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The complex social system known as eusociality is marked by cooperative care of young, overlapping generations, and distinct castes, like workers and breeders. It is found in a variety of organisms such as shrimp, social insects like bees and ants, and mole-rats. Eusociality is a case of kin selection, in which individuals find that helping a relative may provide more overall fitness for themselves than being selfish. In a eusocial society, workers relinquish most, if not all, of their breeding rights to help raise another’s offspring, usually closely related to them, thereby gaining indirect fitness. The mole-rat is an excellent example of how eusociality evolves, as not all species of mole-rats are eusocial. This evolution is best explained by the Aridity Food Distribution Hypothesis which accounts for the environmental constrictions of predators and scarce resources that make helping close kin more beneficial for one’s own genes than risking death in the harsh conditions. Evolution of eusociality has developed distinct tiers of breeders, workers, and dispersers who are essential for reproducing, foraging, and maintaining some genetic drift. The mechanisms behind how these tiers are maintained are not entirely understood, but appear to be a combination of physical factors such as verterbrate length and hormone suppression of reproductive behavior in non-breeder tiers.

Author: Sheena Shah-Simpson


Mole-rats (family Bathyergidae) are a type of rodent found in Africa. They live underground in burrows made up of different chambers for their nests, latrines, food storage areas, alongside chambers for foraging, in which they dig with their incisors and foreclaws. They eat roots, tubers, and bulbs. Most species of mole-rat are solitary, coming together only for mating. A few species, though, form colonies where many mole-rats live together in large complex burrows they have dug because it makes it easier to gather food and reduces predation risks.

Of the species that live in colonies, eusociality has evolved independently at least twice, in Heterocephalus glaber , the naked mole-rat ( [link] ), and separately in Cryptomys damarensis , the Damaraland mole-rat (Allard and Honeycutt 1992, Jarvis and Bennett 1993, Walton et al. 2000, Faulkes et al. 2004). Colonies in both species have three morphologically distinct castes. The first caste is made up of the breeders, usually one female “queen” and her one to three mates, who breed and encourage the workers in their daily tasks. The second caste is formed by the workers, usually mole-rats who are highly related to the queen. These workers are reproductively suppressed by their own hormones that are secreted due to social cues from the queen, keeping them working instead of reproducing. They take care of the offspring, forage for food, and patrol the burrows. The final caste is the dispersal caste, made up of mole-rats of a slightly larger build who act as workers until they leave the colony to either found or join another. The dispersal caste is virtually the only form of gene flow in these mole-rat populations since the colonies are usually separated by a large distance that is dangerous for mole-rats to cross above ground.

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