Mammalian social systems

All levels of social organization occur in mammals. There are solitary species, such as the mountain lion (Felis concolor), in which the male and female adults come together only to mate, and the female remains with her young only until they are capable of living independently. At the other numerical extreme are some of the hoofed mammals, which form herds of thousands of individuals. Other extremes might be considered in terms of specialization for social life. One of the most socially specialized mammals is the naked mole-rat (Heterocephalus glaber) of Africa, which has a eusocial colony structure similar to that of ants, bees, and termites. Between these extremes, there are many variations. No theory accounts for the diversity of mammalian social systems, but two broad generalizations are consistently employed to explain mammalian species’ social organization. These are the environmental context in which the species exists and the mammalian mode of reproduction.

More than any other group of animals, mammals are required to form groups for at least part of their lives. Although in all sexually reproducing animals, the sexes must come together to mate, mammals have an additional required association between mother and youngll species of mammal feed their young with milk from the mother’s mammary glands. This group, a female and her young is the basis for the development of mammalian social groups. In some species, the social group includes several females and their young and may involve one or more males as well.

Twenty-first-century research continues to reveal details of mammalian social systems. One study found that solitary mammals have complex and flexible social structures that are not a primitive stage of evolution, as previously asserted, but a reaction and adaptation to the environment. Additionally, solitary species are less solitary and territorial than once believed. Each animal displays tolerance, aggression, amicable interspecies interactions, and hostile interactions. However, it is essential to note that another study found that species that live solitary lives live shorter lives. In another study, researchers investigated fairness, equality, and hierarchies among sixty-six species—an understudied area of animal behavior research. The results showed that equality and fairness are central to social systems, and social behaviors like sharing food encourage future prosocial, equitable interactions. The scientists also noted the critical role that fairness played in evolution.

The particular social organization adopted by a mammalian species is a response to the environmental conditions under which the species lives. The species’ food supply and the distribution of that food supply are often the predominant determinants, but predators of the species are also important in determining the form of its social organization. The best way to see the effects of these factors on mammalian social structure is by example.

Primate Social Organization

The primates are the most social group of mammals. Monkeys demonstrate the importance of food supply and its distribution in determining social structure. The olive baboon (Papio anubis) occupies savannas, where it exists in large groups of several adult males, several adult females, and their young. Finding fifty or more animals in a group is not uncommon. Individual males do not guard or try to control specific females except when the females are sexually receptive. The group’s food supply is in scattered patches, but each patch contains an abundance of food. The advantage of having many individuals searching for the scattered food is obvious: If any member finds a food-rich patch, there is plenty for all.

Predation probably also plays a role in the olive baboon’s social organization. The savannas they roam have many predators and few refuges for escape. A large group is one defense against predators if hiding or climbing out of reach is not practical. Having many observers increases the chance of early detection, giving the prey time to elude the predator. A large group can also mount a more effective defense against a predator. Large groups of baboons use both of these tactics.

The hamadryas baboon (Papio hamadryas), on the other hand, lives in deserts in which the food supply is not only scattered but also often found in small patches. The hamadryas baboon’s social structure contrasts with that of the olive baboon, perhaps because the small patches do not supply enough food to support large groups. A single adult male, one or a few adult females, and their young make up the basic group of fewer than twenty individuals. Several of these family groups travel together under certain conditions, forming a band of up to sixty animals. Within the band, however, the family groups remain intact. The male of each group herds his females, punishing them if they do not follow him. The bands are probably formed in defense against predators. They break up into family units if predators are absent. At night, hamadryas baboons sleep on cliffs, where they are less accessible to predators. Because suitable cliffs are limited, many family groups gather at these sites. Hundreds of animals may be in the sleeping troop, probably affording further protection against predators.

Though there are exceptions, forest primates consistently live in smaller groups. In many species, fewer than twenty individuals make up the social group at all times. These consist of one or a few mature males, one or a few mature females, and their offspring. The groups are more evenly distributed throughout their habitat than are groups of savanna or desert primates. In forests, the food supply is more abundant and more evenly distributed. Escape from predators is also more readily accomplished—by climbing trees or hiding in the dense cover. Under these conditions, the advantages of large groups are minimal, and their disadvantages become apparent. For example, in small groups, the competition for mates and food is less.

Ungulate Social Organization

The ungulates have all levels of social organization. African antelope demonstrate social organizations that, in some ways, parallel those of the primates. Forest antelopes, such as the dik-dik (Madoqua) and duiker (Cephalophus), are solitary or form small family groups, and they are evenly spaced through their environment. Many hold permanent territories containing the needs of the individual or group. They escape predators by hiding and are browsers, feeding on the leaves and twigs of trees.

Many grassland and savanna antelope, such as wildebeest (Connochaetes), on the other hand, occur in large herds. They outrun or present a group defense to predators and are grazers, eating the abundant grasses of their habitat. In many cases, they are also migratory, following the rains about the grasslands to find sufficient food. The social unit is a group of related females and their young. Males leave the group of females and young as they mature. They join a bachelor herd until fully mature, at which time they become solitary, and some establish territories. The large migratory herds are composed of many female/young groups, bachelor herds, and mature males. The social units are maintained in the herd. Though it may seem strange to speak of solitary males in a herd of thousands, that is their social condition. The male territories are permanent in areas that have a reliable food supply year-round, but they cannot be in regions in which the species is migratory. Under these conditions, the males set up temporary breeding territories wherever the herd is located during the breeding season.

There are parallels with primate social patterns. Large groups are formed in grasslands and roam widely in search of suitable food. The groups are effective in protecting against predators in habitats with few hiding places. Smaller groups are found in forests, where food is more evenly dispersed and places to hide from predators are more readily found.

Rodent Social Organization

Rodents also have all kinds of social organizations. The best known, and one of the most complex, is the social system of the black-tailed prairie dog (Cynomys ludovicianus). The coterie is the family unit in this case, and it consists of an adult male, several adult females, and their young. Members maintain a group territory defended against members of other coteries. Coterie members maintain and share a burrow system. Elaborate greeting rituals have developed to allow the prairie dogs of a coterie to recognize one another. Hundreds of these coteries occur together in a town. The members of these towns keep the vegetation clipped—as a result, predators can be seen from a distance. Prairie dogs warn one another with a “bark” when they observe a predator, and the burrow system affords a refuge from most predators.

The only vertebrate known to be eusocial is the naked mole rat. It occurs in hot, dry regions of Africa. The colony has a single reproductive female, a group of workers, and a group of males whose only function is to breed with the reproductive female. The workers cooperate in an energetically efficient burrowing chain when enlarging the burrow system. In this way, they can extend the burrow system quickly during the brief wet season. Digging is very difficult at other times of the year. The entire social system is thought to be an adaptation to a harsh environment and a sparse food supply.

Carnivore Social Organization

Most carnivores are not particularly social, but some do have elaborate social organizations. Many of these are based on the efficiency of group hunting in the pursuit of large prey or on the ability of a group to defend a large food supply from scavengers. The gray wolf (Canis lupus) and African hunting dog (Lycaon pictus) are examples. In both cases, the social group, or pack, consists of a male and female pair and their offspring of several years. There are exceptions, such as solitary carnivores and carnivores that form temporary family units during the breeding season, such as the red fox (Vulpes vulpes), which hunt prey smaller than themselves. The coyote (Canis latrans) can switch social systems to use the food available most efficiently. It forms packs similar to those of the gray wolf when its main prey is large or when it can scavenge large animals and is solitary when the primary available prey is small.

These examples and many others show that the social groups of mammals are based on the family group. The particular social organization employed by a species is determined by the ecological situation in which it occurs. The specific aspects of the environment that seem to be most important include food abundance, food distribution, food type, and protection from predators.

Fieldwork and Laboratory Studies

Observation has been a very important method of studying mammal societies. One of the reasons that primate and ungulate societies are so well known is that they are large and active during the day and so are easily observable. The observer must take great pains to be inconspicuous or, in some cases, to become a part of the subject’s environment. Small mammals (and sometimes larger mammals) have been kept in enclosures and observed to learn more about their social lives. The observer maps movements, records activities and interactions, and analyzes the data that results.

Simple observation is enhanced by manipulating the subjects in various ways. Individual animals can be marked, or in some cases, they can be identified by natural color patterns, scars, or other marks. These marked individuals can be followed, and their behavior and interactions with other individuals can be observed. Radios and radioactive tracer elements are sometimes implanted in individuals, and these individuals are followed in the field. Much can be learned about a species’ social behavior by following the locations of such tagged animals. In addition, they are more readily locatable for direct observation.

Small mammal species that are not readily observable are trapped live, marked, released, and recaptured. Mutually exclusive use of certain areas, areas used in common, and patterns of multiple captures in individual traps are some types of information from trapping that can be interpreted in terms of social behavior. Experiments are sometimes carried out in the natural context. A group or a specific individual is presented with an artificial situation, and any reactions to it are recorded.

Laboratory studies are also used to supplement the field observations. Psychological and physiological capabilities of organisms can best be studied in the controlled confines of a laboratory experiment. These data, however, must always be put back in the context of the field observations to make a meaningful contribution to the understanding of the species’ social behavior.

Computer simulations and mathematical models have been used to explore the possible reactions of social systems to various environmental pressures. As with laboratory results, it is important to test predictions against the social system in nature before assuming their validity. Comparative studies of all the above types are of great importance. Related species, or different populations of the same species, that occur in different regions are studied and compared; these studies are tantamount to reading the data from a natural experiment.

Social Organization and Food

Mammalian societies are always organized around one or more females and their offspring. Males may also be part of the group, or they may form separate groups. The size and structure of the group are determined by the ecological setting in which it evolves. The particular ecological factors that seem to be of greatest importance in this determination are food supply, the distribution of the food, and predation (including the hiding places and escape routes available in the habitat).

Large groups occur when food is scattered in a patchy distribution. These groups are largest when the patches contain abundant food. Many organisms are more likely to find the scattered patches than a single individual. As long as the patches have enough food for all members of the group, it is to each member’s advantage to search with the group. On the other hand, if food is evenly dispersed in small units throughout the environment, the advantage of a group search is lost. Each individual will be better off searching for themselves, and some strategy involving a very small social group or even solitary existence would be advantageous.

A somewhat similar argument follows for predators. If large prey are taken, a group of predators should be able to subdue the prey and protect its remains from scavengers more efficiently. If small prey are taken, solitary predators have the advantage since the prey is easily dispatched and the predator will have it to itself. Many other factors are involved in determining the final form of a species’ social organization, but the family unit and environmental context are fundamental in determining all mammalian social structures.

Conservation of the mammal species that still exist on Earth requires knowledge of their social organization. Understanding that mammalian social organizations are responses to the environmental context in which they have evolved emphasizes the need to conserve entire ecosystems, not only the individual species that exist within them.

Principal Terms

Browser: An organism that feeds primarily on leaves and twigs of trees and shrubs rather than on grasses

Carnivore: A member of the meat-eating order Carnivora, which includes dogs, cats, weasels, bears, and their relatives

Eusocial: A social system with a single breeding female; other members of the colony are organized into specialized classes (exemplified by bees, ants, and termites)

Grazer: An organism that feeds primarily on grasses

Primates: Members of the order Primates—monkeys, apes, and their relatives

Rodent: A member of the order Rodentia—squirrels, rats, mice, and their relatives

Savanna: A grassland with scattered trees; some ecologists restrict the term to tropical regions

Territory: An area that an animal defends against other members of the species and that often contains food, shelter, and other requirements for the individual or group

Ungulate: A hoofed mammal from the order Artiodactyla (pigs, cattle, antelope, and their relatives) or from the order Perissodactyla (horses, rhinoceroses, tapirs, and their relatives)

Bibliography

Dunbar, Robin I. M. Primate Social Systems. Ithaca, Cornell University Press, 1988.

Eisenberg, John F., and Devra G. Kleiman, eds. Advances in the Study of Mammalian Behavior. Special Publication 7. Lawrence, American Society of Mammalogists, 1983.

Gittleman, John L., ed. Carnivore Behavior, Ecology, and Evolution. Ithaca, Cornell University Press, 2019.

Immelmann, Klaus, ed. Grzimek’s Encyclopedia of Ethology. Hoboken, Van Nostrand Reinhold, 1977.

Macdonald, David W. European Mammals: Evolution and Behavior. New York City, HarperCollins, 1995.

Makuya, Lindelani, and Carsten Schradin. "The Secret Social Life of Solitary Mammals." Proceedings of the National Academy of Sciences, 18 Mar. 2024, www.pnas.org/doi/10.1073/pnas.2402871121. Accessed 10 Sept. 2024.

Nowak, Ronald M., and John L. Paradiso. Walker’s Mammals of the World. 6th ed. 2 vols. Baltimore, The Johns Hopkins University Press, 1999.

Ober, Holly. "Inequality not Inevitable among Mammals, Study Shows." University of California, 3 Oct. 2023, newsroom.ucla.edu/releases/study-shows-inequality-not-inevitable-among-mammals. Accessed 10 Sept. 2024.

Rosenblatt, Jay S., and Charles T. Snowdon, eds. Parental Care: Evolution, Mechanisms, and Adaptive Significance. Advances in the Study of Behavior 25. Cambridge, Academic Press, 1996.

Tamisiea, Jack. "Mammals that Live Together Live Longer." Scientific American, 31 Jan. 2023, www.scientificamerican.com/article/mammals-that-live-together-live-longer. Accessed 10 July 2023.

Vaughan, Terry A. Mammalogy. 6th ed. Philadelphia, Saunders College Publishing, 2015.

Wrangham, Richard W., W. C. McGrew, Frans B. M. De Waal, and Paul G. Heltne, eds. Chimpanzee Cultures. Boston, Harvard University Press, 1994.