Scientist Traces Rat Migration Using Genetics

Oct. 17, 2001 -- Rodney Honeycutt set out to solve a scientific mystery, but he soon found himself up to his eyeballs in rats with a very strange story to tell.

Honeycutt, a professor of biology at Texas A&M, has been studying the evolution of rodents for the past eight years, and he has been particularly interested in the giant rats of South America, some of which can tip the scales at nearly 200 pounds. Genetic evidence shows that these strange rats shared a common ancestor with rats now living in Africa.

For years now scientists have postulated and bickered over how two species of closely related rodents could have found themselves separated by an ocean. Fossil evidence indicates that the rats arrived in South America between 36 million and 40 million years ago.

The Atlantic would have been a bit smaller then, because Africa and South America have been separating for about 100 million years at about the rate of the growth of a human fingernail. It would have still been an ocean, although perhaps about half its current width.

So how did the rats travel across the ocean from Africa to South America?

A Prehistoric Boat Trip?

They could have done it only one of two ways, as far as anybody knows. Either they swam across, which seems preposterous. Or they climbed aboard a raft and followed the currents across the Atlantic to the new world.

"That seems highly improbable," Honeycutt says. But it's the best answer anyone has been able to come up with.The thought of a bunch of rats, manning a raft and sailing across the sea, falls a bit short of an adequate scientific explanation, and it helped set Honeycutt off on a personal expedition several years ago. Perhaps, he reasoned, the South American rats split off from their African cousins much earlier than had been thought, and they were able to cross into South America when the two continents were joined together.

Honeycutt turned to one of the newest tools in the evolutionary sciences, called "molecular evolutionary biology."

When animals or plants adapt to a changing environment they are said to have evolved when the adaptation becomes "fixed" in a DNA molecule, allowing that change to be passed on to succeeding generations. And the rate at which mutations occur remains fairly constant.

"There's a pretty standard rate in which DNA mutates, and it's pretty similar across fairly diverse groups of organisms," Honeycutt says. "Within a group like rodents there's a more or less standard rate at which mutations become fixed."

DNA Timeline May Match Fossil Record

By determining the "rate of fixation" for the rodents on both sides of the Atlantic, Honeycutt came up with a "molecular yardstick, or clock," that allowed him to measure the time since the rats' ancestor diverged into two distinct groups. At that time, all the rats were in Africa, so if he could push that date back quite a bit, it would have been possible for some of the rats to scurry from Africa to South America before the two continents drifted too far apart.

No such luck.

Honeycutt and his graduate student, Diane Rowe, and fellow researcher Ron Adkins of the University of Massachusetts, haven't come up with a precise date, but so far it looks like the evidence will support the fossil record.

Divergence probably occurred around 45 million years ago, long after the two continents drifted far apart. And to that, one expletive comes to mind.

Rats.

"We're still left with trying to explain how on earth they got to South America," Honeycutt says.Since the genetic evidence collected by Honeycutt and his colleagues "clearly supports" the idea that the African and South American rats shared a common ancestor, long after the Atlantic Ocean formed, they must, indeed, have sailed across the sea.

"There doesn't appear to be any other answer," Honeycutt laments.

Of course, that doesn't mean they had modern rafts with navigation equipment. They probably just found themselves trapped on a large chunk of debris, or on logs, and the currents most likely carried them across to their new homes. Once in South America, however, the newcomers set off on a very different course from those they had left behind.

Honeycutt began studying African rodents years ago because he was fascinated by their very complex social behavior. The African rats are similar to pocket gophers in this country, spending most of their time underground. One species called the naked mole rat, he says, "has a social structure that is very similar to bees and ants and termites," known for their hierarchy and sharing of labor.

But once the rats reached South America, they really put on an evolutionary show.

Rats Bigger Than Humans

"These South American rodents are more like ungulates, or large grazing mammals," Honeycutt says. "The ungulates went extinct, and then these rodents experienced major adaptive radiation [spreading into new environments] and more or less filled the ungulate niche.

"So you have some of the largest rodents in the world there, and they are more like big grazing mammals. A lot of them have feet that look very much like deer or an antelope."

And we're talking big here. These rodents, known as "caviomorphs," can weigh more than your average man."That's a pretty good-sized rat," Honeycutt says.

The question over how these critters made it to South America has spawned all sorts of debates with some arguing that they could have made it to Asia and then across to the Americas, but there's no evidence to support that, Honeycutt says. Others have said they shouldn't be considered rodents at all, but the genetic record shows that their close relatives are in Africa, and definitely are rats.

So there it stands.

Unless someone comes up with a very different explanation, we're left with a bunch of rats, somehow sailing across the Atlantic, to start a new life and confound scientists years later.

You've got to love 'em.