Herd

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Boy herding a flock of sheep, India; a classic example of the domestic herding of animals Sheep and herder India.jpg
Boy herding a flock of sheep, India; a classic example of the domestic herding of animals
Wildebeest at the Ngorongoro Crater; an example of a herd in the wild Wilderbeest.jpg
Wildebeest at the Ngorongoro Crater; an example of a herd in the wild

A herd is a social group of certain animals of the same species, either wild or domestic. The form of collective animal behavior associated with this is called herding . These animals are known as gregarious animals.

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The term herd is generally applied to mammals, and most particularly to the grazing ungulates 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 or goats. Large groups of carnivores are usually called packs , and in nature a herd is classically subject to predation from pack hunters.

Special collective nouns 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, the generic term herd can be used for all such kinds of assemblage.[ citation needed ]

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 shepherds (who tend to sheep), goatherds (who tend to goats), and cowherds (who tend to cattle).

A herd of cows in Punjab. A herd of cows in Punjab.jpg
A herd of cows in Punjab.

The structure and size of herds

Herd Of Goats.jpg
Traditional herding of goats in Greece. Overgrazing by poorly managed traditional herding is one of the primary causes of desertification and maquis degradation.
Wildebeest-during-Great-Migration.JPG
Wildebeest in Masai Mara during the Great Migration. Overgrazing is not caused by nomadic grazers in huge populations of travel herds, [1] [2] nor by holistic planned grazing. [3]

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 [4] 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 size is an important characteristic of the social environment of gregarious species.

Costs and benefits of animals in groups

Traffic jam created by a herd of cattle in Addis Ababa, Ethiopia. Traffic Jam with Cattle - Addis Ababa - Ethiopia (8743135833).jpg
Traffic jam created by a herd of cattle in Addis Ababa, Ethiopia.

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 and behavioural ecology. 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 independently multiple times in various taxa 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. [5]

The following includes an outline about some of the major effects determining the trade-offs 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. [6] [7]

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). For fit animals, being in a group with such vulnerable individuals may thus decrease the chance of being preyed upon even further. [5]

Collective vigilance

A snow goose gaggle may contain thousands. Chen caerulescens 32956.JPG
A snow goose gaggle may contain thousands.
Meerkats (Suricata suricatta) take turn to keep guard while other members of the group are resting or feeding. Meerkat (Suricata suricatta) Tswalu.jpg
Meerkats (Suricata suricatta) take turn to keep guard while other members of the group are resting or feeding.

The effect of collective vigilance in social groups has been widely studied within the framework of optimal foraging theory 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. [8]

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. [9]

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. [10] [8] 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. [5] 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, which they are unable to achieve when hunting alone. Working together significantly improves foraging efficiency, meaning the net energy gain of each individual is increased when animals are feeding collectively. As an example, a group of Spinner dolphins is able to corral fish into a smaller volume, which makes catching them easier, as there is less opportunity for the fish to escape. Furthermore, large groups are able to monopolize resources and defend them against solitary animals or smaller groups of the same or different species. It has been shown that larger groups of lions tend to be more successful in protecting prey from hyenas than smaller ones. [5] 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 known to be used by both bees (via a Waggle dance) and several species birds (using specific vocalisations to indicate food).

In terms of Optimal foraging theory, 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). [11]

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. [11] [5]

The obvious disadvantage of foraging in groups is (scramble or direct) competition 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. [5]

Diseases and parasites

Since animals in groups stay near each other and interact frequently, 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. [5] However, some animals have found countermeasures such as propolis in beehives or grooming 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. [12] 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. [5]

Antipredatory behaviour

The collective force of a group mobbing 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. [5]

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. [5]

Domestic herds

A shepherd guiding his sheep through the high desert outside of Marrakech, Morocco Morroco-arid-climate.jpg
A shepherd guiding his sheep through the high desert outside of Marrakech, Morocco

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, with the concept of herd behaviour. However both the term and concepts that underlie its use are controversial.

The term has acquired a semi-technical usage in behavioral finance to describe the largest group of market investors 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 and rational expectations. Other researchers, however, ascribe it to non-rational process such as 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

Related Research Articles

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Ethology is a branch of zoology that studies the behaviour of non-human animals, usually with a scientific focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objective study of animal behavior, usually referring to measured responses to stimuli or to trained behavioral responses in a laboratory context, without a particular emphasis on evolutionary adaptivity. Throughout history, different naturalists have studied aspects of animal behaviour. Ethology has its scientific roots in the work of Charles Darwin and of American and German ornithologists of the late 19th and early 20th century, including Charles O. Whitman, Oskar Heinroth, and Wallace Craig. The modern discipline of ethology is generally considered to have begun during the 1930s with the work of Dutch biologist Nikolaas Tinbergen and Austrian biologists Konrad Lorenz and Karl von Frisch, the three recipients of the 1973 Nobel Prize in Physiology or Medicine. Ethology combines laboratory and field science, with a strong relation to some other disciplines such as neuroanatomy, ecology, and evolutionary biology. Ethologists typically show interest in a behavioral process rather than in a particular animal group, and often study one type of behavior, such as aggression, in a number of unrelated species.

<span class="mw-page-title-main">Predation</span> Biological interaction where a predator kills and eats a prey organism

Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation and parasitoidism. It is distinct from scavenging on dead prey, though many predators also scavenge; it overlaps with herbivory, as seed predators and destructive frugivores are predators.

<span class="mw-page-title-main">Swarm behaviour</span> Collective behaviour of a large number of (usually) self-propelled entities of similar size

Swarm behaviour, or swarming, is a collective behaviour exhibited by entities, particularly animals, of similar size which aggregate together, perhaps milling about the same spot or perhaps moving en masse or migrating in some direction. It is a highly interdisciplinary topic.

<span class="mw-page-title-main">Foraging</span> Searching for wild food resources

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 that studies the foraging behavior of animals in response to the environment where the animal lives.

<span class="mw-page-title-main">Common minnow</span> Species of fish

The Eurasian minnow, minnow, or common minnow is a small species of freshwater fish in the carp family Cyprinidae. It is the type species of genus Phoxinus. It is ubiquitous throughout much of Eurasia, from Britain and Spain to eastern Siberia, predominantly in cool streams and well-oxygenated lakes and ponds. It is noted for being a gregarious species, shoaling in large numbers.

<span class="mw-page-title-main">Scaly-breasted munia</span> Species of bird native to South and Southeast Asia

The scaly-breasted munia or spotted munia, known in the pet trade as nutmeg mannikin or spice finch, is a sparrow-sized estrildid finch native to tropical Asia. A species of the genus Lonchura, it was formally described and named by Carl Linnaeus in 1758. Its name is based on the distinct scale-like feather markings on the breast and belly. The adult is brown above and has a dark conical bill. The species has 11 subspecies across its range, which differ slightly in size and color.

<span class="mw-page-title-main">Anti-predator adaptation</span> Defensive feature of prey for selective advantage

Anti-predator adaptations are mechanisms developed through evolution that assist prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, namely by avoiding detection, warding off attack, fighting back, or escaping when caught.

<span class="mw-page-title-main">Forest tent caterpillar moth</span> Species of insect

The forest tent caterpillar moth is a moth found throughout North America, especially in the eastern regions. Unlike related tent caterpillar species, the larvae of forest tent caterpillars do not make tents, but rather, weave a silky sheet where they lie together during molting. They also lay down strands of silk as they move over branches and travel as groups along these pheromone-containing silk trails. The caterpillars are social, traveling together to feed and massing as a group at rest. Group behavior diminishes as the caterpillars increase in size, so that by the fifth instar (molt) the caterpillars are feeding and resting independently.

<span class="mw-page-title-main">Siberian jay</span> Species of bird

The Siberian jay is a small jay with a widespread distribution within the coniferous forests in North Eurasia. It has grey-brown plumage with a darker brown crown and a paler throat. It is rusty-red in a panel near the wing-bend, on the undertail coverts and on the sides of the tail. The sexes are similar. Although its habitat is being fragmented, it is a common bird with a very wide range so the International Union for Conservation of Nature has assessed its conservation status as being of "least concern".

A mixed-species feeding flock, also termed a mixed-species foraging flock, mixed hunting party or informally bird wave, is a flock of usually insectivorous birds of different species that join each other and move together while foraging. These are different from feeding aggregations, which are congregations of several species of bird at areas of high food availability.

<span class="mw-page-title-main">Flock (birds)</span>

A flock is a gathering of individual birds to forage or travel collectively. Avian flocks are typically associated with migration. Flocking also offers foraging benefits and protection from predators, although flocking can have costs for individual members.

<span class="mw-page-title-main">Mobbing (animal behavior)</span> Antipredator adaptation in which individuals of prey species mob a predator

Mobbing in animals is an antipredator adaptation in which individuals of prey species mob a predator by cooperatively attacking or harassing it, usually to protect their offspring. A simple definition of mobbing is an assemblage of individuals around a potentially dangerous predator. This is most frequently seen in birds, though it is also known to occur in many other animals such as the meerkat and some bovines. While mobbing has evolved independently in many species, it only tends to be present in those whose young are frequently preyed upon. This behavior may complement cryptic adaptations in the offspring themselves, such as camouflage and hiding. Mobbing calls may be used to summon nearby individuals to cooperate in the attack.

<span class="mw-page-title-main">Communal roosting</span>

Communal roosting is an animal behavior where a group of individuals, typically of the same species, congregate in an area for a few hours based on an external signal and will return to the same site with the reappearance of the signal. Environmental signals are often responsible for this grouping, including nightfall, high tide, or rainfall. The distinction between communal roosting and cooperative breeding is the absence of chicks in communal roosts. While communal roosting is generally observed in birds, the behavior has also been seen in bats, primates, and insects. The size of these roosts can measure in the thousands to millions of individuals, especially among avian species.

<span class="mw-page-title-main">Collective animal behavior</span> Animal cognition

Collective animal behaviour is a form of social behavior involving the coordinated behavior of large groups of similar animals as well as emergent properties of these groups. This can include the costs and benefits of group membership, the transfer of information, decision-making process, locomotion and synchronization of the group. Studying the principles of collective animal behavior has relevance to human engineering problems through the philosophy of biomimetics. For instance, determining the rules by which an individual animal navigates relative to its neighbors in a group can lead to advances in the deployment and control of groups of swimming or flying micro-robots such as UAVs.

<span class="mw-page-title-main">Shoaling and schooling</span> In biology, any group of fish that stay together for social reasons

In biology, any group of fish that stay together for social reasons are shoaling, and if the group is swimming in the same direction in a coordinated manner, they are schooling. In common usage, the terms are sometimes used rather loosely. About one quarter of fish species shoal all their lives, and about one half shoal for part of their lives.

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 which every point is nearer to a particular individual than to any other individual. Such antipredator behavior inevitably results in aggregations. The theory was proposed by W. D. Hamilton in 1971 to explain the gregarious behavior of a variety of animals. It contrasted the popular hypothesis that evolution of such social behavior was based on mutual benefits to the population.

Vigilance, in the field of behavioural ecology, refers to an animal's monitoring of its surroundings in order to heighten awareness of predator presence. Vigilance is an important behaviour during foraging as animals must often venture away from the safety of shelter to find food. However, being vigilant comes at the expense of time spent feeding, so there is a trade-off between the two. The length of time animals devote to vigilance is dependent on many factors including predation risk and hunger.

<span class="mw-page-title-main">Pursuit predation</span> Hunting strategy by some predators

Pursuit predation is a form of predation in which predators actively give chase to their prey, either solitarily or as a group. It is an alternate predation strategy to ambush predation — pursuit predators rely on superior speed, endurance and/or teamwork to seize the prey, while ambush predators use concealment, luring, exploiting of surroundings and the element of surprise to capture the prey. While the two patterns of predation are not mutually exclusive, morphological differences in an organism's body plan can create an evolutionary bias favoring either type of predation.

<span class="mw-page-title-main">Primate sociality</span>

Primate sociality is an area of primatology that aims to study the interactions between three main elements of a primate social network: the social organisation, the social structure and the mating system. The intersection of these three structures describe the socially complex behaviours and relationships occurring among adult males and females of a particular species. Cohesion and stability of groups are maintained through a confluence of factors, including: kinship, willingness to cooperate, frequency of agonistic behaviour, or varying intensities of dominance structures.

In ethology and evolutionary biology, group living is defined as individuals of the same species (conspecifics), maintaining spatial proximity with one another over time with mechanisms of social attraction. Solitary life in animals is considered to be the ancestral state of living; and group living has thus evolved independently in many species of animals. Therefore, species that form groups through social interaction will result in a group of individuals that gain an evolutionary advantage, such as increased protection against predators, access to potential mates, increased foraging efficiency and the access to social information.

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