Eusociality represents the most complex level of social organization in the animal kingdom. It is a system built on extreme cooperation, where some individuals reduce their own reproduction to help raise the offspring of others. This arrangement creates highly integrated societies that function as a cohesive unit, often described as a superorganism. The structure of these societies allows for remarkable achievements in building, foraging, and defense.
The Defining Characteristics of Eusociality
A species is classified as eusocial based on three criteria that shape its social structure. The first is a reproductive division of labor, which results in distinct castes. Within a colony, only one or a few individuals, such as a single queen, will reproduce. The rest of the colony members are non-reproductive and take on roles such as foraging for food, maintaining the nest, and defending against predators.
The second defining trait is cooperative brood care, where colony members collectively care for the young. The responsibility of raising offspring is not limited to the mother; instead, the entire community, including older siblings and other non-reproductive adults, participates in feeding, cleaning, and protecting the developing larvae. This communal effort ensures that the queen’s offspring receive constant care and have a higher chance of survival.
Finally, eusocial societies are characterized by overlapping generations of adults living together. This means that offspring grow up alongside their parents and other older relatives. This generational overlap allows maturing individuals to contribute to the colony’s workforce, helping their parents raise the next wave of siblings. The continuous presence of multiple generations ensures that knowledge and labor are passed down, maintaining the stability and growth of the colony.
Eusocial Species in the Animal Kingdom
The most recognized examples of eusociality come from the insect order Hymenoptera, which includes all ants and many species of bees and wasps. In a honeybee hive, for instance, a single queen lays all the eggs while sterile female workers perform every other task. These tasks range from foraging for nectar to feeding the larvae and defending the hive.
While often associated with Hymenoptera, eusociality also evolved independently in other insects, like termites. Unlike bees and ants, termite colonies are founded by a king and a queen who both participate in reproduction. The colony contains distinct castes of sterile workers and soldiers that care for the young, forage for cellulose, and defend their nests. The presence of both male and female workers distinguishes their social structure from the female-dominated societies of Hymenoptera.
Beyond insects, eusociality has been observed in other animals. The naked mole-rat of East Africa is a well-studied eusocial mammal that lives in large underground colonies. These are led by a single reproductive queen, with other individuals acting as workers who dig tunnels and care for her pups. Eusociality also appears in marine environments, with certain snapping shrimp species living in sponge colonies where a single queen reproduces and larger individuals act as soldiers.
The Evolutionary Origins of Eusociality
The evolution of a behavior where individuals sacrifice their own reproduction presents a puzzle for natural selection. The leading explanation is kin selection, which suggests that individuals can indirectly pass on their genes by helping close relatives reproduce. An organism shares genes with its siblings, so ensuring their survival is another way to perpetuate its own genetic legacy. This is effective when the benefit of raising many related individuals outweighs the cost of forgoing personal reproduction.
This genetic incentive is amplified in the Hymenoptera (ants, bees, and wasps) due to a genetic system called haplodiploidy. In this system, males develop from unfertilized eggs and are haploid (one set of chromosomes), while females develop from fertilized eggs and are diploid (two sets of chromosomes). A consequence is that female workers are more closely related to their sisters (sharing, on average, 75% of their genes) than they would be to their own offspring (50%). This high degree of relatedness provides a genetic motivation for a female worker to help her mother, the queen, produce more sisters.
While haplodiploidy is a factor in Hymenoptera, it doesn’t account for eusociality in termites, naked mole-rats, or snapping shrimp, as they do not possess this genetic system. In these species, other ecological factors are believed to have guided the evolution of this social structure. One theory is fortress defense, where a valuable and defensible resource, like a complex nest, makes it beneficial for individuals to stay and defend it rather than dispersing. This communal defense, combined with high relatedness from inbreeding, likely created the conditions for eusociality to emerge.