Naked mole rats, with their underground societies made up of a single breeding pair and an army of workers, look like mammals that make every effort to live like insects. Nearly 300 of the bald, wheat-toothed, and almost blind rodents can traverse the maze of tunnels in a colony.
New research suggests that there is gross power in these numbers: like ants or termites, mole rats go into battle with rival colonies to conquer their lands.
Wild naked mole rats (Heterocephalus glaber) will invade nearby colonies to expand their territory, sometimes hijacking pups to incorporate them into their own ranks, researchers reported Sept. 28 in the Journal of Zoology. This behavior may put smaller and less cohesive colonies at a disadvantage, potentially supporting the evolution of larger colonies.
Researchers came across this phenomenon by accident while monitoring naked mole rat colonies in Kenya’s Meru National Park. The team studied the social structure of this extreme group form living among mammals (SN: 20/06/06).
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For more than a decade, the team has caught and tagged thousands of rats moles from dozens of colonies by implanting small radio frequency transponder chips under the skin or cutting off toes. One day in 1994, while marking mole rats in a new colony, researchers were surprised to find in their tunnels mole rats from a neighboring colony that had already been marked. The queen of the new colony had wounds on her face from the ravages of battle. There seemed to be a war on the ground.
“Naked mole rats are better known for their cooperation within colonies than for competition between colonies,” notes Stan Braude, a biologist at the University of Washington in St. Louis. But throughout the long-term study, Braude and colleagues found that 26 colonies expanded their tunnels by digging into burrow systems occupied by neighboring colonies. In half of these cases, the invaded colony fled to another arm of its tunnel system as the invaders expanded their territory. In the other half of the cases, the invaded colony was completely displaced and the original mole rats were never found there again. In four raids, researchers captured invasive mole rats on the spot, and in three of them, the largest colony made the invasion.
Genetic analysis, which was not available during the original investigation, later confirmed that during the 1994 invasion, the assailants not only evicted the vanquished. They also threw a cochola at least at two young men. The young grew up to become workers in the society of their captors.
This lust for conquest had already been seen in the species, but only in captive colonies. Confirmation that these conflicts occur naturally in the wild means that they can influence the evolution of social life crowded with mole-rats, Braude says. The total evacuation of smaller colonies by larger ones presents a factor that was not previously considered to make it "important for this species to live in a group as large as possible."
None of these “crazy beasts” already surprise evolutionary biologist Chris Faulkes. “The burrow is a very valuable resource because it’s very expensive – in terms of energy – to dig and build,” says Faulkes, of Queen Mary University in London. It makes sense for mole rats not only to defend it, but to try to snatch this resource from others.
Since group size is so important for naked mole rats, Faulkes says it’s interesting that workers from the various colonies don’t get together after an invasion. Only pups are added to the invading colony.
“The number of these kidnapped puppies is really small and it is possible that these kidnapping incidents are not as frequent,” he says. "Given this, I'm not sure how much it really contributes to building large numbers of colonies."
The researchers argue that since there is such a narrow window after birth when a mole rat can be stolen, the fact that these nappies are documented may mean that the behavior is quite common.
Other forces may also be at play to facilitate the growth of large colonies of attached mole rats, such as the uneven distribution of food resources in their hard, dry habitat.
“Staying safe and finding food are absolutely important,” Braude agrees. Invasions can even drive this, contributing to the success of a society of mole rats. For example, inhabiting a larger tunnel system may mean more access to nutritious tubers that mole rats find and feed underground.
War is not the only strategy that naked mole rats have to increase the geographical and genetic influence of their colony. Some naked mole rats are specially equipped with abundant fat reserves that allow them to travel long air distances. These individuals “scattering morphs” mix with members of other colonies and can establish new colonies.