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 (Unmanned Aerial Vehicles).
Examples Examples of collective animal behavior include: Flocking birds Herding ungulates Shoaling and schooling fish Schooling Antarctic krill Pods of dolphins Marching locusts Nest building ants Swarming Stampede
History The basis of collective animal behaviour originated from the study of collective phenomena;Sumpter, D.J., 2010. Collective animal behavior. Princeton University Press. that is, repeated interactions among individuals that produce large scale patterns. The foundation of collective phenomena originates from the idea that collective systems can be understood from a set of techniques. For example, Nicolis and Prigogine (1977)Nicolis, G. & Prigogine, I. 1977. Self-organization in nonequilibrium systems . New York: John Wiley & Sons. employed the use of non-linear thermodynamics to help explain similarities between collective systems at different scales. Other studies aim to use physics, mathematics and chemistry to provide frameworks to study collective phenomena.Wiener, N. 1948. Cybernetics . New York: John Wiley & Sons.Ashby, W. R. 1947. Principles of the self-organizing dynamic system. Journal of General Psychology 125– 128.von Bertalanffy, L. 1968. General system theory . New York: George Braziller.
Proposed functions Many functions of animal aggregations have been proposed. These proposed functions may be grouped into the four following categories: social and genetic, anti-predator, enhanced foraging, and increased locomotion efficiency.
Social interaction Support for the social and genetic function of aggregations, especially those formed by fish, can be seen in several aspects of their behavior. For instance, experiments have shown that individual fish removed from a school will have a higher respiratory rate than those found in the school. This effect has been partly attributed to stress, although hydrodynamic factors were considered more important in this particular study. The calming effect of being with conspecifics may thus provide a social motivation for remaining in an aggregation. Herring, for instance, will become very agitated if they are isolated from conspecifics. Fish schools have also been proposed to serve a reproductive function since they provide increased access to potential mates. Some scientists have provided disadvantages to mating in aggregations by using robotic male crabs; a female is at a higher risk approaching a cluster, has the ability of comparing males, increasing mate competition.
Protection from predators
Several anti-predator functions of animal aggregations have been proposed. One potential method by which fish schools or bird flocks may thwart predators is the 'predator confusion effect' proposed and demonstrated by Milinski and Heller (1978). This theory is based on the idea that it becomes difficult for predators to pick out individual prey from groups because the many moving targets create a sensory overload of the predator's visual channel. Milinski and Heller's findings have been corroborated both in experiment and computer simulations.
A second potential anti-predator effect of animal aggregations is the "many eyes" hypothesis. This theory states that as the size of the group increases, the task of scanning the environment for predators can be spread out over many individuals. Not only does this mass collaboration presumably provide a higher level of vigilance, it could also allow more time for individual feeding.
A third hypothesis for an anti-predatory effect of animal aggregation is the "encounter dilution" effect. Hamilton, for instance, proposed that the aggregation of animals was due to a "selfish" avoidance of a predator and was thus a form of cover-seeking. Another formulation of the theory was given by Turner and Pitcher and was viewed as a combination of detection and attack probabilities. In the detection component of the theory, it was suggested that potential prey might benefit by living together since a predator is less likely to chance upon a single group than a scattered distribution. In the attack component, it was thought that an attacking predator is less likely to eat a particular animal when a greater number of individuals are present. In sum, an individual has an advantage if it is in the larger of two groups, assuming that the probability of detection and attack does not increase disproportionately with the size of the group.
A fourth theory proposes that "predator-prey survival pressure" could directly drive the evolution of collective behavior, as a result of passive space extrusion and polarization induced by the presence of predators.
Enhanced foraging A third proposed benefit of animal groups is that of enhanced foraging. This ability was demonstrated by Pitcher and others in their study of foraging behavior in shoaling cyprinids. In this study, the time it took for groups of minnows and goldfish to find a patch of food was quantified. The number of fishes in the groups was varied, and a statistically significant decrease in the amount of time necessary for larger groups to find food was established. Further support for an enhanced foraging capability of schools is seen in the structure of schools of predatory fish. Partridge and others analyzed the school structure of Atlantic bluefin tuna from aerial photographs and found that the school assumed a parabolic shape, a fact that was suggestive of cooperative hunting in this species (Partridge et al., 1983).
Increased locomotion efficiency This theory states that groups of animals moving in a fluid environment may save energy when swimming or flying together, much in the way that bicyclists may draft one another in a peloton. Geese flying in a Vee formation are also thought to save energy by flying in the updraft of the wingtip vortex generated by the previous animal in the formation. Ducklings have also been shown to save energy by swimming in a line.Fish, F.E. Kinematics of ducklings swimming in formation: consequences of position. (1995) Journal of Experimental Zoology 273:1-11. Increased efficiencies in swimming in groups have also been proposed for schools of fish and Antarctic krill.
Another example can be seen in homing pigeons. When a homing pigeon is released with other individuals from its roost, these pigeon groups showed increased efficiency and decision making to shorten the distance of the route taken to return home, thus saving energy when flying between locations.Biro, D., Sumpter, D.J., Meade, J. and Guilford, T., 2006. From compromise to leadership in pigeon homing. Current Biology, 16(21), pp.2123-2128.
Costs of group living
Ectoparasitism and disease Animals that form colonies form a cost of living in groups. These colonies exhibit a system with close physical proximity and increased contact between individuals, thus increasing transmission of disease and ectoparasites; a universal hazard of animals living in groups.Alexander, R.D., 1974. The evolution of social behavior. Annual review of ecology and systematics, 5(1), pp.325-383.
For example, cliff swallows that are commonly parasitized by swallow bugs incur a cost when forming colonies, as these parasitic bugs increase the mortality rates of cliff swallow nestlings.Brown, C.R. and Brown, M.B., 1986. Ectoparasitism as a cost of coloniality in cliff swallows (Hirundo pyrrhonota). Ecology, 67(5), pp.1206-1218. A study shows that the number of swallow bugs found in cliff swallow nests increased with the increase of cliff swallow colony size, thus reducing overall success of these colonies.
Larger groups of animals tend to harbour an increased number of pathogens and are at a higher risk of epidemics.Loehle, C., 1995. Social barriers to pathogen transmission in wild animal populations. Ecology, 76(2), pp.326-335. This is particularly due to the large amount of waste material produced by larger groups, allowing for a favourable environment for pathogens to thrive.
Intraspecific competition Another cost to group living is the competition over food resources. As individuals group together, there is an increased nutritional requirement of the larger group compared to smaller groups. This causes an increased energetic cost as individuals now travel farther to visit resource patches.Chapman, C.A. and Valenta, K., 2015. Costs and benefits of group living are neither simple nor linear. Proceedings of the National Academy of Sciences, 112(48), pp.14751-14752.
An example of intraspecific competition can be seen within groups of whales and dolphins. Female bottle-nose dolphins with similar home ranges tend to have varied foraging habits in an effort to reduce and negate the intraspecific competition of resources.Rossman, S., Ostrom, P.H., Stolen, M., Barros, N.B., Gandhi, H., Stricker, C.A. and Wells, R.S., 2015. Individual specialization in the foraging habits of female bottlenose dolphins living in a trophically diverse and habitat rich estuary. Oecologia, 178(2), pp.415-425. Benefits of group living on defence from predators is very evident in nature, however in locations of high resource competition poses an effect on the mortality of certain individuals. This can be seen in species of shoaling fish, where the initial aggregation of individuals to a group initially allowed for the protection from predators, however the limiting resources available changes over time, and mortality rates of these fish begin to increase,Ford, J.R. and Swearer, S.E., 2013. Two's company, three's a crowd: Food and shelter limitation outweigh the benefits of group living in a shoaling fish. Ecology, 94(5), pp.1069-1077. showing that resource competition is an important regulator of reef fish groups after the initial benefits of refuge grouping and predatory protection.
Interesting contrasts to the benefit of increased group size on foraging efficiency can be seen in nature particularly due to intraspecific interactions. A study conducted on the Alaskan moose shows that with increasing group size, there is a decrease in foraging efficiency.Molvar, E.M. and Bowyer, R.T., 1994. Costs and benefits of group living in a recently social ungulate: the Alaskan moose. Journal of Mammalogy, 75(3), pp.621-630. This is result of increased social aggression in the groups, as the individuals of the group spent most of its time in alert-alarm postures, thus spending less time foraging and feeding, reducing its foraging efficiency.
Reproduction and development