Eusociality

Eusociality is a collective social system in which some individuals do not reproduce so they can devote themselves to working for the group. Eusocial colonies are often described as superorganisms that comprise groups called castes that specialize in necessary behaviors. Many insects that live in colonies—including some bees, wasps, and almost all ant species—are examples of eusociality. One female, the queen, produces all offspring, and the others in the colony perform tasks such as caring for the queen and her offspring, gathering food, and defending the colony. Eusociality has been found primarily in insects and some crustaceans. A few mammals, notably the Damaraland and naked mole rats, are also eusocial.

Background

The dominant theory of the evolution of eusociality has been kin selection theory, which holds that animals exhibit self-sacrificial actions that benefit their relatives’ genetic fitness. The theory was first proposed in 1963 by W. D. Hamilton, but the term was coined the following year by Maynard Smith. These British evolutionary biologists were considering factors in the evolution of social traits such as altruism and cooperation.

Entomologist Suzanne Batra first used the term eusocial in 1966 when she was studying Halictine bees, commonly called “sweat bees.” She noted that the female founder of a nest would work with her daughters to maintain the colony. This intergenerational interaction is known as subsociality. Batra described their behavior and social organization as eusocial. Several years later, biologist E. O. Wilson expanded the term’s meaning and applied it to other social insects.

Researchers have proposed multiple theories as to how and why eusociality evolved and why certain species are more likely than others to develop this system. Subsocial behavior often precedes eusociality, which has led scholars to ask what conditions lead a daughter to help raise the next brood of her siblings instead of reproducing herself. Researchers have explored behavioral, demographic, ecological, morphological, and other traits in pursuing answers but have not reached a consensus.

Overview

The four characteristics of eusocial species are that adults live in groups, adults cooperatively care for offspring that are not their own, generations overlap, and only specific individuals reproduce. In some species, the reproductive and nonreproductive individuals are physically the same. In others, they may differ anatomically (for example, workers may be sterile). Some colonies, such as termites, have soldier and worker castes. These are insects that have matured to a point before the reproductive stage but have ceased to develop further. This period of suspended development is known as diapause. The castes are physically distinct in other ways, such as that soldiers have larger, stronger jaws. However, termites in diapause remain pluripotent, meaning they have the ability to develop into any caste to meet the colony’s needs. A worker, for example, could become a soldier if the colony needs more soldiers.

A honeybee colony is a typical example of eusociality and castes. The queen is sexually developed and has one primary function: reproduction. Queens produce up to three thousand eggs a day and emit pheromones that keep the colony together. Drones are the largest bees in the colony after the queen. Their only function is to fertilize the queen, after which they usually die. Worker bees care for the hive and offspring and forage for food. They often begin caring for broods as nurses when they are young and later go on to forage. A hallmark of eusociality is evident in honeybee colonies: each individual relies on others to perform their duties; otherwise, the colony will die. In some species, individuals commonly sacrifice themselves for the good of the colony. Honey ant colony workers, for example, sometimes fill their bodies with liquid food, then hang upside down, serving as food storage for the colony.

Eusociality varies among species. Some display a primitive form of eusociality in which workers are not sterile. In some species, a queen is the primary reproducer, while other females produce small broods. Primitively, eusocial colonies tend to be small and have short lives. Superorganismal (advanced eusocial) species’ colonies, in which workers are sterile, are often large, long-lasting, and complex. Some scholars believe that worker sterility is the deciding factor in whether a species has achieved a high level of eusociality. Instead of having an interest in reproducing, sterile workers’ interests are in ensuring the survival of the colony.

The few eusocial mammal species are primitively eusocial. Colonies of naked mole rats contain 70 to 295 members. One queen breeds with a small group of males. Most of the other members of the colony serve as workers, caring for offspring, digging and cleaning tunnels and chambers, defending the burrow system, and collecting food. Although they are fertile, the workers do not reproduce while they are members of the colony unless they replace the previous queen. Damaraland mole rats live in colonies of up to twenty-five members. One large female and one or two large males make up the breeding group. Eusociality allows Damaraland mole rats to survive in environments where precipitation is rare—sometimes several years pass between rains—and many members must work together to find enough food.

Eusociality is related to the fecundity-longevity trade-off, which is the idea that an organism must sacrifice its survival to increase its reproductive activity. However, eusociality provides notable advantages. Queens of superorganismal species may live decades instead of weeks or months while being enormously fertile and passing along their genes. As is evident in naked mole rat and Damaraland mole rat colonies, having many members supporting the reproducing members enables the group to overcome adversity that individuals could not survive alone.

Bibliography

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