Most animals are solitary. Within the vertebrates, and especially among birds and mammals there are species that are social, that is, they form more or less stable associations.
Why are animals social? The reasons why animals form social groups have been explored, and most scientists agree that predation pressure is the most important reason why animals live in groups.
The social environment creates new selection pressures. For example, more dominant and more aggressive animals can make use of a larger share of the resources encountered by the group and thus there is cost for those subordinate or weaker animals. At the same time social partners may become allies or may participate in territorial defense or may help in deterring predators or getting access to resources that individual animals are not able to get access to (e.g. a pack of lions hunting large prey). Basically social interactions can be of two types: competitive or antagonistic ones (e.g. aggression) and cooperative, positive or affiliative ones (e.g. grooming). It is possible to characterize the social structure of social groups by the relationships established by the individuals.
We have seen in previous lectures what happens when two individuals contest a resource, for example a territory, and you have seen how game theory may explain the best strategy to follow.
When animals live in stable groups the superior competitor should consistently win each contest and the inferior competitor should retreat. Thus, we say that this pair of individuals has a dominance relationship. Dominance relationships can be measured by the result of fights in aggressive encounters among two individuals or the direction of threats or submissive gestures. Most social animals have formal signals of aggression or submission that facilitate this type of interactions. For example in most primates the dominant animals signal aggressiveness by showing their opponent s an open mouth and giving particular vocalizations. Animals use signals to indicate their subordination to the more powerful dominant animal. In many primate species individuals signal subordination with a gesture that looks like a smile, associated to particular submissive vocalizations. In fact behavioral biologists have shown that the human smile most probably evolved from these submissive gestures.
Dominance relationships in social animals are maintained by process more complicated than the simple summation of repeated contests over resources. For example, in some species dominance relationships are established even when there are no obvious asymmetries in power between individuals. This is especially clear among female primates. When you see two adult male mammals, e.g. male deer or primates you may sometimes make a good guess of which animal is dominant and which is subordinate because you can compare and rank the animals according to their strength or power and there is usually a good correlation between strength or power and dominance. You can guess who is dominant and who is subordinate just by looking at them. In many primate species, adult females establish dominance relationships among themselves but it is usually not possible to know at first hand by looking at their physical appearance who is the dominant and who is the subordinate. Why do they have this dominance relationship and how they decide who is dominant?
In some species it has been demonstrated that early experience determines if an animal will be dominant or subordinate. Winner-loser effects. e.g. blue footed boobies --experiments with chicks. In this species two chicks are born at each nest. One of them is born earlier and thus has an advantage over the other. It will show aggressive behavior and harass its smaller sibling which in turn will show signals of submission. When two chicks reared in isolation are put together the larger one usually rapidly establishes itself as the dominant one and the smaller one ends up being the subordinate one. On the other hand when chicks reared with their siblings are put together with other chicks also reared with their sibling the one that was dominant consistently beats the other independent of their relative size. Dominant small chicks placed with a subordinate large chick harass the larger animal, which in turn shows submissive signals. This winner-loser effect has also been demonstrated in other birds and rodents.
Dominance hierarchies. Dyadic dominance relationships are those between two individuals. In groups consisting of more than two individuals the animals can be arranged in a dominance hierarchy according to which is dominant to which. For example, if individual A is dominant to B and B is to C and A is also dominant to C, we can arrange the animals in a linear dominance hierarchy, where A is the top dominant animals, B is of intermediate rank and C is the most subordinate animal. How does one determine a dominance hierarchy? What determines the position of an animal in a dominance hierarchy? In some species the rank of an individual is clearly determined by its power, strength or Resource-holding power (RHP). For example in male baboons, young individuals that transfer into a new group have a low rank but very rapidly (within few months) escalate to a top position. In this case there is an inverse relationship between rank and age. This is probably because young males are more powerful than older ones. In other animals the relationship between power or strength is not so clear. For example in spotted hyenas, the dominance rank of adult males is not clearly related to their strength or their age but to the time they have been living in their social group.
One observation that is difficult to explain is why dominance hierarchies are usually linear and stable. By linear we mean that if A is dominant to B and B is dominant to C then A is dominant to C. By stable we mean that the position of the animals in the dominance hierarchy remain constant for long periods of time.
Linear dominance hierarchy Non-linear dominance hierarchy
How does one explain linearity of dominance hierarchies especially when differences in RHP among individuals are low and when group size is large? One explanation is the psychological reinforcement of lower status by winner-loser effects. Another explanation is the use of alliances to maintain the status quo.
Some animals maintain stable and linear social hierarchies in very large groups (e.g. spotted hyenas and female Old World monkeys). In some cases these hierarchies are ordered by specific rules that have nothing to do with RHP. For example, in female macaques daughters rank below their mother in inverse order of age, and the whole matriline ranks above the next most dominant matriline. A matriline is an adult female and all her descendant female offspring. Female alliances are what allows this particular type of rank inheritance. An alliance occurs when two (sometimes more than two) individuals support each other against a third individual (e.g. male baboons). It is very common for primates to have formal signals to solicit aid from allies (e.g. head flagging and screams).
Alliances among kin are very common but what do females gain by intervening in aggressive encounter by non-kin? Experiments on rank determination in Japanese macaques have shown that alliances among mother and their daughters and among females of different matrilines is what keeps the stability of the dominance hierarchy. Three types of alliances: Conservative alliances: between individuals of two dominant matrilines towards an individual of a subordinate matriline. A bridging alliance is that between individuals from a dominant and a subordinate matriline towards a third individual of a matriline of intermediate rank and a revolutionary alliance is that between a two individuals of a subordinate matrilines against an individual from a dominant one. Studies with captive Japanese macaques and other primate species have shown that bridging and revolutionary alliances are rare. Most alliances between non-kin individuals have as a target individuals from a lower matriline. In experimental studies, dominant individuals from a dominant matriline were put alone with a whole subordinate matriline. In almost all cases the dominant individual fell in rank because it did not have access to allies from its own or other dominant matrilines. Those same individuals that lose their rank when placed alone with individuals of a lower matriline, were able to keep their dominant rank when placed together with individuals of a lower matriline and individuals from a dominant matriline. Individuals from different matrilines (non-kin), cooperate to maintain the status quo. This seems to be an ESS.
The ecology of social relationships.
Even among a small group of mammals like the primates there is a lot of variation in the social relationships of individuals. For example, in many species adult females remain in their natal group (they are philopatric) and form strong social bonds. For example they groom each other much more than males do. Social relationships among adult males in these societies are usually antagonistic. This characterizes most Old World monkeys societies. In other species, like the chimpanzee, males are philopatric and maintain strong social bonds (they groom each other more than females do). Females on the other hand spend much less time with other females. Some species are almost solitary, like orangutans. What explain these differences in the social relationships among individuals in different primate societies?
To understand what can account for these differences we can start thinking about what ecological factor is limiting the reproductive success of males and females. Females are limited in their reproductive output by the amount of food they can have access to. It is well known that food availability is what limits female reproduction. In most mammal species where humans have provided extra food females increase their fecundity and start reproducing at an earlier age. This suggests that the amount and type of food competition is what shapes the social relationships of females. Males on the other hand are limited by the number of females they can inseminate. In males mate competition is the dominant factor that determines the type of relationships they will establish among themselves and with females.
Social relationships among females.Lets start assuming that predation risk is what forces animals to live in stable groups. Once groups are formed individuals have to compete for resources if they want to keep together. It is the distribution of food what determines the type of competitive regime that animals will be faced with and the type of social relationships they will establish with other individuals. Since female fitness is more closely related to access to food, food competition will more strongly affect female social relationships than male social relationships. Competition for resources could be of two basic types: scramble competition (also known as indirect competition) or contest or direct competition. Scramble competition occurs when animals have to share a limited resource but there is no way in which an individual can limit the access of that resource to other individuals. All individuals in the group will suffer the same reduction in the food intake. The effect of scramble competition will increase with group size. This type of feeding competition occurs when food occurs in very small quantities and is very dispersed. For example grazing animals feeding in a grassland may suffer the effects of scramble competition. It is also expected to occur in insectivorous and folivorous animals that feed on small prey items (insects or leaves) that are very dispersed. Scramble competition can also occur at food sources that are very large relative to the group spread.
The group size effect of scramble competition
Contest competition, on the other hand occurs when stronger individuals can limit the access to a resource to less powerful individuals. Contest competition occurs because there are differences among individuals in their competitive abilities. Dominant individuals can have access to a larger share of the resource. This type of competition occurs when food occurs in well-defined patches. For example a small fruit tree with ripe fruit is a type of resource that promotes this type of competitive regime.
Competition can also occur between groups. For example, large groups can limit the access to food sources to less powerful smaller groups. Most primate species will suffer the effects of these type of competition to different degrees.
The effect of dominance rank when competition is of the contest type.
Four combinations of types of competition.
A. When within group scramble competition is important but contest competition is not. In folivorous species scramble competition will be more important than contest competition. In folivorous species, since resources are evenly dispersed, females can not use aggression to get a larger share of a resource or to make alliances with other females to keep other females out of that resource. The best strategy a female can follow, given that she has to live in a social group to reduce the risk of predation, is to freely choose to live in a social group of an optimal size. Females will move freely looking for a group where her fitness is maximized (a group where the risk f predation is minimized while scramble competition is minimized). It is thus expected that females in folivorous primates will transfer between groups (they are not philopatric). Since they are not living with relatives they will have peaceful (but not overly friendly) and egalitarian relationships with other females. In this societies females establish shallow dominance relationships. In these species it is very difficult to determine a linear dominance rank, and reversals are frequent. They do not have formal submissive signals and they do not form alliances among females. These societies are called Dispersal-Egalitarian societies. Examples: Howler monkeys, Costa Rican squirrel monkeys, Gorillas
B. When within group contest competition is important but within group scramble competition and between group contest competiton are not.
When within-group contest competition is important, females will tend to use aggression to compete for food. As we have seen females can make alliances with their daughters and sisters to out-compete other females. By aiding closely related females, they are increasing their inclusive fitness, they are helping their genes to pass to future generations. This is a good incentive for females to stay in their natal group and help their kin control access to food. In this type of societies, there is a lot of aggression among females, there are well established dominance hierarchies. This societies are said to be despotic. Females tend to make alliances preferentially with kin. Grooming is also concentrated among kin or allies. Societies where females preferentially aid kin are called nepotistic. This type of societies occurs especially in frugivorous primates and they are called Resident-Nepotistic. Examples: Macaques, baboons, capuchin monkeys.
C In some species, between group contest competition can be more important than within group competition. This may occur when predation risk is reduced and animals do not need to leave in highly cohesive groups but food occurs in patches that are large enough that many individuals can eat at them. If between group contest competition is more important than within group contest competition dominant females should be more tolerant to subordinate ones because if subordinate females receive a lot of aggression and are consistently excluded from food sources they have an incentive to leave the group and join another group where they are tolerated. Since the power of a group depends on the number of females it contains, dominant females should be more tolerant to subordinate ones. Also females will prefer to stay in their natal group to help their mother and sisters to compete against neighbor groups and thus they become philopatric. In this case, aggression will not be frequent within the group. Dominant females will groom subordinate females more often and will tend to groom non-kin also more often. Relationships will thus be more egalitarian and less nepotistic. Females should participate in between group aggressive encounters. This societies are called Resident-Egalitarian. Example: Guenons, patas monkeys, and Hanuman langurs.
D In a few species, both between group contest and within group contest competition.
These species will show characteristics intermediate between the Resident-Egalitarian and the Resident-Nepotistic ones. Females should be philopatric because they will gain by helping their kin in both within- and between-group contest competition. Females will be nepotistic but tolerant. This type of societies are called Resident-Nepotistic-Tolerant.
Nepotistic and despotic
Nepotistic and tolerant
A comparative test of the ecological model of female primate social relationships.
1) Peruvian common squirrel monkeys vs Costa Rican squirrel monkeys.
2) Savannah baboon populations. One population where food occurs in concetrated patches shows despotic relationships, coalitions, female philopatry, and a clear effect of dominance rank on food intake. In the other population, baboons feed on highly dispersed foods. Aggression was very rare, they did not form coalitions and did not have a clear dominance hierarchy. In this population females sometimes transfer between groups.
Social relationships among males
Social relationships among males will be mostly affected by the degree of competition for mates. Unlike food, fertilizations can not be shared among males and thus it is expected that males will not usually form coalitions, and will have mostly agonistic relationships. In some few species, males show strong bonds among them. These are usually species where females transfer and thus males are philopatric and males cooperate to defend a common territory against other male groups. This occurs for example in the chimpanzees.