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A herd is a social group of certain animals of the same species, either
wild Wild, wild or wild may refer to: Common meanings * Wild animal ''Wild Animal'' is the debut solo studio album by Canadian singer Vanity Vanity is the excessive belief in one's own abilities or attractiveness to others. Prior to the 14th centu ...

wild
or
domestic Domestic may refer to: In the home * Anything relating to the human home A home, or domicile, is a space used as a permanent or semi-permanent residence for an individual, group or family. It is a fully or semi sheltered space and can h ...
. The form of
collective animal behavior Collective animal behavior is a form of social behavior Social behavior is behavior Behavior (American English) or behaviour (British English; American and British English spelling differences#-our, -or, see spelling differences) is the Act ...
associated with this is called ''
herding Herding is the act of bringing individual animals together into a group (herd), maintaining the group, and moving the group from place to place—or any combination of those. Herding can refer either to the process of animals forming herds in ...

herding
''. The term ''herd'' is generally applied to mammals, and most particularly to the grazing
ungulate Ungulates ( ) are members of the diverse Ungulata which primarily consists of large mammals with . These include s such as s, es, and s; and s such as , s, s, s, , , and es. s such as , , and are also classified as even-toed ungulates, althoug ...
s that classically display this behaviour. Different terms are used for similar groupings in other species; in the case of birds, for example, the word is '' flocking'', but ''flock'' may also be used for mammals, particularly
sheep Sheep (''Ovis aries'') are quadrupedal, ruminant mammals typically kept as livestock. Like all ruminants, sheep are members of the order (biology), order Artiodactyla, the even-toed ungulates. Although the name ''sheep'' applies to many species ...

sheep
or
goat The domestic goat or simply goat (''Capra hircus'') is a domesticated species of typically kept as . It was from the (''C. aegagrus'') of and . The goat is a member of the animal family and the subfamily , meaning it is closely related ...

goat
s. Large groups of
carnivores A carnivore , meaning "meat Meat is animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organ ...
are usually called '' packs'', and in nature a herd is classically subject to
predation Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

predation
from
pack hunter A pack hunter or social predator is a predatory animal which hunts its prey by working together with other members of its species. Normally animals hunting in this way are closely related, and with the exceptions of chimpanzees where only males no ...
s. Special
collective noun In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign language) and writing. Most languag ...
s may be used for particular taxa (for example a flock of geese, if not in flight, is sometimes called a ''gaggle'') but for theoretical discussions of behavioural
ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms In biol ...
, the generic term ''herd'' can be used for all such kinds of assemblage. The word ''herd'', as a noun, can also refer to one who controls, possesses and has care for such groups of animals when they are domesticated. Examples of herds in this sense include
shepherd A shepherd or sheepherder is a person who tends, herds, feeds, or guards herds of sheep Sheep (''Ovis aries'') are quadruped The zebra is a quadruped. Quadrupedalism is a form of terrestrial locomotion where a tetrapod Tetrapods ( ...

shepherd
s (who tend to sheep),
goatherd 240px, A man herding goats in Tunisia A goatherd or goatherder is a person who herds goat The domestic goat or simply goat (''Capra hircus'') is a domesticated species of goat-antelope The subfamily Caprinae is part of the ruminant ...
s (who tend to goats), and cowherds (who tend to cattle).


The structure and size of herds

When an association of animals (or, by extension, people) is described as a herd, the implication is that the group tends to act together (for example, all moving in the same direction at a given time), but that this does not occur as a result of planning or coordination. Rather, each individual is choosing behaviour in correspondence with most other members, possibly through imitation or possibly because all are responding to the same external circumstances. A herd can be contrasted with a coordinated group where individuals have distinct roles. Many human groupings, such as army detachments or sports teams, show such coordination and differentiation of roles, but so do some animal groupings such as those of eusocial insects, which are coordinated through pheromones and other forms of animal communication. A herd is, by definition, relatively unstructured. However, there may be two or a few animals which tend to be imitated by the bulk of the herd more than others. An animal in this role is called a "control animal", since its behaviour will predict that of the herd as a whole. It cannot be assumed, however, that the control animal is deliberately taking a leadership role; control animals are not necessarily socially dominant in conflict situations, though they often are.
Group sizes. Many animals, including humans, tend to live in groups, herds, flock (birds), flocks, bands, Pack (canine), packs, Shoaling and schooling, shoals, or Bird colony, colonies (hereafter: groups) of conspecific individuals. The size of these groups, a ...
is an important characteristic of the social environment of gregarious species.


Costs and benefits of animals in groups

The reason why animals form herds can not always be stated easily, since the underlying mechanisms are diverse and complex. Understanding the social behaviour of animals and the formation of groups has been a fundamental goal in the field of
sociobiology Sociobiology is a field of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology ...
and
behavioural ecology Behavioral ecology, also spelled behavioural ecology, is the study of the evolutionary basis for ethology, animal behavior due to ecology, ecological pressures. Behavioral ecology emerged from ethology after Niko Tinbergen outlined Tinbergen's fo ...
. Theoretical framework is focused on the costs and benefits associated with living in groups in terms of the fitness of each individual compared to living solitarily. Living in groups
evolved Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolved
independently multiple times in various
taxa In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
and can only occur if its benefits outweigh the costs within an evolutionary timescale. Thus, animals form groups whenever this increases their fitness compared to living in solitary.Majolo, B., & Huang, P. (2020)
Group living
In J. Vonk & T. Shackelford (Eds.), Encyclopedia of Animal Cognition and Behavior.
The following includes an outline about some of the major effects determining the
trade-off A trade-off (or tradeoff) is a situational decision that involves diminishing or losing one quality, quantity, or property of a set or design in return for gains in other aspects. In simple terms, a tradeoff is where one thing increases, and another ...
s for living in groups.


Dilution effect

Perhaps the most studied effect of herds is the so called dilution effect. The key argument is that the risk of being preyed upon for any particular individual is smaller within a larger group, strictly due to the fact that a predator has to decide which individual to attack. Although the dilution effect is influenced by so called selfish herding, it is primarily a direct effect of group size instead of the position within a herd. Greater group sizes result in higher visibility and detection rates for predators, but this relation is not directly proportional and saturates at some point, while the risk of being attacked for an individual is directly proportional to group size. Thus, the net effect for an individual in a group concerning its predation risk is beneficial. Whenever groups, such as shoals of fish, synchronize their movements, it becomes harder for predators to focus on particular individuals. However, animals that are weak and slower or on the periphery are preferred by predators, so that certain positions within the group are better than others (see
selfish herd theory The selfish herd theory states that individuals within a population attempt to reduce their predation risk by putting other conspecifics between themselves and predators. A key element in the theory is the domain of danger, the area of ground in whi ...
). For fit animals, being in a group with such vulnerable individuals may thus decrease the chance of being preyed upon even further.


Collective vigilance

The effect of collective vigilance in social groups has been widely studied within the framework of
optimal foraging theory Optimal foraging theory (OFT) is a behavioral ecology model that helps predict how an animal behaves when searching for food. Although obtaining food provides the animal with energy, searching for and capturing the food require both energy and ti ...
and animal decision making. While animals under the risk of predation are feeding or resting, they have to stay vigilant and watch for predators. It could be shown in many studies (especially for birds) that with increase in group size individual animals are less attentive, while the overall vigilance suffers little (many eyes effect). This means food intake and other activities related to fitness are optimized in terms of time allocation when animals stay in groups.Lima, S. L., & Dill, L. M. (1990)
Behavioral decisions made under the risk of predation: a review and prospectus.
Canadian Journal of Zoology, 68(4), 619–640.
However, some details about this concepts remain unclear. Being the first to detect predators and react accordingly can be advantageous, implying individuals may not fully be able to rely only on the group. Moreover, the competition for food can lead to the misuse of warning calls, as was observed for great tits: If food is scarce or monopolized by dominant birds, other birds (mainly subordinates) use antipredatory warning calls to induce an interruption of feeding and gain access to resources. Another study concerning a flock of geese suggested that the benefits of lower vigilance concerned only those in central positions, due to the fact that the possibly more vulnerable individuals in the flock's periphery have a greater need to stay attentive. This implies that the decrease in overall vigilance arises simply because the geese on the edge of the flock comprise a smaller group when groups get large. A special case of collective vigilance in groups is that of sentinels. Individuals take turn in keeping guard, while all others participate in other activities. Thus, the strength of social bonds and trust within these groups have to be much higher than in the former cases.


Foraging

Hunting together enables group-living predators, such as wolves and wild dogs, to catch large prey, what they are unable to achieve when hunting alone. Working together significantly improves
foraging Foraging is searching for wild food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce. Foraging theory is a branch of behavioral ecology Behavioral ecology, also spel ...
efficiency, meaning the net energy gain of each individual is increased when animals are feeding collectively. A group of
Spinner dolphin The spinner dolphin (''Stenella longirostris'') is a small dolphin Dolphin is a common name of aquatic mammals within the infraorder Cetacea. The term dolphin usually refers to the extant families Delphinidae (the oceanic dolphins), Plat ...
s is for instances able to force fish into a small confined space, which makes capturing prey particularly easy, as there is no way for the latter to flee. Furthermore, large groups are able to monopolize resources and defend them against solitary animals or smaller groups of the same or different species. It was shown that larger groups of lions tend to be more successful in protecting prey from hyenas than small ones. Being able to communicate the location and type of food to other group members may increase the chance for each individual to find profitable food sources, a mechanism which is commonly known to be used by bees, which use a so called
Waggle dance Waggle dance is a term used in beekeeping Beekeeping (or apiculture) is the maintenance of bee colonies, commonly in man-made beehive, hives, by humans. Most such bees are honey bees in the genus ''Apis (insect), Apis'', but other honey-produc ...

Waggle dance
, and several species birds using food calls. In terms of
Optimal foraging theory Optimal foraging theory (OFT) is a behavioral ecology model that helps predict how an animal behaves when searching for food. Although obtaining food provides the animal with energy, searching for and capturing the food require both energy and ti ...
, animals always try to maximize their net energy gain when feeding, because this is positively correlated to their fitness. If their energy requirement is fixed and additional energy is not increasing fitness, they will use as little time for foraging as possible (time minimizers). If on the other hand time allocated to foraging is fixed, an animal’s gain in fitness is related to the quantity and quality of resources it feeds on (Energy maximizers).Pyke, G. H., Pulliam, H. R., & Charnov, E. L. (1977). Pyke etal-1977. Optimal foraging-a selective review of theory and tests. the quarterly review of biology. In The Quarternarly Review of Biology (Vol. 52, Issue 2, pp. 137–154). Since foraging may be energetically costly (searching, hunting, handling, etc.) and may induce risk of predation, animals in groups may have an advantage, since their combined effort in locating and handling food will reduce time needed to forage sufficiently. Thus, animals in groups may have shorter searching and handling times as well as an increased chance of finding (or monopolizing) highly profitable food, which makes foraging in groups beneficial for time minimizers and energy maximizers alike. The obvious disadvantage of foraging in groups is (scramble or direct)
competition Competition is a rivalry A rivalry is the state of two people or groups engaging in a lasting competitive relationship. Rivalry is the "against each other" spirit between two competing sides. The relationship itself may also be called "a ri ...
with other group members. In general, it is clear that the amount of resources available for each individual decreases with group size. If the resource availability is critical, competition within the group may get so intense, that animals no longer experience benefits from living in groups. However, only the relative importance of within- and between-group competition determines the optimal group size and ultimately the decision of each individual whether or not to stay in the group.


Diseases and parasites

Since animals in groups stay near each other and interact frequently,
infectious An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxin A toxin is a harmful substance produced within living cells ...

infectious
diseases and parasites spread much easier between them compared to solitary animals. Studies have shown a positive correlation between herd size and intensity of infections, but the extent to which this sometimes drastic reduction in fitness governs group size and structure is still unclear. However, some animals have found countermeasures such as
propolis Propolis or bee glue is a resin In polymer chemistry and materials science, resin is a solid or highly Viscosity, viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of ...

propolis
in beehives or in social animals.


Energetic Advantages

Staying together in groups often brings energetic advantages. Birds flying together in a flock use aerodynamic effects to reduce energetic costs, e.g. by positioning themselves in a V shaped formation.Portugal, S. J., Hubel, T. Y., Fritz, J., Heese, S., Trobe, D., Voelkl, B., Hailes, S., Wilson, A. M., & Usherwood, J. R. (2014)
Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight.
Nature, 505(7483), 399–402.
A similar effect can be observed when fish swim together in fixed formations. Another benefit of group living occurs when climate is harsh and cold: By staying close together animals experience better thermoregulation, because their overall surface to volume ratio is reduced. Consequently, maintaining adequate body temperatures becomes less energetically costly.


Antipredatory behaviour

The collective force of a group
mobbing Mobbing, as a sociological term, means bullying of an individual by a group, in any context, such as a family, peer group, school, workplace, neighborhood, community, or online. When it occurs as physical and emotional abuse in the workplace, such ...
predators can reduce risk of predation significantly. Flocks of raven are able to actively defend themselves against eagles and baboons collectively mob lions, which is impossible for individuals alone. This behaviour may be based on reciprocal altruism, meaning animals are more likely to help each other if their conspecifics did so earlier.


Mating

Animals living in groups are more likely to find mates than those living in solitary and are also able to compare potential partners in order to optimize genetic quality for their offspring.


Domestic herds

Domestic animal herds are assembled by humans for practicality in raising them and controlling them. Their behaviour may be quite different from that of wild herds of the same or related species, since both their composition (in terms of the distribution of age and sex within the herd) and their history (in terms of when and how the individuals joined the herd) are likely to be very different.


Human parallels

The term herd is also applied metaphorically to human beings in
social psychology Social psychology is the scientific Science () is a systematic enterprise that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about the universe."... modern scienc ...

social psychology
, with the concept of
herd behaviour Herd behavior is the behavior Behavior (American English) or behaviour (British English; American and British English spelling differences#-our, -or, see spelling differences) is the Action (philosophy), actions and mannerisms made by indivi ...
. However both the term and concepts that underlie its use are controversial. The term has acquired a semi-technical usage in
behavioral finance Behavioral economics (also, behavioural economics) studies the effects of psychological, cognitive, emotional, cultural and social factors on the decisions of individuals and institutions and how those decisions vary from those implied by c ...
to describe the largest group of market
investor An investor is a person that allocates capital with the expectation of a future financial return (profit) or to gain an advantage (interest). Through this allocated capital most of the time the investor purchases some species of property. Type ...
s or market speculators who tend to "move with the market", or "follow the general market trend". This is at least a plausible example of genuine herding, though according to some researchers it results from rational decisions through processes such as
information cascade An Information cascade or informational cascade is a phenomenon described in behavioral economics Behavioral economics (also, behavioural economics) studies the effects of psychological Psychology is the science of mind and behavior. P ...
and
rational expectations In economics, "rational expectations" are model-consistent expectations, in that agent (economics), agents inside the model (economics), model are assumed to "know the model" and on average take the model's predictions as valid. Rational expectat ...
. Other researchers, however, ascribe it to non-rational process such as
mimicry In evolutionary biology, mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry f ...

mimicry
, fear and greed contagion. "Contrarians" or contrarian investors are those who deliberately choose to invest or speculate counter to the "herd".


See also


Literature

*Krause, J., & Ruxton, G. D. (2002). Living in groups. Oxford: Oxford University Press.


References

{{Authority control Ethology Group processes Herding