How Do Naked Mole-Rats Choose a Queen?
Five years into her reign, the naked mole-rat colony at the Smithsonian’s National Zoo and Conservation Biology Institute lost its 11-year-old queen Nov. 30. The cause of death? Another naked mole-rat.
The sudden, violent demise of the queen is not uncommon for naked mole-rats, both in the wild and in human care. Naked mole-rats are eusocial—with a single dominant female presiding over an underground colony, these wrinkled rodents have a social structure more in common with ants and termites than other mammals.
So what happens after the queen dies? Get the answer from Kenton Kerns, assistant curator of the Small Mammal House.
What makes a female naked mole-rat a ‘queen’?
The queen calls the shots for the colony. She is the only female that breeds, while the majority of the group (both males and females) spend their entire lives working for the colony. We believe the queen suppresses the ability of the other females to mate.
Physically, the queen is larger than the rest of the colony members. With each pregnancy, the queen’s intervertebral disc space (the space between her vertebrae) grows, which causes her body to lengthen over time. This means the queen gets larger with each pregnancy and is capable of holding more babies while pregnant. A new queen will typically give birth to 10 to 15 babies, but a large queen can give birth to as many as 30 individuals in a single litter.
Dominant females use their large relative size to fend off potential challengers, but sometimes, another challenging female will inflict fatal damage on the queen.
How do naked mole-rats fight?
With their sharp teeth and powerful jaws. A naked mole-rat can move each of its large incisor teeth separately—like a pair of chopsticks. Nearly one-quarter of their muscle mass is contained in their jaws. Most of the time, they use their teeth to dig their underground tunnels (their hairy mouths prevent them from swallowing dirt). But, they can also use them to fight and injure their opponents—sometimes with fatal consequences.
What happens when a queen naked mole rat dies?
If a queen dies—or is dethroned, which is what happened this fall at the Small Mammal House—several of the group’s females will fight and challenge each other for the throne.
Any female can technically become a queen. But from what we’ve seen, the new queen tends to be whichever female mole-rat is the best at defending herself and the best at killing other potential queens who challenge her. It also seems the position of queen might carry some degree of political sway among the rest of the colony, since we've seen other mole-rats fighting who are not even challengers for the top position.
We don’t know exactly what causes a mole-rat power struggle to begin. Our best guess is that challengers will kill the queen when they think they can do a better job of ruling the colony.
What happened to the old queen?
The 11-year-old queen who died in November had been the dominant female of our 58-member colony since claiming the throne in December 2018, when she had her first litter. She had been attacked three times this fall by unknown individuals. A third attack in November left her grievously injured, and after a consultation with the veterinary team, animal care staff humanely euthanized her.
About the Zoo’s Naked Mole Rat Cam... what’s happening on camera with the colony right now?
Naked-mole-rat colonies are supposed to have a queen, so the fighting in the colony is mole-rats working out who should become the next queen.
You might see individuals with bite wounds on camera, or wounds that are scabbed and healing. But like other animals, naked mole-rats also play-fight to socialize, so not every fight you might see is a violent struggle for the throne.
If one of the successors gets rejected, how does a new challenger step in?
Because mole-rats look so similar to each other, it can be difficult to determine which individuals are doing the most fighting. When we notice another mole-rat has been killed, we think a claimant to the throne has challenged or defended herself.
It’s hard to really know what happened in every circumstance, since the attacks happen so quickly that they’re hard to observe.
If only one female can reproduce, isn’t the colony at risk for inbreeding?
Certain species of animals, like rodents, have a higher genetic tolerance for inbreeding. We believe mole-rats have a high tolerance for inbreeding, which results in a high degree of genetic similarity among colony members, both in the wild and in human care. All of the members of our colony are closely related.
Do keepers ever intervene in this process?
It can be emotionally difficult to watch the animals we care for fight, but we try to let them behave as naturally as possible. As keepers, we closely monitor the colony, and if we notice an individual is badly wounded to the point where the wound might not heal on its own, we’ll alert our veterinary team about staging an intervention, or in the worst cases, humane euthanasia.
Of course, we would never intervene to select a queen. We still don’t really know how mole-rats choose their queen, so we try to be hands-off so they can choose a new queen that the whole colony will support. We just hope they choose a new one soon.
Why are naked mole rats like this?
Naked mole-rats have evolved with this social structure in place because it’s an ecologically efficient way to live in a complex underground society. When large populations live and work in close proximity, having a single member calling the shots can guarantee both cooperation and an even division of labor among the majority of individuals. It seems weird to us because they’re mammals, but eusocial systems are common in the animal kingdom—thousands of species of insects operate with the same social structures. But we don’t judge insects in the same way because they’re not mammals like we are.