Sociality

Herd of American bison at Genesee Park

Backgrounds

Sociality refers to how individuals within a population interact with social groups and form cooperative societies.

This behavior often arises as a response to evolutionary challenges. For instance, when a mother wasp stays near her larvae, it reduces the chance of parasites attacking them. Biologists believe that these types of behaviors in wasps from the Vespidae family have evolved due to pressures from parasites and predators.

A key feature of social behavior in animals is parental investment—any effort or resource (such as time or energy) a parent devotes to its offspring’s well-being. This investment comes at the expense of future reproductive opportunities or support for other relatives. Animals that care for their young without other social behaviors are classified as subsocial.

Animals exhibiting a high degree of sociality are known as social animals. The most advanced form of sociality, known as eusociality, is characterized by overlapping generations, division of reproductive roles, cooperative offspring care, and in some cases, a caste system within the species.

Types of Sociality

Subsociality

Subsocial behavior is commonly observed across various animal species. In subsocial groups, parents provide care to their offspring for a certain period. Even if this care is brief, the species is still categorized as subsocial. When adult animals interact with other adults, they are classified differently based on their social interactions. If a species' most social behavior involves occasional interaction or nesting with other adults, they are referred to as solitary but social.

Subsociality is prevalent among winged insects and has independently evolved multiple times. Insect groups with at least some subsocial species are highlighted in bold italics on a phylogenetic tree of the Neoptera (with many non-subsocial groups excluded).

Solitary but social

Solitary but social animals typically forage alone but may share sleeping locations or nests with others. In many species, female home ranges often overlap, while males tend to have distinct, non-overlapping territories. Male individuals usually avoid each other and are often driven out once they reach maturity. However, some species, like cassowaries, display the opposite behavior. Among primates, this social structure is particularly common in nocturnal strepsirrhines and tarsiers. Examples of solitary but social animals include mouse lemurs, lorises, and orangutans.

Certain cetaceans also exhibit solitary but social behavior, living separately from their species but engaging with humans. This has been observed in various species, including bottlenose dolphins, common dolphins, striped dolphins, belugas, Risso’s dolphins, and orcas.

Parasociality

Sociobiologists group communal, quasisocial, and semisocial animals into a larger category called parasocial. What these parasocial taxa have in common is their display of parental investment and their socialization within a single, cooperative habitat.

The distinctions between these groups lie in how they manage communal living and care for offspring. In communal groups, adults share a nest, but each individual takes care of their own young. In quasisocial species, not only do they cohabit, but they also share responsibilities for brood care, as seen in certain Hymenoptera, spiders, and other invertebrates. Semisocial animals have all the traits of communal and quasisocial groups, but they also introduce a caste system, where labor is divided based on reproductive ability.

Beyond parasociality is eusociality, often seen as the pinnacle of social organization in species like ants or honeybees. Eusocial societies feature overlapping adult generations that cohabit and collectively care for the young. As a result, multiple generations live together, and older generations actively help raise the newest offspring—a true example of cooperative care across time.

This framework of social organization can bring to mind the words of Margaret Mead: "Never doubt that a small group of thoughtful, committed citizens can change the world." Whether it's eusocial insects or parasocial animals, their cooperation ensures survival in ways that echo through the broader natural world.

Eusociality

Eusociality is the highest organizational level of sociality. Eusocial societies are characterized by overlapping adult generations, cooperative care of the young, and the division of reproductive roles. In species where individuals are born into specific physical castes that do not change throughout their lives, this represents the highest level of social organization. Eusociality has evolved across multiple insect orders. Common examples include Hymenoptera (ants, bees, wasps, and sawflies) and Blattodea (specifically Isoptera, termites). Eusocial behavior is also found in certain Coleoptera (like the beetle Austroplatypus incompertus), Hemiptera (such as Pemphigus spyrothecae), and Thysanoptera (thrips). Eusocial species without fixed physical castes are referred to as primitively eusocial.

In mammals, two potential examples of primitively eusocial species are the naked mole-rat (Heterocephalus glaber) and the Damaraland mole-rat (Fukomys damarensis). These diploid, highly inbred animals help raise their siblings, all of whom are born from a single reproductive queen. Typically inhabiting harsh environments, these mole-rats show occasional outbreeding and colony formation when resources allow, as suggested by a 2008 study by O'Riain and Faulkes.

Eusociality has also been observed in some crustaceans. For example, Synalpheus regalis, a type of snapping shrimp, demonstrates fortress defense behavior in tropical reefs and sponges. These shrimp live in colonies with one breeding female, who is protected by male defenders equipped with large snapping claws. The non-breeding members of the colony work together to defend their shared habitat, mirroring the communal structure seen in other eusocial species.