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CHAPTER 7

Primate Sociality, Social Behavior, and Culture

A lecture on primate behavior has to start with Jane Goodall. Goodall’s pioneering work with wild chimpanzees that began in the 1960s led not only to important new insights about our closest primate relatives but to new questions that researchers continue to try to answer today. Goodall studied a group of chimpanzees at the site of Gombe, located on the shores of Lake Tanganyika in Tanzania (map to the right). Some time after Goodall arrived at the site in 1960, the chimpanzees had been habituated; in other words, they had gotten used to her to the extent that she could observe their behavior up close. She made some of the first observations of chimpanzee social organization, competition, cooperation, diet, and intelligence—many of the topics we will cover in this lecture. Goodall made the surprising discovery that chimpanzees make and use tools (something previously thought to be unique to humans) and she discovered that chimpanzees were not vegetarians, as was thought. They hunted small antelopes, warthogs, and even monkeys. This lecture will explore some of what scientists have learned about primate behavior, and how we can use modern primate behavior as a model to help us understand the evolution of human social behavior.

Primate Sociality, Social Behavior, and Culture: Questions

Questions addressed in this chapter:

Why are primates social?

What is special about primate societies and social behavior?

How do primates acquire food?

How do primates communicate?

In this lecture, we will explore the questions that primatologists ask and attempt to answer. Why are primates social? This is an important question because humans are primates, and humans are also quite social. By understanding the benefits of being social in other species of primates, we might begin to understand the selective forces that encouraged social behavior in human ancestors as well. We will look at the different group structures, or societies, that primates live in, and how reproductive strategies differ in different primate societies. Central to understanding primate behavior is a study of how primates acquire the food they need to survive. Finally, we will look at some case studies that help us understand how primates communicate with one another. No other primate possesses the language capacities of modern humans, but the origins of human language can be better understood by examining how different primate species communicate. Let’s start with the complex social groups that primates live in.

Primate Societies: Diverse, Complex, and Long-Lasting

Primates express themselves socially through a range of behaviors.

Many primate societies are complexly organized.

Primates form various long-term social relationships.

Primatologists have made some general observations about social behavior in our primate relatives. First, primates have a wide range of behaviors that serve as social cues. They touch one another, groom, fight, greet, lip smack, and even hug. These behaviors serve as a form of social “currency,” which helps establish, break, or reinforce bonds between individuals in a group. Primatologists have also documented the many different, and often highly complex, ways in which different primates form alliances with one another. Variables determining who gets along with whom include kinship, rank, age, and sex. Finally, unlike those in many other species, primate social relationships are long term. Establishing long-term bonds means that one individual can assist another in the short term, only to be rewarded later. This kind of cooperation between individuals is more likely to evolve in species that have long lives, and have the brain power to remember specific individuals and events.

Sociobiology: The Evolution of Behavior

Natural selection favors behaviors that enhance survival and reproduction.

Why are primates social at all? And how can we understand social behavior in an evolutionary context? The key here is to recognize that primate behavior is in many ways dictated by natural and sexual selection. Those behaviors that enhance our chance of survival and reproduction will be selected for, and will evolve. It is a misconception to think that only anatomies, like bird beaks and tooth size, can evolve. Behaviors follow the same rules of evolution by means of natural selection. There is variation that is, in part, heritable, and this variation leads to differential survival and reproduction. The idea that behaviors can evolve by means of natural selection is the central tenet of the science of sociobiology—the brainchild of the Harvard biologist E. O. Wilson. Of course, sociobiology does not claim that all behaviors are innate. Many behaviors are learned, and, in fact, behaviors, like all biological products, are a function of both our genetic predispositions and our particular context, or environment. This combination gives primates, including humans, a range of behavioral flexibility that has allowed us to adapt to a wide variety of environmental situations.

Primate Social Behavior and Sexual Dimorphism

Male reproductive strategies

Compete with other males for access to females

Female reproductive strategies

Compete with other females for access to resources

Most primate behavior can be understood in the context of reproduction. Behaviors that increase the fitness of an individual will be passed on to the next generation, and as we’ve already learned, fitness can be increased by having more offspring. Primate males and females necessarily have different reproductive strategies because of the simple fact that primates are mammals. It is the female that produces a single, valuable egg during her reproductive cycle, gestates the energetically expensive infant, and nurses the growing newborn. What the female needs most to assure that her offspring survives is adequate access to food. Therefore, females will compete for access to high-quality food resources. Males, on the other hand, have a very different strategy. Sperm are very plentiful (and therefore, energetically quite cheap), and male primates often contribute very little to the raising of the offspring. Therefore, the limiting factor to male reproductive success is adequate access to females. Males will therefore compete with other males for access to females. This reality of being a mammal results in interesting differences between males and females—something called sexual dimorphism. For instance, as shown in the graph here, males that compete individually have more sexual dimorphism. Those individuals who happen to be large win aggressive encounters with other males, acquire a higher rank, reproduce more often, and pass on the genes underlying the large canines and body size. But in primate groups where males and females form long-term pair bonds, canine and body size doesn’t matter as much and selection has not resulted in big differences between the males and females. Notice that we could use this information about primates today to infer what the mating system was like in extinct primates if we have fossil evidence for how big the body sizes were in males and females of the same species.

Primate Residence Patterns

The graph on the previous slide implied that there were different primate residence patterns; in fact, it is even more complicated than that. Different primate species tend to live in the various social systems shown here. But even within a species, these residence patterns can change depending on the food availability. Nevertheless, there are generally six different primate residence patterns recognized by primatologists. In the upper left is the one-male/multifemale group, called a polygynous group. The one breeding male is usually exceptionally large and he constantly has to ward off aggressive challenges from other males. This one male reproduces with all the females of reproductive age. Gorillas often will live in a one-male/multifemale group. The opposite can happen too, where one breeding female breeds with several males of reproductive age. This group is polyandrous, and is rarer. Only some species of New World monkey live in this kind of social grouping. Many species, like chimpanzees and baboons, live in a multimale/multifemale, promiscuous mating system. Rank is particularly important in these groups, with higher-ranking males gaining more opportunities to reproduce. At the bottom left is a residence pattern known as all-male. In some primate groups, like baboons, bachelor males form temporary groups before individuals settle into a multimale/multifemale residence. Gibbons and some New World monkeys, like marmosets and owl monkeys, form one-male/one-female monogamous pair-bonds. Finally, some primates live solitary lives, interacting only for reproductive purposes. Orangutans and some prosimians tend to be solitary.

Primate Reproductive Strategies

Sexual selection

A form of natural selection based on the attractiveness of an individual’s traits to the opposite sex

Infanticide

Seen primarily in one-male, multifemale group

Dominance

How a primate acquires resources and access to mates

Because females invest more in their offspring than males do, males and females have evolved different reproductive behaviors. For instance, males compete with one another to gain access to reproductively receptive females. Those males who happen to be larger, have larger canines, or have the louder or more melodious vocalization, reproduce more often, passing on to the next generation the very features that gave them reproductive success. This form of natural selection is called sexual selection and can lead to differences in size, coloration, and appearance between males and females of a species. There is another male behavior that is not nearly as pleasant to think about but which can still be understood in an evolutionary framework. It is called infanticide. Because a female primate who has a nursing infant cannot become pregnant (something called lactational amenorrhea), males who have recently become the dominant individual will sometimes kill an infant that is not his own. This causes the female to resume ovulating, and the dominant male can mate with her, thereby increasing his own fitness. This behavior was first observed in langur monkeys by the primatologist Sarah Blaffer Hrdy, and has since been observed in many different primate species, from howler monkeys to gorillas. It is inaccurate, however, to think that females do not have reproductive choice, or do not compete with one another. They do. However, they do not compete for access to males, as there are plenty of willing males to go around. Instead, they compete for access to high-quality resources to support their offspring. There is therefore a relationship between rank or dominance (which leads to access to better food resources) and the number of surviving offspring. This has been measured in gelada baboons. As can be seen in this graph, the higher-ranking monkeys have more births per year than the lower-ranking monkeys. This trend is being driven by access to enough food to support a healthy baby.

Cooperation in Primates

Altruism

Kin selection: William Hamilton

Mutualism

Up to this point in the lecture, we’ve been discussing competition among primates. But primates are also remarkably cooperative. Chimpanzees, for instance, will hunt together and share food with one another. Many primates, including humans, groom one another, as can be seen in the image at the bottom of this slide of grooming chimpanzees (left) and humans (right). This practice can serve a hygienic function, but grooming also helps to reaffirm social bonds or to communicate relative rank (subordinates often groom dominants). Why would one individual ever help another one? It is difficult to understand cooperation in the context of natural selection. Cooperative behaviors like grooming are altruistic, meaning that they help one individual, but only at the expense of another. For instance, a baboon may give a warning call to alert his fellow baboons that a leopard is nearby. But by doing that, he draws attention to himself and might get eaten! Why would natural selection ever favor such a behavior? In the early 1960s, an evolutionary biologist by the name of William (W. D.) Hamilton hypothesized that altruistic acts can evolve if the act benefits related individuals in the group. By helping individuals related to you, you are indirectly helping your own genes survive long enough to get passed on to the next generation. This is known as kin selection, and it helps us understand altruistic acts like female alliances in baboons and macaques (where females live in their natal groups), and territorial patrolling in male chimpanzees (where males lives in their natal groups). But cooperation is more complicated than that, since unrelated individuals will often help one another. In addition to kin selection, a phenomenon known as “mutualism” is also at work. This is the “I’ll scratch your back if you scratch mine” idea, and because of the long lives and elevated intelligence of primates, they will engage in long-term exchanges of cooperative acts, like food-sharing, that cannot be explained solely by kin selection.

Chimpanzee Hunting Behavior

Most of you probably know that chimpanzees are quite clever. What you may not know is that one of their favorite things to actively hunt are monkeys. Chimpanzees often hunt in groups, which increases their success rate, and their hunting often appears planned and coordinated. They go after young antelopes (called bushbuck), wild forest pigs, and, as I’ve already mentioned, monkeys. Sometimes when the prey (often a juvenile red colobus monkey) is captured, it is shared with other members of the hunting party. Some researchers have also found that a male might share meat with a female to increase the chance that the female will mate with him when she is in estrous. In addition to hunting behavior, chimpanzee males also will conduct territorial patrols and sometimes engage in warlike raids of their neighbors’ territory. These raids can result in the brutal beating or killing of rival males. Chimpanzees can be wonderfully cooperative and clever, but they can also be brutally violent and competitive. In many ways, these sides of chimpanzee behavior mirror the seemingly contradictory behaviors of humans—wonderfully compassionate at times, and horrifically ruthless at others.

Sociality and Group Living

The reason primates even have the opportunity to be cooperative is because they are social and often live in large groups. Why do primates live in groups in the first place? Not every animal lives in groups, and group living has its costs. For instance, in groups members must compete with other individuals for access to mates and to food. So why would sociality evolve? Researchers studying this question suspect that group living evolves in animals that have an elevated risk of being eaten. Primates are preyed upon by many different species: large cats, snakes, birds of prey, and even other primates. When there is the elevated risk of predation, group living has its benefits. Primatologists have observed warning calls (which you will learn about later in this chapter) and physical defense, but it is difficult to observe direct defense of each other to a predator due to the nature of studying primates. Primate predators tend to be afraid of humans and avoid primate groups being studied. In studying the nests of eagles, which are common predators for primates, primatologists were able to show that predation rates are lower among larger primate groups than among smaller primate groups. This suggests that there is an increase in survival based on group size. Mutual cooperation, or sociality, is favored due to the protection it provides. But it also provides access to mates.

Food Acquisition in Primates

Primate foraging time

Up to 50% of waking time

Female burden

Good nutrition leads to:

Earlier first birth

Healthier infants

Short interbirth interval

Longer lifespan

Food factors

Quality

Distribution

Availability

Primates not only need to avoid becoming food, they need to gather enough food to survive, grow, maintain their bodies, and ultimately reproduce. This is not a trivial subject. Primates spend up to 50% of their waking day foraging for food. As we’ve already discussed, this burden is particularly high on female primates, and especially on mothers, since they have to feed themselves as well as support a growing, dependent offspring. Studies have shown that success foraging for food translates directly into reproductive success for female primates, since females with good nutrition reproduce at an earlier age, have healthier babies, have a shorter time between births (called an interbirth interval), and live longer. Collectively, these advantages add up to a rather large increase in overall fitness.

A primate’s strategy for successful feeding depends on three factors: the quality of the food, the distribution of it, and its seasonal availability. Different primates have evolved different strategies to extract energy from the food they eat. Nevertheless, some food, like fruit and meat, has more energy per unit size than other things, like leaves or grasses.. The cellulose and fiber in leaves and grasses are difficult to digest, but they are quite plentiful, leading to an interesting trade-off in quantity versus quality of food items. The distribution of food also drives foraging patterns. Primates need food, and they have to maintain a net gain of energy relative to the energy expended acquiring the food in the first place. Behaviors that minimize the costs of acquiring food are therefore beneficial. Ideally, primates can sit in a fruit tree and eat all day. But fruit is not as plentiful as grass and leaves. Therefore, different primates have evolved different group sizes, or strategies for breaking the group into smaller foraging parties. Finally, food availability fluctuates seasonally. Because primates eat mostly forest foods (leaves and fruit), they are confined to equatorial regions.

Do Nonhuman Primates Have Material Culture?

Humans obviously rely on much more than their hands and mouths to acquire and process food. We use technology, and the different technologies we use to acquire different foods around the world result in many cultural differences in food acquisition in our species. But what about other primates? In the 1960s, Jane Goodall made one of her most profound discoveries when she observed a chimpanzee modifying a twig, inserting it into a termite mound, and eating the insects that clung to this termite fishing pole. By observing this behavior in chimpanzees, we can infer that the earliest human ancestors were at least capable of this kind of tool construction. Continued work at chimpanzee sites throughout Africa have revealed three central features about chimpanzee tool use. First, chimpanzees are exceptionally intelligent. Much of the work on chimpanzee intelligence has been done in captive settings. Nevertheless, chimpanzees can learn how to crack open nuts with stones, or to use a sharp object to cut twine from around a box containing food. They not only learn these things, they can pass this behavior on to other members of a group, often from mother to offspring. Second, most of the tool-making behaviors in chimpanzees are associated with food. For example, the primatologist Jill Pruetz recently observed chimpanzees making spearlike objects (left image) to skewer little strepsirrhines living in the hollows of tree trunks. Finally, these tool-making behaviors are localized, meaning they vary from site to site across Africa. For instance, chimpanzees in West Africa use hard stones to crack open highly nutritious nuts. This behavior is not practiced elsewhere in Africa, even though there are stones and nuts present. These local traditions, passed on from generation to generation through social learning, are rudimentary “cultures.” And it is not just found in chimpanzees. Dietary and tool-making traditions have been observed in various orangutan and even capuchin monkey groups as well.

Primate Communication

Primate vocalizations are complex

Playback experiment

Let’s finish our discussion of primate behavior with some information on how primates communicate with one another. For a long time, it was thought that primate vocalization was a simple, emotional response to some stimulus. But more recent research is showing that primate calls are much more complex, and may provide a model for understanding how human language evolved. Researchers translate the meaning of different primate vocalizations by recording the vocal repertoire of a primate group, as is shown in this image of Dawn Kitchen recording the alarm calls of a chacma baboon group responding to a nearby lion. Sometimes, though, the context of the call is not as obvious as a lion in the vicinity, and the meaning of these calls is deciphered using “playback experiments” in which a recorded call is broadcast to a group of primates through hidden speakers, and the listener’s reaction is carefully observed by primatologists. For instance, the primatologists Dorothy Cheney and Robert Seyfarth played the vocalization of a screaming infant vervet monkey to a group of mothers whose infants had all wandered off. Two things of note occurred. First, only the mother of the particular infant whose scream was artificially broadcast looked toward the speakers. Second, the other mothers looked not toward the direction of the hidden speaker but at the specific mother of the screaming infant. This experiment supports the hypothesis that primates know each other and their distinct vocalizations and also know who is related to whom in a group.

Primate Communication: Baboons and Howler Monkeys

Subtle vocalizations can have important meanings. Cheney and Seyfarth have also recorded the sounds made by chacma baboons in Botswana. They found that subordinate baboons will move off if a dominant approaches silently, or produces a particular sound called a threat-grunt. However, there is a second, quite different, grunt that the dominant sometimes makes, and it results in the subordinate NOT moving away—a sort of “It’s OK, I’m not going to hurt you” grunt. There are also very subtle differences between two other calls—an alarm and a separation call. These subtle differences cannot be detected by human ears but clearly are distinguished by baboons, who respond differently to these different vocalizations. Of course, sometimes the calls are not subtle at all. Chacma baboons make “wahoo” calls that are very loud, and are used during male-vs-male aggressive competitions. These calls accurately communicate the physical status of the individual, allowing lesser males to avoid confrontations with much stronger ones who make more impressive “wahoo” calls. Similarly, howler monkeys produce loud, long calls to establish their territories. Howler monkeys can count the number of long callers in a group to assess the strength of their neighbors. Using playback experiments, it has been shown that howler monkeys that are outnumbered will retreat, while groups that outnumber the long calls they hear will advance.

Primate Communication: Diana Monkeys and Chimpanzees

Primate vocalizations can get even more complicated than counting numbers of individuals. Diana monkeys, for instance, give different calls depending on whether the predators are aerial (like an eagle) or terrestrial (like a leopard). Vervet monkeys do this too, and are born with the ability to make predator-specific alarm calls. However, the primatologists Cheney and Seyfarth found that baby vervet monkeys make the aerial alarm call when anything at all comes from the sky, whether a raptor or a falling leaf. As they age, they begin to refine their use of these vocalizations, refraining from giving the aerial alarm call unless there truly is a raptor present. Diana monkeys also respond quite differently depending on whether the threat is a pursuit hunter or an ambush hunter. Stealth and ambush predators are met with a barrage of loud vocalizations to warn others of their presence. Monkeys that live in the same forest, Campbell’s monkeys, also have an alarm call for ambush hunters like leopards—a loud “krak.” However, if the initial alarm was made by Diana monkeys, the Campbell monkeys note this “linguistically” with an “oo” added to the end of the call, for a “krak-oo.” This is sort of like adding an “ed” to the end of a verb to indicate something happened in the past. When Diana monkeys encounter chimpanzees, they slink away, so as not to alert the chimps to their presence. Of course, chimpanzees themselves are extremely vocal and also have a wide range of sounds meaning different things. Many of these calls are food related, and are given depending on the type and amount of food they encounter in their forest.

Primate Communication: Types

Vocal dialects

Humans: new vocalizations throughout their lives

Nonhuman primates: preprogrammed vocalizations present at birth

Gestural communication

Hand clasp

Cultural variants

Ape language studies

Kanzi ASL

Let’s end this lecture with a closer look at communication in our closest living relative, the chimpanzee. In the wild, chimpanzees communicate not only vocally but with gestures. For instance, the hand-clasp grooming technique shown on the right is one of many different forms of gestural communication documented in wild chimpanzees. Interestingly, this form of grooming is found in only some chimpanzee communities (Mahale, Tanzania, for instance) but not in others (the nearby Gombe site where Jane Goodall worked, for instance). Ecology and genetics do not explain the pattern of hand-clasp grooming across Africa, and therefore researchers suspect that this is yet another example of a cultural tradition practiced by some chimps in some regions of Africa but not by others. Chimpanzee groups can even vary vocally, almost the way human dialects vary from region to region. The extraordinary communication abilities of the apes are best documented in captive-ape language studies. Many apes—including orangutans, gorillas, and chimpanzees—have been taught American Sign Language. Kanzi, a bonobo, has been taught to communicate using symbols on a keyboard (lexigrams). He has the same level of communication skills found in a human two-year-old, and is shedding important light on the communication abilities of our early human ancestors, before the evolution of spoken language. While humans routinely invent new vocalizations throughout their lives, nonhuman primates appear to have preprogrammed calls that are available shortly after birth.

CONCEPT QUIZ: Question 1

A polyandrous society means

there are several adult males, several adult females, and their offspring.

there is one reproductive-age female, several adult males, and their offspring.

there are multiple adult females and males and their offspring.

there is one adult male, one adult female, and their offspring.

B. Polyandrous refers to a social group that includes one reproductively active female, several adult males, and their offspring.

Thinking Anthropologically: Discussion Topic 1

Describe some of the observations about chimpanzees noted by Jane Goodall. How might the habituation of the chimpanzees have affected her observations?

Thinking Anthropologically: Discussion Topic 2

Describe the differences between male and female primate reproductive strategies. Why are their strategies different?

Credits

This concludes the Lecture PowerPoint presentation for Our Origins: Chapter 7.

For more resources, please visit https://wwnorton.com/instructor-resources/9780393680881.