Territoriality and aggression (zoology)

Any field or forest inhabited by animals contains countless invisible lines that demarcate territories of individuals of many species. Humans are oblivious to these boundaries, yet have quick perception of human property lines. Other animals are equally oblivious to human demarcations. Most organisms, in fact, appear to attend only to the territorial claims made by members of their own species. If separate maps of individual territories could be obtained for each species in the same habitat and superimposed on one another, the resulting hodgepodge of boundaries would show little consensus on the value of particular areas. Yet, basic similarities exist in why and how different species are territorial.

Causes of Territoriality

The existence of aggression and territorial behavior in nature hardly comes as a surprise. Even casual observations at a backyard bird feeder reveal species that are commonly perceived as friendly being highly aggressive. The observation of birds at feeders can lead to interesting questions concerning territorial behavior. For example, bird feeders usually contain much more food than any one bird could eat. Why, then, are aggressive interactions so common? Moreover, individuals attack conspecifics more often than birds of other species, even when all are eating the same type of seeds.

Aggressive defense of superabundant resources is not expected to occur in nature, but bird feeders are not a natural phenomenon. Perhaps the observed aggressive encounters are merely artifacts of birds trying to forage in a crowded, novel situation, or perhaps bird feeders intensify aggressive interactions that occur less frequently and less conspicuously, in nature. While the degree to which aggression observed at feeders mirrors reality is open to question, the observation of a greater intensity of interactions between conspecifics reflects a natural phenomenon. Members of the same species are usually more serious competitors than are members of different species because they exploit the same resources. Members of different species might only share a few types of resources. Despite the ecological novelty of artificial feeders, noting which individuals win and lose in such an encounter can provide valuable information on the resource-holding potential of individuals that differ in various physical attributes, such as body size, bill size, or even sex. For organisms that live in dense or remote habitats, this type of information can often be obtained only by observations at artificial feeding stations.

Territorial defense can be accomplished by visual and vocal displays, chemical signals, or physical encounters. The sequence of behaviors that an individual uses is usually predictable. The first line of defense may involve vocal advertisement of territory ownership. One function of bird song is to inform potential rivals that certain areas in the habitat are taken. If song threats do not deter competitors, visual displays may be employed. If visual displays are also ineffective, residents may chase intruders and, if necessary, attack them. This sequence of behaviors is common in territorial interactions because vocal and visual displays are energetically cheaper than fighting and involve less risk of injury to the territory owner.

It may be less obvious why fighting is a necessary component in territorial interactions for both territory owners and intruders. Without the threat of bodily injury, there is no cost to intruders that steal the resources of another individual. This would severely hamper an owner’s ability to control an area. On the other hand, if intruders never physically challenge territory owners, it would pay for all territory owners to exaggerate their ability to defend a resource. Thus, physical aggression may be essential. Animals do not frequently kill their opponents, however, so there must be something that limits violence. Various species of animals possess formidable weapons, such as large canine teeth or antlers, that are quite capable of inflicting mortal wounds. Furthermore, a dead opponent will never challenge again. Yet, fights to the death are rare in nature. When they occur, some novel circumstance is usually involved, such as a barrier that prohibits escape of the losing individual. Restraint in normal use of weapons, however, probably does not indicate compassion among combatants. Fights to the death may simply be too costly, because they would increase the chance that a victor would suffer some injury from a loser’s last desperate attempts to survive.

Functions of Territory

Territories can serve various functions, depending on the species. For some, the area defended is only a site where males display for mates; for others, it is a place where parents build a nest and raise their offspring; for others, it may be an all-purpose area where an owner can have exclusive access to food, nesting sites, shelter from the elements, and refuge from predators. These different territorial functions affect the area’s size and the length of time an area is defended. Territories used as display sites may be only a few meters across, even for large mammal species. Territorial nest sites may be smaller, such as the densely packed nest sites guarded by parents of many colonial seabirds. All-purpose territories are typically large relative to the body size of the organism. For example, some passerine birds defend areas that may be several hundred meters across. Although all three types of territories may be as ephemeral as the breeding season, it is common for all-purpose territories to be defended year round.

The abundance and spatial distribution of needed resources determine the economic feasibility of territoriality. On one extreme, if all required resources are present in excess throughout the habitat, territory holders should not have a reproductive advantage over nonterritory holders. At the other extreme, if critical resources are so rare that enormous areas would have to be defended, territory holders might again have no reproductive advantage over nonterritory holders. If needed resources, however, are neither superabundant nor extremely rare and are somewhat clumped in the habitat, territoriality might pay off. That is, territorial individuals might produce more offspring than nonterritorial individuals.

Studies of territoriality raise more questions than biologists can answer. Researchers investigate how large an area an individual defends and whether both sexes are equally territorial. They seek to determine whether the territories of different individuals vary according to quality. The density of conspecifics may influence territoriality, or territoriality itself may serve to regulate population size, although evidence suggests that this is an incidental effect.

All-purpose territories vary considerably in size, depending on the resource requirements of the individuals involved and the pattern of temporal variation in resource abundance. In some organisms, individuals only defend enough area to supply their minimum daily requirements. In others, individuals defend a somewhat larger area—one that could still support them even when resource levels drop. In others, individuals defend territories that vary in size depending on current resource levels. For example, pied wagtails (European songbirds) defend linear territories along riverbanks about six hundred meters long during the winter. The emerging aquatic insects they consume are a renewable resource, but renewal rates vary considerably during the season. Rather than adjusting territory size to match the current levels of prey abundance in the habitat, wagtails maintain constant territory boundaries. This inflexibility persists even though territories that extend for only three hundred meters could adequately support an individual for about one-third of the season. In contrast, the territory size of an Australian honeyeater varies widely during the winter. Nectar productivity of the flowers visited by honeyeaters varies considerably during the season. By adjusting territory size to match changing resource levels, individual birds obtain a relatively constant amount of energy each day (about eighteen kilocalories).

Territorial Roles

In some species, only males are territorial. In other species, both sexes defend territories, but males defend larger territories than females do. In some mammals in which both sexes are territorial, males are aggressive only to other males, and females are only aggressive to other females. In these species, male territories are sufficiently large to encompass the territories of several females. Presumably, these males have increased sexual access to the females within their territories. Perhaps the most curious example of sex-specific territorial behavior is observed in a number of coral reef fish, in which all individuals in the population are initially female and not territorial. As the individuals grow older and larger, some develop into males. Once male, they engage in territorial behavior.

Within a species, significant variation in territory quality exists among individuals. Studies on numerous species have demonstrated a relationship between territory quality and an individual’s resource-holding potential. For example, larger individuals tend to control prime locations more often than smaller individuals. In addition, possession of higher-quality territories often results in increased reproductive success. For some species, this occurs because individuals with better territories obtain mates sooner or obtain more mates than individuals with poorer territories. In other species, possession of superior territories increases the survival chances of the owner.

As the density of conspecifics increases, the ability of individuals to control territories decreases. In some species, the territorial system may break down completely, with all individuals scrambling for their share of needed resources in a chaotic fashion. In other species, the territorial system is replaced by a dominance hierarchy. All competitors may remain in the area, but their access to resources is determined by their rank in the hierarchy. For example, elephant seal males can successfully defend areas containing from eighty to a hundred females from other males. Very dense clusters of females, however (two hundred or more), attract too many males for one male to monopolize. When this happens, one male—usually the largest male—dominates the rest and maintains disproportionate access to females.

Territoriality's adaptive function is to increase the survival and reproductive success of individuals. Territoriality can also have several possible incidental effects, one of which was once considered an adaptive function: serving as a means of population regulation. The reasoning behind this hypothesis is simple. The number of territories in a habitat would limit the number of reproducing individuals in a population and would thereby prevent overpopulation that could cause a population crash. Support for this hypothesis would include demonstration that a significant number of nonbreeding adults exist in a population. Indeed, for several species, experimental removal of territory owners has revealed that “surplus” individuals quickly fill the artificially created vacancies. In most of the species studied, however, these surplus individuals are primarily males. Population growth can be curbed only by limiting the number of breeding females, not the number of breeding males. Furthermore, the population regulation argument assumes that some individuals abstain from reproduction for the good of the population. If such a population did exist, a mutant individual that never abstained from reproducing would quickly spread, and its descendants would predominate in future generations.

Territoriality in the Field

Territoriality is typically investigated in the field using an observational approach. Initial information collected includes assessing the amount of area used by each individual, how much of that area is defended from conspecifics, and exactly what is being defended. It is relatively easy to discern the spatial utilization of animals. For many species, all that is required is capturing each individual, marking it for field identification, and watching its movements. For species that range long distances, such as hawks or large mammals, and species that are nocturnal, radio telemetry is frequently used. This methodology requires putting radio transmitters on the individuals to be followed and using hand-held antennas, or antennas attached to cars or airplanes, to monitor movements. For fossorial species (animals that are adapted for digging), animal movements are often determined by repeated trapping. This method involves placing numerous baited live traps above the ground in a predetermined grid.

Knowing the spatial utilization of an animal does not document territoriality. Many types of animals repeatedly use the same regions in the habitat but do not defend these areas from conspecifics. Such “home ranges” may or may not contain areas that are defended (that is, territories). Territorial defense can be readily documented for some animals by simply observing individual interactions. These data often need to be supplemented by experiments. Behavioral interactions might only occur in part of the organism’s living space because neighbors do not surround it. For these individuals, researchers play tape-recorded territorial vocalizations or place taxidermy mounts of conspecifics in different locations and note the response of the territory holder. For other species, such as fossorial rodents, direct estimates of territory size cannot be obtained because aggressive interactions cannot be observed; as a result, territory boundaries must be inferred from trapping information. Regions in which only the same individual is repeatedly trapped are likely to be areas that the individual defends. This is an indirect method, however, and can be likened to watching the shadow of an organism and guessing what it is doing.

It is often difficult to determine exactly what an animal is defending in an all-purpose territory where organisms use many different types of resources. Which resource, that is, constitutes the “reason” for territorial defense? On the other hand, several resources may contribute in some complex way. For many species these things simply are not known. This uncertainty also complicates estimates of territory quality. For example, red-winged blackbirds in North America have been particularly well studied for several decades by different investigators in various parts of the species range. Males defend areas in marshes (or sometimes fields), and some males obtain significantly more mates than others. Biologists think that males defend resources that are crucial for female reproduction. Some males may be more successful at mating than others because of variation in territory quality. Yet, the large number of studies done on this species has not yielded a consensus on what the important resources are, whether food, nest sites, or something else. The perceived value of the resource being protected also influences the territorial behavior of animals. Generally, frugivores are more aggressive and more territorial than animals that eat plants or foliage because fruit only grows in specific spots. It is much more easily monopolized than plants.

Theoretical investigations of territorial behavior often employ optimality theory and game theory approaches. Optimality theory considers the benefits and costs of territorial defense for an individual. Benefits and costs might be measured simply as the number of calories gained and lost, respectively. Alternatively, benefits might be measured as the number of young produced during any one season; costs might be measured as the reduction in number of future young attributable to current energy expenditures and risks of injury. For territorial behavior to evolve by means of natural selection, the benefits of territorial behavior to the individual must exceed its costs.

Game theory analyses compare the relative success of individuals using alternative behaviors (or “strategies”). For example, two opposing strategies might be “defend resources from intruders” and “steal resources as they are encountered.” In the simplest case, if some individuals only defend and other individuals only steal resources, the question would be which type of individual would leave the most offspring. Yet, defenders interact with other defenders as well as with thieves, and the converse holds for thieves. By considering the results of interactions within and between these two types of individuals, a game theory analysis can predict the conditions under which one strategy would “win” or “lose” and how the success of each type of individual would vary as the frequency of the other increases in the population. A complete understanding of territoriality involves not only empirical approaches in the field but also the development of testable theoretical models. Considerable advances have been made recently merging these two methodologies. Future investigations will no doubt include experimental control over resource levels that will allow definitive tests of predictions of alternative theoretical models.

Territoriality and Aggression

The importance of investigating any biological phenomenon might be measured by its contribution to understanding nature and humankind in general, making aggression and territoriality essential topics for scientists. Some animal species are highly aggressive in defending their living space, and others ignore or tolerate conspecifics in a nearly utopian manner. Some species exhibit sex-specific aggressions, and some groups of animals, like ants, chimpanzees, bees, and spider monkeys, display aggressive behaviors for the group's betterment. Some animals are territorial during only part of the annual cycle, and some only in specific areas they inhabit; others remain aggressive at any time and place. Thus, an important research focus in animal behavior is to unravel the ecological and evolutionary conditions that favor aggressive behavior and territoriality.

Aggression and territorial behavior have evolved in various organisms because, in the past, aggressive and territorial individuals outreproduced nonaggressive and nonterritorial ones. An implicit assumption of behavioral biologists is that animals other than humans do not interact aggressively because of conscious reasoning, nor are they consciously aware of the long-term consequences of aggressive acts. Should these consequences be detrimental, natural selection will eliminate the individuals involved, even if this means total extinction of the species. Humans are different. They are consciously aware of their actions and the consequences of such actions. They need only use conscious reasoning and biological knowledge of aggressive behavior to create conditions that can reduce conflict between individuals and groups.

A physiological component of territoriality and aggression, hormones, differ between species and sex. The primary hormones that trigger aggression, which may make territoriality behaviors more violent, include testosterone, estrogen, sex steroid precursors like dehydroepiandrosterone (DHEA), melatonin, and others. Some animals experience hormonal fluctuations during mating season that increase their territoriality, like some nesting birds. Their territoriality behaviors decrease or are nonexistent in non-breeding months.

Principal Terms

Adaptive Function: The reason that a characteristic evolved by means of natural selection

Conspecifics: Members of the same species

Dominance Hierarchy: A social system, usually determined by aggressive interactions, in which individuals can be ranked in terms of their access to resources or mates

Home Range: An area that an animal frequently uses but does not defend

Population Regulation: Long-term stability of population size at a level that prohibits overexploitation of resources

Resource-Holding Potential: The ability of an individual to control a needed resource relative to other members of the same species

Strategy: A behavioral action that exists because natural selection favored it in the past (rather than because an individual has consciously decided to do it)

Territoriality: The active defense of an area that is required for survival and/or reproduction

Bibliography

Alcock, John, and Dustin R. Rubenstein. Animal Behavior. 11th ed., Sinauer Associates, 2019.

Allen, Colin, and Marc Bekoff. Species of Mind: The Philosophy and Biology of Cognitive Ethology. MIT Press, 2000.

Davies, Nicholas B., and John R. Krebs. An Introduction to Behavioral Ecology. 4th ed., Wiley-Blackwell, 2014.

Dennen, J. van der, and V. S. E. Falger, editors. Sociobiology and Conflict: Evolutionary Perspectives on Competition, Cooperation, Violence, and Warfare. Springer Verlag, 2013.

Fuxjager, Matthew J., et al. “Why Animals Fight: Uncovering the Function and Mechanisms of Territorial Aggression.” APA Handbook of Comparative Psychology: Basic Concepts, Methods, Neural Substrate, and Behavior, 2017, pp. 853–75, doi.org/10.1037/0000011-041.

Greggor, Alison. "Territoriality: What, How, and Why It Matters in Species’ Reintroductions." San Diego Zoo Wildlife Alliance, 2019, science.sandiegozoo.org/science-blog/territoriality-what-how-and-why-it-matters-species’-reintroductions. Accessed 10 Sept. 2024.

Nordell, Shawn E., and Thomas J. Valone. Animal Behavior: Concepts, Methods, and Applications. 4th ed., Oxford UP, 2024.

Ratcliffe, Derek A. The Peregrine Falcon. 2nd ed., T&AD Poyser, 2010.

Rubenstein, Dustin R. Animal Behavior. 12th ed., Oxford UP, 2023.

Rubenstein, Dustin R., and John Alcock. Animal Behavior. 11th ed., Oxford UP, 2019.

Wilson, Edward O. Sociobiology. The Belknap Press of Harvard UP, 1975.