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Swarm describes a behaviour of an aggregate of animals of similar size and body orientation, often moving en masse in the same direction. "Swarming" is a general term that can be applied to any animal that swarms. The term can be applied to insects, birds, fish, various microorganisms such as bacteria, and people. The term applies particularly to insects. "Flocking" is the term usually used for swarming behaviour in birds, while "shoaling" or "schooling" refers to swarming behaviour in fish. The swarm size is a major parameter of a swarm.
Many of the larger birds fly in flocks. Flying in flocks helps in reducing the energy needed. Many large birds fly in a V-formation and individual energy savings have been estimated in the range 12–20 %. Red Knots Calidris canutus and Dunlins Calidris alpina were found in radar studies to fly 5 km per hour faster in flocks than when they were flying solitary.
The term locust refers to the swarming phase of the short-horned grasshoppers of the family Acrididae. The origin and apparent extinction of certain species of locust—some of which reached 6 inches (15 cm) in length—are unclear.
These are species that can breed rapidly under suitable conditions and subsequently become gregarious and migratory. They form bands as nymphs and swarms as adults—both of which can travel great distances, rapidly stripping fields and greatly damaging crops.
Research at Oxford University has identified that swarming behaviour is a response to overcrowding. Increased tactile stimulation of the hind legs causes an increase in levels of serotonin. This causes the locust to change color, eat much more, and breed much more easily. The transformation of the locust to the swarming variety is induced by several contacts per minute over a four-hour period. It is estimated that the largest swarms have covered hundreds of square miles and consisted of many billions of locusts.
In a paper in the 2009-01-30 edition of the AAAS magazine Science, Anstey & Rogers et al. showed that when desert locusts meet up, their nervous systems release serotonin, which causes them to become mutually attracted, a prerequisite for swarming.
In further independent research from the University of Oxford, published in the 2009-04-07 edition of PNAS, Yates et al. suggested that an individual locust's response to a loss ofalignment in the group is increased randomness of its motion, until an aligned state is again achieved. They claim that noise-induced alignment appears to be an intrinsic characteristic of collective coherent motion.
In many cultures, termites are used for food, particularly the alates. The alates are nutritious, having a good store of fat and protein, and are palatable in most species with a nutty flavour when cooked. They are easily gathered at the beginning of the rainy season in West, Central and Southern Africa when they swarm, as they are attracted to lights and can be gathered up when they land on nets put up around a lamp.
Mosquitoes: Adult mosquitoes usually mate within a few days after emerging from the pupal stage. In most species, the males form large swarms, usually around dusk, and the females fly into the swarms to mate.
Shoal can describe any group of fish, including mixed-species groups, "school" is reserved for more closely knit groups of the same species swimming in a highly synchronized and polarized manner.
Fish derive many benefits from shoaling behaviour including defense against predators (through better predator detection and by diluting the chance of capture), enhanced foraging success, and higher success in finding a mate. It is also likely that fish benefit from shoal membership through increased hydrodynamic efficiency.
Fish use many traits to choose shoalmates. Generally they prefer larger shoals, shoalmates of their own species, shoalmates similar in size and appearance to themselves, healthy fish, and kin (when recognized).
The "oddity effect" posits that any shoal member that stands out in appearance will be preferentially targeted by predators. This may explain why fish prefer to shoal with individuals that resemble them. The oddity effect would thus tend to homogenize shoals.
One puzzling aspect of shoal selection is how a fish can choose to join a shoal of animals similar to themselves, given that it cannot know its own appearance. Experiments with zebrafish have shown that shoal preference is a learned ability, not innate. A zebrafish tends to associate with shoals that resemble shoals in which it was reared (that is, a form of imprinting).
Other open questions of shoaling behaviour include identifying which individuals are responsible for the direction of shoal movement. In the case of migratory movement, most members of a shoal seem to know where they are going. In the case of foraging behaviour, ethologist Stephan Reebs, writing in the journal Animal Behaviour, reported that captive shoals of golden shiner (a kind of minnow) were led by a small number of experienced individuals who knew when and where food was available.
Other marine animals
Small marine animals, such as copepods, can also swarm under certain conditions. Antarctic krill, small shrimp-like crustaceans live in large swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic meter. Jellyfish are also said to "swarm". Large marine mammals, such as whales, dolphins and dolphins, also form social groups called pods, but perhaps short of what would be called a "swarm".
A group of people can also exhibit swarm behaviour, such as soldiers swarming over the parapets. In Cologne, Germany, two biologists from the University of Leeds demonstrated a flock like behaviour in humans. The group of people exhibited a very similar behavioural pattern to that of a flock, where if five percent of the flock would change direction the others would follow suit. If one person was designated as a predator and everyone else was to avoid him, the flock behaved very much like a school of fish. The mathematical modelling of flocking behaviour is a common technology, and has found uses in animation. Flocking simulations have been used in many films to generate crowds which move realistically.
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- New York times article on investigations into swarming
- Christian Jacob, Ph.D — swarm computer simulation
- Lessons from Mother Nature on How to Manage Traffic — Daily Planet on Discovery Channel