Animal aggregation

Some animals spend most of their time alone because the presence of other conspecifics would interfere with the use of a particular resource or a suite of resources. These animals only come together with another solitary individual to pair for reproduction. Others form groups ranging from pairs of animals to large herds. Finally, some animals are brought together by phenomena over which they have no control (winds, currents, or tides) and are simply clumped in space. Groups formed for whatever reason can be either temporary or permanent and are generally theorized as being beneficial for a variety of reasons. Some associations are simply the result of congregating around a common food resource. Other associations arise for specific functions, such as finding mates, caring for the young, providing a learning environment for developing young, providing protection from the elements, thermoregulation or huddling, locomotory efficiency (swimming or flight), locating and subduing food items, resource defense against other groups or competing species, division of labor, population regulation, predator vigilance, and reduced predation risk via dilution, confusion, encounter, or group defense effects. While potential benefits can be many, aggregation can also result in distinct disadvantages for individuals making up the group. Such disadvantages include increased competition for resources (such as mates, food, or shelter), increased risk of disease and spread of parasites, and interference in reproductive behaviors.

Reproduction and Rearing Young

Living in groups can increase an individual’s chance of finding a mate, but it often results in increased aggression between males who must compete for females. Because all females may come into estrus within a short time, grouping together at specific mating territories ensures that all females will be mated. In such species, courtship rituals are common. These rituals serve a dual purpose: They provide the male with information about the sexual receptivity of the female, and they allow the female to assess the quality of the male prior to pairing with him. In some cases, the rituals also serve to bind the pair together for a breeding season or longer in some species. In other cases, males simply congregate at display grounds to attract and court females. Females leave the display grounds after mating to nest elsewhere, while the males remain to court other females. This latter grouping strategy is known as lekking.

In some species, which both aggregate during mating seasons and form pair bonds, both males and females can care for the young. In some cases, a mating pair may have helpers at the nest—other members of the species (usually offspring) who aid the parents in raising the young. Helpers at the nest greatly contribute to the breeding success of the parental birds and gain experience themselves in rearing young. They can then use their experience to be successful parents in the next breeding season. Other examples, such as lions and elephants, include kin groups (generally sisters or a mother and her daughters) that help to raise whatever offspring are present in the group.

Colonial nesting and, thus, synchronous egg-laying produces offspring in large numbers, who are vulnerable to predators for only a short time. In this way, each parent lessens the chance that their offspring will be the ones taken by any predator (dilution effect). However, colonial nesting also presents the possibility that offspring may grow more slowly or run the risk of parasitism from fleas and mites. Some evidence exists that the offspring of bank swallows gained weight more slowly if they were from large colonies versus small colonies, suggesting that large colonies were depleting their resources more rapidly. Nests from large colonies were also more often infested with fleas than those in small colonies.

Rearing young in a group gives the young opportunities to learn from more than one adult. It can also provide them with practice in tasks that later prove important when the offspring are on their own. Cooperative hunting can provide young with the opportunity to learn hunting skills from their elders. Generally, this benefit results in longer-lived species that produce only a few young per year.

Survival

Cold temperatures can cause physiological problems, particularly for animals that are ectothermic (relying upon the environment for heat). During the day, such animals can bask in the sun and maintain high body temperatures. At night, however, they may have difficulty staying warm, so many huddle together in large groups. This type of behavior allows ectotherms to continue to digest their food (a process requiring heat).

Ectothermic species are not the only ones that huddle—many birds and mammals do so as well. Some birds flock together on trees on particularly cold nights to reduce the surface area of their body that is exposed to the elements. Voles, which are normally asocial, huddle in groups during the winter to keep warm. Marmots can huddle underground in groups of twenty for up to seven months to avoid freezing temperatures in the mountain regions they occupy.

The social context of group sleeping and the environment animals sleep in is a growing area of research among animal behaviorists. Decision-making, coordination, and cooperative inclinations are believed to be linked to group sleep patterns and behaviors. In captivity, bees have been observed avoiding sleep when they have young offspring, and as the group size of olive baboons increases, they sleep less. Using wearable devices and video monitors to track animals in the wild, data may reveal the evolutionary impacts of social sleep on animal groups and the function of sleep as a mechanism of evolutionary development.

Lowered individual predation risk is theorized as a primary benefit leading to the evolution of gregarious behavior, particularly in the absence of kinship between group members. Three likely outcomes of grouping that can reduce rates of attack are the dilution effect, the confusion effect, and the encounter effect. In the dilution effect, the probability of a particular individual being killed or injured is reduced by the presence of other group members that might be attacked first. This helps to explain why ostriches lay eggs in communal clutches, which only the first laying female incubates. The first female is not necessarily acting in an altruistic manner because she is diluting the chance that predators will find her eggs and eat them or, after the chicks hatch, that predators will capture her chicks instead of those of another female. Individuals in a group may also benefit by putting other animals between themselves and the predator (the selfish herd effect). Grouping provides the opportunity to decrease the area of danger around each individual. If individuals within the group are acting in a selfish herd manner, the groups formed tend to be tightly clumped, as all individuals attempt to put other individuals on all sides around them.

Creating confusion benefits group members because predators may have difficulty in fixing upon a particular individual for attack. The time it takes for the predator to discern a particular individual from a mass of surrounding individuals may be enough for the entire group to scatter and further confuse the predator.

In contrast, the encounter effect results when it is more difficult for a predator to find a single group of prey than an equal number of scattered individuals due to the apparent rarity of the grouped individuals. However, the actual result of aggregation may be increased risk of detection if the group becomes a more conspicuous entity, which is detrimental to the individuals making up the group, rather than beneficial.

Groups can also benefit by collective defense, which is seen in animals possessing weaponry such as horns or other piercing appendages. Commonly, these groups will form a line (phalanx) or a rosette (circular structure) with weapons pointed outward toward the attacker(s). Common examples of these defensive groups include musk oxen, elephants, and spiny lobsters. The use of weapons in these defensive lines may enhance the probability of survival above that of mere dilution since each defending animal is capable of inflicting damage on a predator.

An individual must partition its time between foraging and being vigilant for potential predators. However, if that individual is within a group, it can spend more time on foraging and less time on vigilance because its scan frequency for predators can decrease proportionate to the number of members of the group. Furthermore, because of the many eyes within the group, detection of predators with subsequent alerting of the danger is enhanced. Some species go so far as to alert other group members to danger by alarm calling. This may have evolved for several reasons: The caller benefits because it knows where the predator is and can position itself appropriately within the group to avoid being a target; the caller may enhance the probability of becoming the target, but at the same time, it reduces the probability that its kin will be targeted (this works in kin groups); and the caller may attract attention to itself at the time of the call, but if it survives, it is entitled to payback at a later date from some other individual in the group doing the same.

Communal Foraging and Hunting

Frequently, food being sought for exploitation is clumped in space. As a result, the animals that feed upon this food are also clumped, and this promotes aggregation into group structures, such as herds. Because animals can observe others of their species feeding, they can follow successful foragers to feeding groups and group for no other reason. This phenomenon may allow such foragers to exploit food resources in a more systematic way. Some evidence suggests that grasses clipped by herbivore herds actually grow faster and are more productive than grasses not so clipped, and by proceeding from one patch to another, the herds actually allow the grasses to grow back in a systematic fashion. This allows the food time to replenish itself before the herd passes by it again and would be less likely to happen if individuals exploited the resource, isolated in time and space.

Other animals form groups to facilitate hunting and capture success. However, some of these groupings can be highly variable in time, based on the food supply and how a kill is partitioned between group participants. For example, the sociality of lions is controlled by food supply. When wildebeests or zebras are especially numerous, lions concentrate their efforts upon capturing them, but solitary hunters are successful only 15 percent of the time, while groups of five are 40 percent successful. Furthermore, groups of five lions are better able to protect their kill from scavengers than are groups of two or three lions, even though a group of two or three lions will maximize their daily intake of food per successful kill, while a group of five will only secure the minimum daily requirement of food. Despite their propensity for sociality, lions will hunt alone or in pairs when the migratory wildebeests or zebras leave, and only resident gazelles are left. Again, the success rate for a solitary lion hunting a gazelle is only 15 percent, while that for a pair is 30 percent; however, a gazelle can provide the minimum daily requirement of food for only two lions.

Similarly, groups of dogs (the African wild dog, Asian dhole, dingoes, or wolves) can kill prey larger than themselves by cooperatively hunting. These packs are composed of kin (parents and their offspring) and cover large distances to hunt their prey. The individual dogs will spread out around the prey in a phalanx and then approach until one member selects a victim and runs after it. Other members of the pack will run after the prey in relays until the prey is exhausted and can be subdued.

Communal spiders build webs larger than a single individual could spin and capture prey larger than any one could capture alone. They communally feed on the captured prey. Most spider species have a short interval after the spiderlings have hatched where they remain clumped in a group and live in a communal web. After a period of several days, the spiderlings disperse to take up a solitary life. In communal species, however, adults of the same species come together to form colonies of up to one thousand individuals.

Coordinated group hunting is also known in marine mammals, particularly killer whales (orcas) and humpback whales. Orcas live in matriarchal societies (pods) of two to twelve members and hunt other marine mammals. Single orcas will charge sea lions in the surf of a beach, while other pod members will wait offshore to ensnare any sea lions that respond to the charge by entering the water. Adults will also train juvenile orcas, in the process of play-stranding, to capture seals or sea lions by throwing a dead seal to a beached juvenile. Orcas will also attack baleen whales by surrounding them and biting and holding the whale underwater so that it will drown. Humpback whales are known for bubble feeding, where members of the pod surround a school of small fish and release bubbles while spiraling upward. This bubble net concentrates the terrified fish into a tight ball, which the whales then eat as they approach the surface and open their mouths. Groups of dolphins herd fish into shallow waters and surround them so they can be easily picked off.

Even groups of mixed species, called mixed-species animal groups (MSGs), are known to cooperatively hunt. This is common in shorebirds, where one species might herd the prey while another species either dives to prevent the prey’s escape from below or stabs at the prey to prevent its escape along the banks of the water. Pelicans, cormorants, and gulls, as well as grebes and egrets, are known to form these associations. Additionally, robins have been observed feeding raising the young of other bird species, such as pied wagtails.

Colonial species, particularly those that are sessile and need space to spread out, need to be able to defend their resources. Among invertebrates, larger colonies may have the competitive advantage in excluding newcomers from unoccupied space, as well as pushing other colonial species out of the way, so that the space once occupied can be overrun. Corals are well known for their warlike actions as the space on a reef becomes scarce—they will use their stinging cells to attack adjacent coral species in an attempt to kill the polyps making up the other colony. Dense colonies of bryozoans are much more able to withstand overgrowth by other invertebrates than are bryozoan colonies that are more spread out. Mussel beds can overrun barnacle colonies in the intertidal zone if their main predator is not present in sufficient numbers to keep the mussel population low.

Division of Labor

In eusocial insects, colony members divide the labor amongst themselves. In worker bees, the labor done is dependent on the age of the bee. After emergence, a worker bee’s first job is cleaning the hive. She then tends the brood, builds up the honeycomb, and guards the nest. Her final task is to forage for pollen and nectar. The change in her duties correlates to physiological changes in her nurse glands and wax glands.

Ant colonies comprise thousands of individuals, divided into workers, brood, and queen. In army ants, workers vary in size, and these size differences determine their role in the colony. Smaller workers spend most of their time tending to and feeding the larval broods; medium-sized workers make up the majority of the population and are responsible for making raids on other colonies and locating food. The largest workers are called soldiers because of their powerful jaws; they accompany the raiding parties but carry no food.

The naked mole-rat and the Damaraland mole-rat are the only known vertebrates that live in colonies with an advanced degree of social organization similar to those of the social insects like ants, wasps, bees, and termites. Only one female breeds. Larger individuals remain in the colony and huddle to keep the entire colony warm. Smaller individuals are the worker caste and are responsible for nest building and foraging.

These examples serve to illustrate that grouping behavior of animals can serve a myriad of purposes or can simply be the result of phenomena beyond the animal’s control (patchy food resources, physical forces of nature). The functions that lead to cooperative activities are usually best explained by groups being composed of kin; however, cooperation can also evolve in the absence of kinship, provided the benefits to group members outweigh the costs.

Principal Terms

Competition: Interactions among individuals that attempt to utilize the same limited resource

Conspecifics: Members of the same species

Dilution Effects: The reduction in per capita probability of death from a predator due to the presence of other group members

Encounter Effects: The reduction in the probability of death from a predator due to a single group of N members being more difficult to locate than an equal number of solitary individuals

Interference: The act of impeding others from using some limited resource

Predation: The act of killing and consuming another organism

Resource Defense: The control of a resource indirectly or directly

Sociality: The tendency to form and maintain stable groups

Bibliography

"Aggregation (Biota)." WetlandInfo, Department of Environment, Science and Innovation, 30 Aug. 2023, wetlandinfo.des.qld.gov.au/wetlands/ecology/processes-systems/aggregation. Accessed 20 Sept. 2024.

Bertram, B. C. R. “Living in Groups: Predators and Prey.” In Behavioral Ecology: An Evolutionary Approach, edited by J. R. Krebs and N. B. Davies, Sinauer Associates, 1978.

"For Many Animals Sleep Is a Social Activity, but It's Usually Studied as an Individual Process." ScienceDaily, 5 Sept. 2024, www.sciencedaily.com/releases/2024/09/240905120854.htm. Accessed 20 Sept. 2024.

Goodale, Eben, et al. “Mixed Company: A Framework for Understanding the Composition and Organization of Mixed-Species Animal Groups.” Biological Reviews, vol. 95, no. 4, 2020, pp. 889–910, doi.org/10.1111/brv.12591.

Halliday, Tim, editor. Animal Behavior. U of Oklahoma P, 1994.

Hamilton, W. D. “Geometry for the Selfish Herd.” Journal of Theoretical Biology, vol. 31, 1971, pp. 294-311.

Janson, C. H. “Testing the Predation Hypothesis for Vertebrate Sociality: Prospects and Pitfalls.” Behavior, vol. 135, 1998, pp. 389-410.

Krebs, J. R., and N. B. Davies. An Introduction to Behavioral Ecology. 4th ed., Blackwell Scientific, 2014.

Philippe, Anne-Sophie, et al. “Genetic Variation in Aggregation Behaviour and Interacting Phenotypes in Drosophila.” Proceedings: Biological Sciences, vol. 283, no. 1827, 2016, pp. 1–8. doi.org/10.1098/rspb.2015.2967.

Pulliam, H. R., and T. Caraco. “Living in Groups: Is There an Optimal Group Size?” In Behavioral Ecology: An Evolutionary Approach, edited by J. R. Krebs and N. B. Davies, 2nd ed., Sinauer Associates, 1984.