Biology
The Evolution of Eusociality in Naked Mole-Rats
Quick fact
Naked mole-rats are the only eusocial mammals—they live in colonies where only one female breeds, and most individuals spend their lives as non-reproductive workers, a system driven by extreme inbreeding and kin selection.
Why this is interesting
You've heard of eusocial insects like ants and bees, but what if a mammal—a rodent with a backbone—also lived in a colony with a single breeding queen and countless workers?
Read the full explanation
Understanding The Evolution of Eusociality in Naked Mole-Rats
Naked mole-rats (Heterocephalus glaber) live in large underground colonies in East Africa, typically numbering 20–300 individuals. Within each colony, a single queen is the only female that reproduces, mating with one to three males. All other colony members are workers that forage, dig tunnels, defend the nest, and care for the young. This system mirrors the eusociality seen in ants and bees, but with a crucial twist: it evolved in a mammal. How could such a system evolve? The key lies in relatedness. Naked mole-rats are highly inbred—they often mate with siblings and parents—so all colony members are genetically very similar. This means that when workers help the queen reproduce, they are still passing on a large fraction of their own genes through their close relatives. This is the logic of kin selection: helping relatives can be a better strategy than attempting to breed independently, especially when the environment makes independent survival risky. In the harsh, arid savanna where burrows are hard to dig and food is patchy, staying with the colony and helping relatives may offer better fitness returns than trying to disperse and start a new colony.
A deeper explanation
The evolution of eusociality in naked mole-rats is driven by a combination of ecological constraints and kin selection. The African savanna where they live is hot, dry, and unpredictable, making the energy cost of digging a new burrow system enormous. Dispersal is risky, and the chance of successfully founding a new colony is low. This 'ecological constraints hypothesis' suggests that when independent breeding is tough, individuals may gain more by staying in their natal group and helping rear relatives. In naked mole-rats, inbreeding further boosts relatedness: because they often mate with siblings, the relatedness among colony members can approach 0.8 or higher, compared to the 0.5 seen between parents and offspring in outbreeding species. This makes altruism easier to evolve because the indirect fitness benefits (getting your shared genes into the next generation through relatives) are very high. The queen also plays a role: she suppresses reproduction in other females through aggressive behaviors and pheromonal signals, ensuring that most colony members remain non-reproductive. This reproductive skew is maintained because subordinates get inclusive fitness benefits and also improve their personal chances of becoming the next queen if the current queen dies or is replaced. Interestingly, naked mole-rats are also ectothermic to some degree, which reduces individual metabolic costs, allowing them to allocate more resources to the colony. Their eusociality is a striking counterexample to the assumption that eusociality is confined to insects, showing that the same evolutionary logic—high relatedness, ecological constraints, and cooperative care—can produce similar social systems in mammals.