Group living
In ethology and evolutionary biology, group living is defined as individuals of the same species, 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.
Important aspects of group living include the frequency and type of social interactions between individuals of a group, the group size, and the organization of group members in the group.
Terminology of animal groups also varies among different taxonomic groups. Groups of sheep are termed herds, whilst groups of birds are referred to as colonies, or flocks.
Most studies on group living focus strictly on groups comprising a single species. However, many mixed-species groups commonly occur in nature. Examples of mixed-species groups include wildebeests forming groups with zebras, and different species of birds that form large foraging flocks.
Group living may sometimes be confused with collective animal behavior. Collective animal behavior is the study of how the interactions between individuals of a group give rise to group level patterns and how these patterns have evolved. Examples include the marching of locusts and flocks of migrating birds. Group living however focuses on the long-term social interactions between individuals of a group and how animals have evolved from solitary living.
Definition of group living
It is extremely difficult to distinguish between solitary living and group living. Distinctions between the two are relatively artificial. This is because many species of animals who spend a majority of their life alone, at some point in their life, will join a group or engage in social behavior. Some examples of this happens during mating, parental care of their offspring, or even aggregations of conspecifics to an area to exploit resources of food or shelter. Therefore, multiple definitions of group living have been proposed. Differences in group living definitions vary dependent on the frequency and type of social interactions that members of a group display and the level of coordination and cohesion of group members. For example, Wilson defines a group as “any set of organisms, belonging to the same species, that remain together for a period of time while interacting with one another to a distinctly greater degree than with other conspecific organisms." This definition cannot be applied to situations such as moths drawn to a lamp, or when animals aggregate around a watering hole, as they are not exampling of a social aggregation. Most definitions however agree that a fundamental characteristic of group living is that individuals need to show spatial proximity over time to be considered a group. Therefore, the working definition of group living is where two or more individuals display a degree of spatial proximity over time, emphasizing the importance of mechanisms of social attraction to maintain these groups.Evolution of group living
There have been multiple different hypotheses proposed to explain how group living evolved in animals. Research shows that grouping habits may differ between individuals, and this tendency to group can be inherited. Research also shows that grouping tendency depends heavily on the interaction of many genes, as well as experiences gained by an individual and the environmental conditions surrounding the individual. Other studies argue that the main driving force of the evolution of social grouping is phylogenetic inertia alongside ecological pressure. However, it is still unclear how exactly animals have evolved from the ancestral state of solitary life.Benefits of group living
Information access and transfer
A key advantage to group living is the ability for individuals in a group to access information gained by other group members. This ability to share information can benefit many aspects of a group’s success, such as increased foraging efficiency and increased defenses against predators.Foraging efficiency
An advantage of information access from group living is increased foraging efficiency. When individuals form a group, they can more effectively locate high quality resources in their environment. Foraging efficiency can be increased by the sheer area of space individuals occupy as well as a greater number of individuals searching for food. Once a high-quality resource is found, the individuals may produce signals or cues that guides other members of the group to the location of the resource. The cues and signals produced thus helps individuals of a group discriminate between low- and high-quality resources. An example of this information transfer to benefit foraging efficiency can be seen in honey-bee colonies, in which waggle dances performed by honey-bees share information on where these dancing bees foraged nectar. The waggle dance thus guides other bees to the location of highly productive flowers. In some species, for instance the forest tent caterpillar, foraging behaviors change depending on food source. On less favorable food sources, caterpillar groups tend to splinter, thereby potentially increasing the risk for predation, but increasing the potential of finding a more favorable food source.Increased defense from predators
Another advantage of living in a group is seen in many prey species in their ability to increase defenses against predatory animals.A way that a group may increase its defenses against predators is through the ‘many-eyes effect’. This effect states that larger groups of animals are better at detecting predators compared to smaller groups. This allows the individuals within a group to more effectively identify predators, allowing these individuals to flee or adopt postures to alert the predators that their presence is known. An example can be seen in a study conducted by Siegfried and Underhill on laughing doves, in which large groups react to a mock-predator much more rapidly than smaller groups.
Another way in which a group may have decreased risk of predation is through the dilution effect. The dilution effect shows the idea that an individual in a large group will have a reduced risk of predation compared to an individual in a small group or a solitary individual. Hence the risk is ‘diluted’ among the other members in a group. It is important to note however; this effect only occurs where predators are unable to capture all individuals in a group. For example, a flock of birds preying on a large group of caterpillars will not have any dilution effect, as these birds can rapidly consume all caterpillars at once. All individuals in a large group however, may not benefit from the dilution effect, and thus the selfish herd theory was developed. The selfish herd theory states that individuals in the periphery of a group is more likely to be preyed upon than those in the center of the group
Breeding
It is hypothesized that reproductive success of a female is determined by the number of eggs she can produce, while reproductive success of a male is determined by the number of females he mates with. Furthermore, according to Bateman's principle, it is expected that females of a population will select mates that result in the best quality offspring, while males compete among each other to mate with a female.Group living provides the presence of social information within the group, allowing both male and female members to find and select potential mating partners. Alongside this, living in a group allows for higher reproductive success as individuals have access to a greater number of potential mates, and the possibility to choose between them. Therefore, individuals living in groups have a higher chance of finding a mate and successfully reproducing, on the basis that larger groups present a greater number of accessible mates nearby.