Chapter 7. Primate Sociality, Social Behavior, and Culture

Transcription

1 Chapter 7 Primate Sociality, Social Behavior, and Culture

2 Overview We study primates because we are a primate species and so by studying those species closest to ourselves we gain understanding. We use this comparative method to explain physiological and behavioral systems evolved as adaptations. We will look at the primates in a very general sense, but with enough detail that we will see certain patterns emerge. Primatologists are: Scientists who study the evolution, anatomy, and behavior of nonhuman primates. Those who study behavior in free-ranging animals are usually trained as physical anthropologists. The three most famous are Jane Goodall (chimpanzees), Dian Fossey (mountain gorillas), and Biruté Galdikas (orangutans) -- all protégés of Louis Leakey (famous paleoanthropologist) In this lecture, we will explore the questions that primatologists ask and attempt to answer. Why are primates social? This is an important question, since humans are primates and are also quite social. By understanding the benefits of being social in other species of primate, 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.

3 Primate Societies 1 Primate residence patterns Different primate species tend to live in various social systems. But, even within a species, these residence patterns can change depending on food availability. Nevertheless, there are generally six different primate residence patterns recognized by primatologists. 1. One-male, multifemale group (once 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 of the reproductive-age females. Gorillas often will live in a one-male, multifemale group. 2. One-male, multimale group. (once called polyandrous) is rarer. Only some species of New World monkey live in this kind of social grouping: Tamarins ands marmosets 3. Multimale, multifemale group 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. 4. All-male. In some primate groups, like baboons, bachelor males form temporary groups before individuals settle into a multimale, multifemale residence. 5. One-male, one-female group (also called pair bonding or monogamous) 1. Gibbons and some New World monkeys, like marmosets and owl monkeys from pair bonds. 6. Solitary. Some primates live solitary lives, only interacting for reproductive purposes. Orangutans and some prosimians tend to be solitary.

4 Residence Patterns

5 Primate Societies 2 Agonistic relationships are those indicating unfriendly and often, aggressive relationships. Conflict within a group develops out of competition for mating partners and food items. This can be as serious as death or more display-based. Most intra-group aggression occurs in the form of signals and displays within the context of a dominance hierarchy. Primates resolve the majority of tense situations using submissive and appeasement behaviors. Primates are essentially social creatures with variation in size and structure of their primary social groups. Nonhuman primate societies rank individuals in terms of their relative dominance in the group. Dominance A dominance hierarchy is the ranking system within the society and reflects which individuals are most and least dominant. They are found in most nonhuman societies but vary widely in importance. Usually ruled by those with the greatest access to food or sex or those who control the behavior of others to the greatest extent. When there is strong male dominance, there is also often the expression of sexual dimorphism in body size. There is a strong competition for females. Males are larger and those pass this on to offspring. Whose genes? In a study of baboons, 81% were the offspring of the top-ranking male. This is a short-term finding, as males gain/lose position over time.

6 Primate Societies 3 What affects dominance rank? Individual s size, age, and ability to form alliances. In some species, the social position of the mother is important. In some species, female hierarchies are more stable and can be strong, even with strong male hierarchies in place. Territory Between groups, aggression is used to protect territories. Primate groups are associated with a home range (area where they remain permanently) Within the home range is a core area. The core area is portion of a home range containing the highest concentration and most reliable supplies of food and water. The core area is defended. Generally, territoriality is associated with species whose ranges are small enough to be patrolled and protected. Some species are not territorial. Some are not aggressive to other primate species. Dispersal and social behavior Dispersal occurs when an individual leaves their birth group and moves to another social group. It is important because it reduces inbreeding, and introduces new genetic material. It also changes social relationships and may reduce competition of males for mates. In many primate groups males move, but females move in other groups, and even both males and females of a species can move. If one refuses to go, they are aggressively pushed out.

7 Primate Societies 4 Diversity of primate societies Primatologists have made some general observations about social behavior in our primate relatives. Primates have a wide range of behaviors that serve as social cues. Primatologists have also documented the many different, and often highly complex, ways in which different primates form alliances with one another. Unlike many other species, primate social relationships are long-term. Primate social behavior But, 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 one s chance of survival and reproduction will be selected for, and will evolve. That behaviors can evolve by means of natural selection, is the central tenet of the science of sociobiology, the brain child 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 our genetic predispositions and our particular context, or environment. This combination gives primates, including humans, a wide range of behavioral flexibility, which has allowed us to adapt to a wide variety of environmental situations.

8 Primate Societies 5 Primate social behavior (continued) According to sociobiology, 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. Interestingly though, primate males and females necessarily have different reproductive strategies because of the simple fact that primates are mammals. Concept: Competitive males, choosy females. Males, therefore, will compete with other males for access to females. 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. Problem: Oversimplifies and ignores other factors. Primate studies by the mid-1980 s had shifted toward behavioral ecology (the study of behavior from an ecological and evolutionary perspective strategies). Sexual selection is studied, and so are environmental factors. Among areas of study: Dispersal affects social relationships that develop between kin. Studies of the behavior and ecology of individual primate species reveals a great deal of social behavior. Primates are highly social and have strong mother infant bonds. All primates strengthen social bonds through grooming. Primate societies have dominance hierarchies, although their influence on social interactions is variable.

9 Primate Societies 6 There is another male behavior that is not nearly as pleasant to think about but still can 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. Sarah Hrdy conducted work with langurs (monkeys) where there is social group composed of only 1 male. When a new male takes over the group, he often kills all the infants It is thought that the single adult male kills all infants fathered by a previous male to increase his own fitness. Also females go back into estrus more quickly if they are not parents Other hypotheses have been proposed to explain this behavior as well. Overcrowding has been suggested as a trigger for deviant behavior Insanity has also been suggested What does the data show? In one study of 16 langur deaths all were among infants not related to the dominant male. This supports the adaptive strategy explanation. While advantageous for the individual male, is counterproductive for the species. 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; there are plenty of willing males to go around. Instead, they compete for access to high-quality resources to support their offspring.

10 Primate Societies 7 Cooperation in primates Affiliative relationships are those indicating strong and friendly bonds and are important to maintaining large social groups Why form groups and cooperate? More individuals means more eyes and the higher likelihood that at least one of the primates will spot the potential predator. Many individuals can join together and defend themselves by mobbing the predator. a behavior engaged in by many primates against their potential predators. The third benefit is one of statistical probability. Group living certainly has its costs, but in animal groups that are heavily preyed upon (like primates), it can increase survival of the individuals in the group and, therefore, can evolve. Grooming is a type of behavior among all nonhuman primates involving the practice of handling and cleaning another individuals fur. It is a form of social activity. Only occasionally is grooming allogrooming (self-grooming); it is usually a way to cement social bonds. Play behavior in primates serves several functions including development of motor skills and learning to behave with others. Play behavior in primates serves several functions including development of motor skills and learning to behave with others. It allows the infant to develop motor skills Social skills are learned, including one s place in the social hierarchy.

11 Primate Societies 8 Altruism Altruism is a form of risking oneself in social groups and is often analyzed in relation to evolutionary fitness. Why would natural selection ever favor altruism? William (W. D.) Hamilton suggested that altruistic acts could evolve if the act benefits related individuals in the group. This is known as kin selection. 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 (also called reciprocal altruism) is also at work. This is the I ll scratch your back if you scratch mine idea. 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. Fairness and equality Frans de Waal is well known for his primate behavior work. He has studied the concept of morality in non-human primates. He suggests they have a sense of fairness and equality. A clip from a TED talk:

12 Primate Societies 9 The mother-infant bond Evolutionary explanations often focus on the maximizing of fitness. A related concept is that of parental investment, the set of behaviors that increases the probability that offspring will survive The mother-infant bond is the single most important primate behavior and represents a huge energy investment. This bond often lasts beyond infancy. This bond is more than the need for nourishment and has been shown by Harlow s work. Psychologist Harry Harlow s maternal depravation studies with rhesus monkeys and surrogate mothers confirmed the importance of motherhood beyond nutrition. Warmth and comfort were necessary as well. Harlow s experiments clearly demonstrated the importance of social contact with peers, showing abnormal behaviors in monkeys kept from other infants. Alloparenting Parents are not the only individuals who care for an infant. When one watches primate social groups you can see the evidence of this When an individual other than a parent cares for an infant this is called alloparenting. Alloparents are those who care for an infant but are not their biological parents. Primate females are often alloparents. More often those who have not given birth than those who have. This is often adaptive if the natural mother dies. Also gives a break to the mother with alloparental care Alloparenting increases one s genes being passed to next generation I often joke that I am engaged in alloparenting when I give presents to my nieces and nephews.

13 Resources Primates spend up to 50% of their waking day foraging for food. Female burden As we ve already discussed, this burden is particularly high on female primates, and specifically. So on mothers, since they not only have to feed themselves but they have to support a growing, dependent offspring as well. Females with good nutrition: 1) reproduce at an earlier age, 2) have healthier babies, 3) have a shorter time between births (called an interbirth interval), and 4) 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: 1. The quality of food: 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.. 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. 2. The distribution of food: The distribution of food also drives foraging patterns. Not only do primates need food but 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, therefore, are 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. 3. Food availability fluctuates seasonally. Because primates eat mostly forest foods (leaves and fruit), they are confined to equatorial regions.

14 Culture? The basic primate learning pattern provides a means by which new behavior is passed on from one generation to the next. We call this culture in humans. In nonhuman primates this is referred to as protoculture. Interesting film: The cultured ape 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 (like the one in the upper middle image) 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. 1. Chimpanzees not only learn these things, but they can pass on this behavior to other members of a group, often from mother to offspring. 2. Most of the tool-making behaviors in chimpanzees are associated with food. For example, the primatologist Jill Pruetz recently observed chimpanzees making spear-like objects (bottom left image) to skewer little prosimians living in the hollows of tree trunks. 3. These tool-making behaviors are localized, meaning they vary from site to site across Africa.

15 Communications 1 Communication is the larger category, with language a subset of this category. Communication is defined in different ways: As the sending of signals. As both sending and receiving of signals. Sending, receiving and responding is communication according to some researchers. 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.. Researchers translate the meaning of different primate vocalizations by recording the vocal repertoire of a primate group. Sometimes, though, the context of the call is not obvious as a lion in the vicinity, and the meaning of these calls is deciphered using playback experiments. For instance, 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. 1. Only the mother of the particular infant whose scream was artificially broadcast looked toward the speakers. 2. 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. Cheney and Seyfarth have also recorded the sounds made by chacma baboons in Botswana.

16 Communications 2 Alarm calls Four key characteristics that distinguish language from call systems: Call systems most often communicate emotions or occur in response to immediate stimuli. Call systems are stimuli-dependent, Animal calls are distinct and not combined or modified to produce calls with a different meaning. Animal call systems are instinctual 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, 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 s 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 past. When Diana monkeys encounter chimpanzees, they slink away, so as to not alert the chimps to their presence.

17 Communications 3 Chimpanzees 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. In the wild, chimpanzees not only communicate vocally, but also 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 (in 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. Even vocally, chimpanzee groups can vary almost like human dialects vary from region to region. The extraordinary communication abilities of the apes is 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 2-year-old human, and is shedding important light on the communication abilities of our early human ancestors, prior to the evolution of spoken language.