Herd

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.

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).

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

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.

The following includes an outline about some of the major effects determining the trade-offs for living in groups.