Scientist Traces Rat Migration Using Genetics

"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.

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