Living in groups has costs and benefits

Animals are social for a variety of reasons. In some cases, offspring remain with their parents to form family groups and do not disperse. In other cases, individuals are attracted to each other for breeding. Individuals can also aggregate because they are independently attracted to the same habitat or resource. For instance, vultures gather around a carcass and dung flies congregate on cow pies. In this section, we will examine the costs and benefits of living in social groups and then discuss how animals use territories and dominance hierarchies in social interactions.

Benefits of Living in Groups

Animals generally form groups to increase their survival, rate of feeding, or success in finding mates.

Survival

While an individual might not be able to fend off the attack of a predator alone, a group of individuals can be quite effective at doing so (Figure 10.1). Another survival mechanism available to social groups is a phenomenon known as the dilution effect. The dilution effect refers to the reduced, or diluted, probability of predation to a single animal when it is in a group. In an aggregation of prey, the predator has many prey choices, so the individual living in a group has a lower probability of being caught. The dilution effect is an important benefit of large groups such as herds of mammals, flocks of birds, and schools of fish.

Figure 10.1 Group defense Adult muskox (Ovibos moschatus), like these from Victoria Island, Canada, form an outward-facing circle and place the calves inside the circle where they are safe from approaching predators.

Photo by Eric Pierre/NHPA/Science Source.

A lower probability of predation in groups also allows individual prey animals to spend less time watching for predators. Consider the case of the European goldfinch (Carduelis carduelis), a small bird that feeds on the seed heads of plants in open fields and hedgerows. If you watch closely as the birds feed, you will notice that they raise their heads and look around for predators. The total number of head raises conducted by the group increases with flock size; as you can see in Figure 10.2a, the larger the group, the more eyes are on the lookout for predators. As flock size increases, however, each individual can raise its head less frequently, as shown in Figure 10.2b. Because each individual spends less time looking for predators, it can spend more time feeding. The data in Figure 10.2c show that when a goldfinch spends less time looking for predators, it can husk a seed much faster and therefore consume seeds more quickly.

Figure 10.2 Increased vigilance when living in a group In the European goldfinch, an increase in flock size results in (a) an increase in the total number of head raises performed by the flock, (b) a decrease in the number of head raises performed by an individual, and (c) a decrease in the time required to husk a seed.

Data from E. Glück, Benefits and costs of social foraging and optimal flock size in goldfinches (Carduelis carduelis), Ethology 74 (1987): 65–79.

Feeding

Living in groups can also help animals locate and consume resources. Having many conspecifics all searching for food means that there are many more sets of eyes that might find food when it is rare. In some cases, animals may find food easily but have difficulty in capturing and killing it when they are alone. In lions, for example, a lone female has a low probability of capturing and killing a zebra, but if she hunts with many other lions, the probability of a capture goes up dramatically.

Mating

Socializing can also provide mating benefits since being social makes it easier to locate potential mates. An extreme example of socializing for mating benefits occurs when animals aggregate in large groups to attract members of the opposite sex by making calls or displaying in ways that capture the attention of potential mates. The location of the aggregation, known as a lek, is used only for displaying. The site has no other value either to the displaying sex or to the attracted sex. For example, males of the ruff (Philomachus pugnax)—a medium-sized wading bird that lives in northern Europe and Asia—come together at a lek and participate in mating displays to attract females. On the island of Gotland in Sweden, researchers observed ruff leks to determine whether lek size affected ruff mating. As you can see in Figure 10.3a, males in larger leks were more successful at attracting females. In addition, as illustrated in Figure 10.3b, males in larger leks experienced a higher percentage of successful copulations with females, which confirms that forming social groups provides fitness benefits to the male birds.

Figure 10.3 Breeding benefits in a lek . Among displaying ruff males, those displaying to females in larger groups (a) are more likely to attract females and (b) have a higher probability of successfully copulating.

Data from J. Högland, R. Montgomerie, and F. Widemo, Costs and consequences of variation in the size of ruff leks, Behavioral Ecology and Sociobiology 32 (1993): 31–39.

Costs of Living in Groups

The benefits of group living can certainly be substantial for many species, but group living can also come with costs that include predation and competition.

Predation

Groups of animals are much more conspicuous to predators than are individual animals. In a grassland, for example, it is easier for a predator to spot a herd of antelopes than an individual antelope. Given the propensity of antelopes to live in herds, this cost of being detected is outweighed by the benefits of the dilution effect and of more eyes to detect approaching predators.

The risk of parasites and pathogens can also increase when living with conspecifics. Many species of parasites and pathogens spread from one host to another. High population density can increase the rate that disease spreads and can lead to epidemics. For instance, coral reefs that experience fishing pressure generally have fewer fish compared to reefs that are protected from fishing. In 2008, researchers reported the results of a study on fish parasites from a protected and unprotected coral reef in the central Pacific Ocean. As shown in Figure 10.4, they found that fish on the protected reef were infested with a higher number of parasite species than the same species living in a coral reef that was fished. In addition, fish living in the protected reef frequently carried higher numbers of each species of parasite.

Figure 10.4 Parasite occurrence in coral reef fishes Coral reefs protected from fishing have higher fish densities. A survey of five different species of fish found that fish living in a reef that is subject to fishing pressure contained fewer species of parasites than the same species living in a reef that is protected from fishing. Error bars are 95% confidence intervals.

Data from K. D. Lafferty, J. C. Shaw, and A. M. Kuris, Reef fishes have higher parasite richness at unfished Palmyra Atoll compared to fished Kiritmati Island, EcoHealth 5 (2008): 338–345.

The parasite and disease costs of group living can also be readily observed in modern aquaculture operations, which farm aquatic species for human consumption. These operations raise oysters, salmon, catfish, shrimp, and other edible species at very high densities. Under such conditions, a single infected individual can rapidly spread parasites and pathogens to the rest of the group.

Increased transmission of parasites and diseases to groups that live in higher densities makes it undesirable for people to feed wild animals such as deer. When there is a steady, easily available source of food, deer form large aggregations around the food. This aggregating behavior makes it more likely that the animals will experience outbreaks of parasites compared to when they live in smaller family groups. Similar concerns exist for livestock operations in which animals are raised under very high densities. In this situation, diseases can jump to native wildlife populations and have dramatic effects with such diseases as rinderpest, avian influenza, and West Nile virus.

Competition

Another major cost of living in groups is competition for food. While larger groups are better at locating food, the food must then be shared among all of the individuals in the group. Returning to the example of the European goldfinch, which experiences benefits from living in large groups, Figure 10.5 shows one consequence of sharing the food. Larger flocks consume the seeds in an area much faster than small flocks, so larger flocks have to spend more time flying between patches of seeds. This causes each bird to spend more time and energy looking for food.

Figure 10.5 Large groups face increased competition for food In the European goldfinch, larger flocks run out of food faster and must spend extra time and energy flying to find new patches of food.

Data from E. Glück, Benefits and costs of social foraging and optimal flock size in goldfinches (Carduelis carduelis), Ethology 74 (1987): 65–79.

Each species that has evolved to live in groups faces different costs and benefits, which depend on the ecological conditions under which it lives. Assuming a genetic component for such social behavior, we expect natural selection to favor the evolution of group sizes that balance the costs and benefits for each species.

Territories

Many species of animals have evolved to manage living near other conspecifics by establishing a territory or a dominance hierarchy. A territory is any area defended by one or more individuals against the intrusion of others. Territories can be either transient or relatively permanent, depending on the stability of the resources in the territory and how long an individual needs those resources. For example, many migratory species establish summer breeding territories and defend them for several months. Defending a high-quality territory generally assures greater resources, such as abundant food or nest sites. This typically improves the attractiveness of a territory holder as a mate, and therefore its fitness. When breeding is complete for the season, migratory species move on to their wintering grounds where they establish new territories. Shorebirds that stop at several points along the way of their long migration defend feeding areas for a few hours or days and then continue their migratory trip. Hummingbirds and other nectar feeders defend individual flowering bushes and abandon them when those bushes cease producing flowers. As long as a resource can be defended and the benefits of defending the resource outweigh the costs, animals are likely to maintain territories.

Dominance Hierarchies

In some situations, defending a territory is impractical. This can occur when an individual is surrounded by so many conspecifics that it becomes impractical to defend against them all, when resources are available for only short periods of time, or when the benefits of living in a group override the benefits of defending a territory. In such circumstances, individuals of many species form dominance hierarchies. A dominance hierarchy is a social ranking among individuals in a group, typically determined through fighting or other contests of strength or skill. Once individuals order themselves into a dominance hierarchy, subsequent contests among them are resolved quickly in favor of higher-ranking individuals. In a linear dominance hierarchy, the first-ranked member dominates all others, the second-ranked dominates all but the first-ranked, and so on down the line to the last-ranked individual, who dominates no one else in the group.