENTURES OF STANLEY AND LIVINGSTONE. New York: Random House, 2003.

Estes, Richard D. THE SAFARI COMPANION: A GUIDE TO WATCHING AFRICAN MAMMALS INCLUDING HOOFED MAMMALS, CARNIVORES, AND PRIMATES. White River Junction: Chelsea Green, 1999.

Martelli, George. LIVINGSTONE’S RIVER: A HISTORY OF THE ZAMBEZI EXPEDITION, 1858-1864. New York: Simon & Schuster, 1970.

Nolting, Mark W. AFRICAN SAFARI JOURNAL. Fort Lauderdale: Global Trade Publishing, 2003.

——. AFRICA’S TOP WILDLIFE COUNTRIES, 6th ed. Fort Lauderdale: Global Trade Publishing, 2003.

Sapolsky, Robert M. A PRIMATE’S MEMOIR: A NEUROSCIENTIST’S UNCONVENTIONAL LIFE AMONG THE BABOONS. New York: Simon & Schuster, 2001.

Smuts, Barbara B. SEX AND FRIENDSHIP IN BABOONS. Cambridge: Harvard University Press, 1999.

Strum, Shirley C. ALMOST HUMAN : A JOURNEY INTO THE WORLD OF BABOONS. Chicago: University of Chicago Press, 2001.

Teede, Jan, and Fiona Teede. THE ZAMBEZI: RIVER OF THE GODS. London: Andre Deutsch Ltd., 1991.

Van Schaik, Carel, and Charles H. Janson. INFANTICIDE BY MALES AND ITS IMPLICATIONS. Cambridge: Cambridge University Press, 2000.

As we learn in NATURE’s Murder in the Troop, life for a chacma baboon is defined by strict hierarchical structure. Among the most highly socialized of animal species, chacmas rely on a complex and varied communication system in order to candidly express themselves to family, friend, and even foe. The following are some examples of the visual signals, vocal gestures, and body language they use to get their message across.

Chacma baboons draw upon a rich repertoire of vocal signals to express themselves. Adult males may assert their dominance in a troop by emitting relatively soft, but threatening “uh huh” sounds that are referred to by primatologists as “grunts.” Grunting commonly precedes a “two-phase bark,” which is a deep, loud “wa-hoo” call, emitted at intervals of 2 to 5 seconds. This call is frequently directed in aggression toward other males or an approaching predator.

But antagonism on the part of a male may evoke the equally powerful vocal response known as “screeching.” In a series of high-pitched screams, male and female baboons of all ages use this vocal tactic in their defense, as it appears to inhibit the hostile behavior of the aggressor.

When retreating from a threatening animal, subadult and adult baboons may emit a short, sharp “yakking” call, while infants and juveniles produce a chirplike “clicking.” These sounds are often coupled with a “fear-grimace” in which the animal retracts its lips and exposes its clenched teeth. This display serves as a pacifying gesture and is used to avert an aggressive encounter. Infants in distress may also emit a cry known as an “ick-ooer,” a two-phased call made through pursed lips.

In the case of immediate danger, all members in a troop, with the exception of the adult males, engage in a “shrill bark” — an explosive, single, sharp cry of alarm that alerts troop-mates to flee. Higher in pitch than the shrill bark is the “doglike bark,” sent out by subadult and adult baboons when members are thought to be separated from the troop.

But not all calls are designed to communicate aggression, fear, or danger. The animals articulate friendly intentions through low, soft “rhythmic grunts,” interpreted as an amiable greeting when one individual approaches another. Juveniles at play are often heard emitting nasal “chattering,” which involves rapid murmurs that signal, at least for the moment, all is well in their world.

Whether on the giving or the receiving end of a glance, nod, smile, or stare, we can all appreciate the power of visual communication. A look, as they say, is worth a thousand words. Adult male chacmas have earned a “tough guy” reputation. And while violent fights do occur, they can be costly. Injuries sustained during conflict may lead to infection and the inability to forage or travel, and could leave the troop more vulnerable to predators or rivals. Therefore, it is in the baboons’ best interest to avoid coming to blows. Visually assertive displays may serve to get a message across before the situation escalates into a physical brawl.

“Staring” sends a crystal-clear warning. In this penetrating glare, the baboon raises his eyebrows while keeping his eyes trained on his subject. To enhance the menacing effect, he will move his ears back so that the skin on his face and scalp is stretched taut, revealing a differently colored fur beneath his eyes. It is a signal that the recipient would be wise to heed.

A “canine display” is sometimes presented by a lower-ranking male to test a male of higher rank who is consorting with a female. A quick flash of the teeth serves as a threat and is often accompanied by “eyebrow raising,” which is viewed as an antagonistic gesture in itself. In order to avoid a scrap, a baboon on the receiving end of these visual cues may respond by “rapid glancing,” turning his head in the opposite direction and shifting his eyes to ease the tension of the situation. Another means to avert an aggressive encounter is through a “fear grimace,” in which the baboon retracts its lips and exposes its clenched teeth. This expression is usually accepted by the aggressor as a pacifying gesture.

But hostile displays aren’t only directed at troop-mates; they are also useful in defending the group. When a predator or rival troop approaches, an adult male is often effective in repelling them by opening his mouth widely in a “tension yawn,” fully revealing his formidable canine teeth.

Nonaggressive visual cues are also highly important in chacma society. “Social presenting” is a submissive act in which a female or juvenile exhibits their hindquarters to a higher-ranking male. A female may also present to a mother with an infant as a gesture of respect. She often embellishes this act by “lip smacking” to signal her affiliation with the troop. In this display of reassurance, the baboon protrudes and smacks its lips together repeatedly. Along with “teeth chattering,” this is also performed by a dominant male when a female or juvenile is presenting to him.