June 8, 2000 At face value, blind naked mole-rats appear to be poster rodents for generations of inbreeding. In fact, many biologists who study the adorably ugly, pink, toothy, hairless critters that are the big rage at our nation's zoos long have thought that themselves. There is direct, observable evidence that they will tolerate inbreeding in laboratories and zoos.
But now a biologist at Washington University in St. Louis reports evidence that the rodents are far more adventuresome and sociable than previously thought.
Stanton Braude, Ph.D., Washington University lecturer in biology, studied 54 colonies of the subterranean rodents in Kenya since 1986. Braude has captured, tagged and released more than 8,000 mole- rats, in whole colonies -- some as large as 290 animals. Over a 12-year time span, Braude caught 21 of the tagged animals that had dispersed from their original colonies and had either formed new colonies or migrated into neighboring ones. He also confirmed the development of 20 nascent (new) colonies made of no more than a breeding pair and their fist litter of young. The findings strongly indicate that blind naked mole-rats leave their natal, or birth, colony, and seek new partners with whom to breed.
Braude's discovery is all the more interesting because blind naked mole-rats are eusocial -- they are the only vertebrate species known to have, like bees,ants wasps and termites, a queen, worker-breeder hierarchy. Researchers had assumed that this system goes hand-and-glove with inbreeding, and that splitting of an existing colony was the only way to form new colonies. Moreover, researchers had found scant evidence that small colonies even existed, or that a single breeding pair could be found alone, on the verge of starting a big family. Braude found proof of both scenarios because he is the only researcher to have followed wild colonies for so long.
"The classic explanation for eusociality in ants, bees and wasps is based on higher genetic relatedness between workers than between a queen and her workers," Braude says. "We initially looked to inbreeding in naked mole-rats because it could lead to similarly skewed relatedness patterns."
Braude says that most workers never become sexually mature, another puzzling feature of life in a naked mole-rat colony. Those that do become capable of reproducing are hand-picked for breeding by the queen, the biggest rat in the colony, and the baddest. She will kill unwanted suitors as well as female competitors. In the naked mole-rat scheme of things, there is only one queen per colony, and usually only one breeding male, though some queens will take on a second breeder.
Queens are known for their longevity; they can live for at least 15 years, with one laboratory queen living to 25, believed to be a record for small rodents. Some breeding males last as long as the queens do, but workers generally give out after two or three years.
"My results along with those of fellow researchers at the university of Capetown and Michigan demonstrate that outbreeding is the breeding system of choice for mole-rats," says Braude. "They may tolerate inbreeding because their populations go through frequent bottlenecks, and that selects out genes that would contribute to inbreeding depression."
A population bottleneck occurs in naked mole-rats when a population becomes really small, and then a disperser pair goes out to start a new colony at a distant site.
The mole-rats live in burrows generally a half-foot to roughly a yard below the surface. They have never been observed in daylight in the wild They disperse at night, traveling as far as a mile, an incredible voyage considering their size (roughly between four and eight inches), their blindness and their normal below-ground environment. After capturing a colony, Braude weighs and measures animals and also records length, sex and breeding status. Then he tags and releases the animals.
While most of the dispersers found in the laboratory are male, Braude found that females also start new colonies alone (roughly half of the 21 wild dispensers were female). Previous laboratory research had noted that certain males in colonies are fatter and lazier than others -- they refuse to work. It was theorized that, if mole-rats do disperse and start or join other colonies, they take time to build up a fat reserve for the long voyage. Not the typical picture of a Don Juan in the human world!
Indeed, Braude found that his dispersers were fatter prior to their release than workers of similar length, corroborating the laboratory observation. The proof of the success of outbreeder mole-rats outbreeding for naked mole-rats is now resounding. In 1998, Braude recaptured a disperser who had become the breeding male of a new colony; he had originally dispersed from his birth colony between 1988-89.
"The fact that this fellow was not recaptured for 10 years suggests that we will discover additional cases of successful dispersal as monitoring of this population continues," he says.
Braude reported his findings in the Feb., 2000 issue of Behavioral Ecology. The St. Louis Zoo is supporting his research for another two years.
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