Herds

Herds are associations of animals, usually the same species, that keep, feed, or move together. Generally, the word “herd” is reserved for large, terrestrial animals. Animal herds serve several purposes. A herd offers protection against predators as members search for food. When food is spaced irregularly into patches and cannot be easily defended, individuals of a particular species will form a herd as they gather to feed on the same food resource in the same location. Large grazing mammals are notable for forming the largest and most dense herds. The most familiar include African ungulates (wildebeests, zebra, gazelle, wild horses, rhinos, hippopotami). Subungulates (elephants, sirenians, the extinct mastodons) also form herds. In North America, herds are comprised of bison, buffalo, caribou, elk, moose, and deer. In Australia, kangaroo herds are common. In Eurasia, typical herds contain deer, elk, moose, antelope, wild horses, sheep, goats, pigs, boars, and peccaries. In addition to large mammalian herbivores, small grazers that exploit vegetation in herds include marine and freshwater snails, tortoises and turtles, geese, and hyrax. Herds also provide camouflage, as individuals may be harder to identify when formed into a large group. Herding also allows animals a higher degree of safety as they forage for food. It enables them to focus on finding and eating sustenance yet be alerted if a predator’s presence is detected.

Grazers that feed indiscriminately tend to form herds that vary in density with season and foliage density. Furthermore, these herds vary in numbers and vegetation utilization patterns depending on the type of vegetation exploited and its productivity potential. In seasons or areas where rainfall is common, herds tend to congregate together and exploit small patches of grass, which they clip into grazing lawns. Maintaining grazing lawns increases the productivity of the plants via increased nitrogen content and improved digestibility in freshwater, marine, and terrestrial environments. When rainfall is scarce, the herds disperse over vast areas, which allows some grasses to grow into tall meadows. The benefits to individuals in a herd from grazing a larger area and creating a grazed lawn are greater than those to a lone individual who can only graze a small patch of grass. In this situation, there would be a greater risk of predation than if many others surrounded the individual. Furthermore, if the lone individual were subject to a greater risk of predation, there would be less likelihood of it being able to return to the same patch repeatedly to keep it as a grazed lawn. Thus, by grouping, an individual benefits multiple times from the gain in nutrition from a grazed lawn and again from the dilution effect, where the risk of predation is diluted by the number of members forming the group.

Herding to Avoid Predation

Large herbivores that most commonly form herds are clearly visible to predators. The risk of predation is diminished when these animals form a herd, which is again known as the dilution effect. The probability of a particular individual being killed or injured is reduced by the presence of other group members that might be attacked first. In other words, there is safety in numbers. Individuals in a group may also benefit by putting other animals between themselves and the predator. This is known as “the selfish herd effect.” Grouping thus provides the opportunity to decrease the area of danger for each individual. If individuals within the group are acting in a selfish herd manner, the groups formed tend to be tightly clumped, as all individuals attempt to put other individuals on all sides around them and move into a central location. The formation of a herd can make it more difficult for a predator to select one specific individual from the crowd for attack. Once an attack is initiated and herd members scatter during flight, these large numbers can make it difficult for the predator to decide which to chase.

Herd members also benefit from reducing the time necessary to scan for predators while foraging. An individual foraging has to split their time between consuming as much food as possible and avoiding becoming another animal’s meal. Thus, a solitary individual must be much more vigilant than a group member who can rely on many other eyes and, therefore, reduce its scan rate. Because of this advantage and the reduction of the per capita predation rate, even herds that simply congregate around a resource greatly benefit.

Migratory and Social Herds

Many large herbivores migrate in response to food availability in different seasons. Red deer, caribou, wildebeest, mountain goats, northern fur seals, and humpback whales are examples of animals that all migrate in response to seasonal changes in rainfall or food abundance. Some of these migrations are over incredible distances: Wildebeests travel about six hundred miles; northern fur seals and humpbacks can travel three to four thousand miles. During these migrations, small herds unite with bigger herds to form even larger herds. While terrestrial herbivores migrate to follow food, some marine mammals, such as gray whales and humpbacks, migrate south to calf and breed (but do not feed, living off food reserves instead) and migrate north to abundant feeding grounds.

Some herding species display social organization beyond that expected by mere association. This social organization tends to break a herd into smaller units (matrilineal groups, harems, or small territories controlled by one or several males) that are clustered within the entire herd. These species include horses, zebras, pronghorn sheep, walruses, sea lions, seals, and elephants. Horse and zebra herds comprise several small groups of females and their foals. A single stallion oversees these individual groups; young males leave these groups to form bachelor herds. Group members distinguish each other via a “corporate smell.” Stallions generally control a group of females and will fight with challenging stallions in elaborate rearing displays. If a stallion is challenged and loses after inseminating the females in his group, the challenger will mount the females and rape them to induce them to abort. They will then come into estrus quickly, and the new stallion will remate them. Stallions will groom females to cement their relationship with them; likewise, mares will groom their foals for the same purpose. Male impalas and gazelle maintain harems during the breeding season; male wildebeest do the same, but if the herd is large, the defense of the harem may be accomplished by several males rather than one. Male elk are divided into four main categories: Primary bulls are the first to establish harems, but they can become exhausted from the challenges of herding and mating with females. They may be unable to eat during these activities. At this point, the harems can be taken over by secondary bulls. Once the secondary bulls become exhausted, tertiary bulls take over. The fourth category, opportunistic bulls, only mate with females by chance.

Pinniped herds (sea lions, seals, and walruses) form breeding herds, where males establish territories. In some groups, females are herded together in harems to remain in the male’s territory; in other groups, females are free to move from territory to territory. Males vigorously defend their territories against intruding males.

Sirenians, such as the dugong and manatee, have proved challenging to study, but dugongs often form large herds for unknown reasons. During mating, a series of males follow a receptive female to form a cluster of up to twenty animals. Males then initiate fighting to determine who will mount the female. After mating, the main social unit is the female and her calf. A similar situation exists in the manatee, with females and their calves being the main social unit and females in estrus becoming the focus of a mating herd of males.

Herding serves important purposes for animals: utilization of a common resource (usually food) and predator reduction via a safety-in-numbers principle. Herds can also be formed during migrations or for breeding purposes. While many herds are simply loose associations of animals, some are highly organized into several social units, such as harems or matrilineal groups. Casual socialization, the bonds of community, and the opportunity for thermoregulation in cold climates are reasons animals form herds. Many animals that one would not normally associate with herds exhibit herding behavior, including flamingos, penguins, and schools of fish.

Principal Terms

carnivores: animals that eat the flesh of other animals

dilution effects: the reduction in per capita probability of death from a predator due to the presence of other group members

herbivores: animals that eat plants and show specializations of teeth and digestive tracts to do so

phalanges: the free toes of the foot; some can be modified to bear claws, hoofs, or nails

subungulates: non-hoofed mammals that support their weight on more than the terminal phalanges; some, such as elephants and hyraxes, have pads under their metatarsals, and others, such as the sirenians, have forelimbs modified into flippers

ungulates: hoofed mammals that support their weight only on the hoof-clad terminal phalanges and have teeth specialized for clipping vegetation

Bibliography

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