Two thousand miles east of the coast of Argentina, Gough Island rises out of the Atlantic Ocean in an awesome display of ancient volcanic activity. A green carpet of windswept mosses and grasses covers 35 square miles of jagged peaks and steeply sloping valleys. Waterfalls spill out of craggy cliffs and fall hundreds of feet to the sea, which runs uninterrupted for another 1,700 miles before crashing into the tip of South Africa. It is one of the most remote places on our planet.

Four miles west of the University of Wisconsin– Madison campus, the Charmany Instructional Facility is a low-slung labyrinth of concrete hallways lined by bright fluorescent lights and permeated with a smell that is equal parts animal and antiseptic. Part of the UW School of Veterinary Medicine, Charmany is nearly half a world away from Gough Island (pronounced “Goff ”). Yet the two locations share a common trait— they both are home to the largest mice on Earth.

In terms of body size and weight, Gough Island mice are twice the size of their mainland cousins, notes Bret Payseur, a geneticist with a joint appointment in CALS and the School of Medicine and Public Health. “The amazing thing about them being twice the size is that they’ve only been on the island a couple of hundred years,” he says. The island’s early rodent settlers were a more moderate-sized strain of Mus musculus, house mice stowaways in the holds of sealing ships from Western Europe. But somewhere along the line, Gough Island mice outgrew that ancestry—doubling in size over the course of only a few hundred generations. “That’s incredibly rapid evolutionary change,” Payseur says. “It’s some of the most rapid that I know about.”

In the canon of origin stories, however, this tale reads more like a mystery. How did the Gough Island mice get so big so quickly? It could be that a genetic mutation proved so advantageous that huge mice became the norm. Or maybe conditions on the island favored preexisting genetic traits that had lain dormant until the mice became castaways. For the time being, however, the Gough mouse story is transcribed only in A’s, T’s, C’s and G’s—the nucleic acids that write genetic code. Payseur hopes to translate that text. What he finds could not only shed light on evolution in action. It could also help illuminate the genetic mechanisms underlying human metabolic diseases like obesity and diabetes.

The Island Rule

While Gough Island mice are unusually large, it isn’t unusual for small animals on islands to grow bigger than their mainland counterparts. The phenomenon is often referred to as the “island rule,” which states that, in general, small animals tend to get bigger and large animals tend to get smaller once they’ve been island castaways for some period of time. There are, of course, exceptions. But from giant Komodo dragons to extinct pygmy mammoths, examples of the island rule run throughout the animal kingdom.

The gigantism effect of this rule seems to be especially pronounced in rodents. Human history is full of daring adventure on the high seas involving fearless mariners and the obligatory stowaways—mice and rats. As a result, the world’s islands are full of transplanted rodents. Biologist J. Bristol Foster first posited the island rule in a 1964 paper in the journal Nature, titled “The Evolution of Mammals on Islands.” In his study, Foster looked at 69 populations of island mice off the coasts of Western Europe and North America. The mice in 60 of those populations were measurably larger than their mainland cousins. Since that study, time and again, scientists find mice and rats on islands that are markedly bigger than genetically similar mainland populations.

This is notable because, in evolution, random genetic mutations or suddenly shifting environmental conditions can lead a species down a certain path. Which means that chance plays a big role in charting a species’ history. “If you ‘run the tape’ once and go back and run it again,” Payseur says, “you would expect different outcomes because of that role of chance.” When patterns like the island rule appear in evolution, he says, “People get very excited. It suggests that what underlies the patterns is a common mechanism that would tell us something important about how evolution works.”

Payseur’s scientific background is anchored in evolutionary biology, and the natural history of species on islands has fascinated him throughout his career. After early work with primates in Madagascar, Payseur realized that, while there is a lot one can do in primate research, keeping captive colonies of lemurs in a lab and breeding the thousands of crosses needed to actually get at answers wasn’t one of them. So he turned his attention to mice.

“The great thing about house mice—and I know most people don’t think house mice are great—is that the strains or lines of mice that people study in the lab are descended from wild house mice, including the wild mice that often inhabit islands,” Payseur says. “So they’re kind of cousins evolutionarily and share a lot of the same traits. That means we can use the genetic tools developed for the lab strains of mice to understand what’s happening in wild mice.”

He’s looking to these small creatures to answer some very big questions. “In the very long term, what I would like to answer with this research is, ‘What types of genetic changes are responsible for the extreme body size on islands?” Payseur says. “Are they the same on different islands? Do we see the same genes popping up over and over again, or do organisms take different paths to get big?”

Knowing that he would have the time, money and resources to deal with only a single strain of island mouse at a time, Payseur decided to start with the most extreme example of the island rule that he could find. He turned to colleagues who studied house mice in the field—and every one of them pointed him to Gough Island.

An Incredible Journey

Most researchers simply order mice via catalog, usually from what Payseur calls “the world center for mouse genetics,” the Jackson Laboratory in Maine. A copy of their glossy catalog lets researchers pick trait-specific lines of mice, from body size and coat color to preassigned conditions like immunodeficiency. Then, simply place an order and wait a few days for the mail to arrive. Gough Island mice aren’t in that catalog. Which means that Payseur had to figure out a way to get mice from an incredibly remote island with a grand total of six to eight full-time human residents, all of whom were busy with their year-long stint staffing the South African National Antarctic Programme’s weather station.

The solution came in the form of an unusual and macabre adaptation of behavior in Gough Island mice. In addition to developing bigger bodies in their few hundred years on the island, they have also developed an appetite for bigger food—the chicks of nesting seabirds, which they, quite literally, nibble to death. Luckily for Payseur, there are quite a few people concerned about those seabirds.

Gough Island is officially a possession of Britain and part of the Dependency of Tristan de Cunha. It is also listed as a World Heritage Site by the United Nations Educational, Scientific and Cultural Organization, which recognizes Gough as a pristine, primarily untouched ecosystem. Its towering cliffs, according to the UNESCO description of the island, “host some of the most important seabird colonies in the world,” from the endangered Tristan albatross to the Atlantic petrel to the Northern Rockhopper penguin. Under such circumstances, a population of non-native, quick-breeding, bird-eating mice is of grave concern—especially to the governments and scientists tasked with preserving the island’s biodiversity.

Peter Ryan, director of the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town, South Africa, says that, especially where petrels and albatrosses are concerned, Gough Island mice are a threat to breeding populations. Ryan has been an honorary conservation officer in the Tristan de Cunha islands since 1989 and has witnessed the decline in seabirds firsthand. When Payseur reached out to him in 2008, Ryan was working with Richard Cuthbert, a scientist at the Royal Society for the Protection of Birds, on a census of sorts to help the British government plan an intervention—or, rather, an eradication.

The mice “were easy enough to catch,” Ryan wrote in an email recalling Payseur’s request. “They occur at very high densities and we’d been live-catching lots of mice to estimate their movements and densities and to conduct poison trials to ensure that all were susceptible to the poison bait.” Ironically, in order to study how best to kill them, the researchers had the live traps, food, bedding and other paraphernalia needed to keep the mice alive for study.

The “big issue” Ryan recalls, was shipping them. Eventually, the crew of the S.A. Agulhas, a South African Antarctic research vessel, agreed to give the mice a lift, but “Even this was a bit tricky, because we had to convince them that the mice wouldn’t be able to escape.” In the fall of 2008, 50 Gough Island mice boarded a boat and took the return trip to the mainland, specifically Cape Town, South Africa. After a lot of paperwork they were sent to Johannesburg, with inspections and quarantines and mountains of paperwork piling up as they made their way by plane to Europe, then to Chicago and, in a final car ride, to the campus of the University of Wisconsin–Madison, where postdoctoral researcher Melissa Gray was waiting.

That September, Gray had just begun her stint in Payseur’s lab. The idea of working with mice excited her, since, as with Payseur’s initial study of primates in Madagascar, the Channel Island foxes she had been working on promised to be a difficult study organism. When a mentor suggested she reach out to Payseur, Gray says, “It was a perfect connection.” She had a background working on island populations and the genetics of size and “Bret already had this project and nobody to work on it.” Plus, she wouldn’t have to wait long to get going. “I started in Bret’s lab in September,” Gray recalls, “and the mice arrived in late October.”

Immediately upon their arrival, the Gough Island mice alleviated any concerns about their suitability as a study subject. “Basically it was a cardboard box with some breathing holes and food stuffed inside,” Gray recalls. But when she opened the box, “It was amazing,” she recalls. Ryan had sent 50 mice off to Wisconsin. Forty-five survived the trip and, even better, they’d managed to produce a couple of litters along the way. They hadn’t even begun their experiment, and already the Payseur Lab was growing a colony of Gough mice. “In a way, we ended up with more than we started with, which is crazy with the amount of stress they were under,” Gray says.

After that initial excitement wore off, the real work began. First, Gray had to randomly breed several sets of mice to ensure that their large size was genetic and not the result of conditions on the island. When those lines came out as big as the wild-born mice, she could turn her attention to creating the first lab-raised line of Gough Island mice, inbreeding some promising strains of mice to create lines that were genetically identical, which makes gene mapping much easier. These mice would then serve as the lab’s breeding colony, slated as mates for lab mice with a mainland heritage.

One way to think about the process—to borrow a metaphor from Mark Nolte, a current postdoctoral researcher in the Payseur Lab—is to imagine two decks of playing cards, one red and the other blue, where each card is a gene. Each deck represents a chromosome, a long strand of DNA wrapped around proteins that carries genetic instructions from a parent to its offspring. When sexual reproduction occurs, each parent contributes a copy of one of their two chromosomes to their offspring.

Imagine the Gough Island mice as having two blue decks of cards—one deck for each chromosome—and the mainland mice as having two red decks. Their initial mating yields what’s called a “filial generation one,” or an F1 baby mouse with two distinct chromosomes, one with all blue cards and the other with all red cards. But when an F1 mouse mates with another F1 mouse, those decks get shuffled. These “filial generation 2,” or F2 mice, hold the first key to untangling the riddle of the evolution of Gough Island’s giant mice.

Breaking the Code

In a small, windowless room at the Charmany Instructional Facility, doctoral candidate Michelle Parmenter lifts two wriggling brown mice out of separate plastic cages by the base of their tails. One is from a line of laboratory mouse with a lineage that runs, if one looks far enough back, to a population of U.S. house mouse. The other is also a strain of laboratory mouse, although it’s of the lab’s own creation—its Gough Island heritage evident in the way it dwarfs its companion when nestled side by side in Parmenter’s hand.

Parmenter, Nolte and a half-dozen Payseur Lab undergrads spend a large portion of their time taking measurements, plopping each of the 480 mice in the room—increasingly inbred descendants of the original Gough mice—one by one into an empty container of French onion dip and putting it on a scale.

Parmenter has slipped on tough blue “bite gloves” before handling the mice— and one mouse’s attempted nibbles remind her why she needs them. “Okay, you’re trying to bite me,” she announces, putting the critter down. “These bite gloves are good, but they’re only so good.”

A smaller mouse, on the other hand, sits meekly in her palm. Parmenter and Nolte say there are a lot of anecdotal differences in behavior between the Gough line of mice and their mainland counterparts. Gough mice scrabble at the corners of their clear plastic cages and frantically scale the grates near their water bottles like monkey bars. The mainland mice spend more time quietly nestled in the shredded paper bedding provided for burrows. When working with the mice, Parmenter and Nolte put them in deep plastic basins, since the Gough mice seem to be strong jumpers and more aggressive. In comparison, says Nolte, “I could work with classical laboratory strains of mice on a level surface and they wouldn’t go anywhere. They wouldn’t even try to escape.”

While they enjoy discussing the potential evolutionary drivers behind some of this observed behavior, what is really exciting to Parmenter and Nolte is what these mice are now telling them at a genetic level.

By crossing mice from Gough and the mainland strain, the Payseur Lab has produced about 1,400 F2 mice. They’ve extracted DNA from each one, sent those samples to a lab for analysis and, in return, received a genomic portrait of each mouse’s DNA. Combing through all of that is a slow process, says Parmenter, but already they are finding hints of the genetic code responsible for their remarkable size.

“Imagine I take the two decks of cards—or ‘chromosomes’—and spread them out, and I can go down each row and say, ‘Oh, there’s a mainland chunk of DNA,’ or ‘Hey, that one came from Gough,’” Nolte says. When you do this enough, patterns begin to appear. “If you take your largest mice and spread their decks, you notice that at the same position on the chromosome they all share the same Gough DNA.” When a big enough percentage of large mice show the same chunk of genes at the same position on the genome, Nolte says, it indicates that, somewhere in the region, there is a gene responsible for size.

That strong association, however, isn’t exactly a smoking gun. When the project began, says Payseur, a prevailing thought was that the rapid evolution in Gough Island mice would be the result of mutations in just a couple of key genes. But in a September 2015 paper in the journal Genetics, the lab published its first genetic mapping results from the F2 crosses, reporting that 19 different sections of the genome appear to play some role in the rapid and extreme size evolution of Gough Island mice. Each of those 19 sections is comprised of anywhere from 400 to 1,400 genes, which means there is much more work to do.

Right now, the process “is not getting at a specific gene,” says Gray, who was the lead author of the Genetics paper. “It’s saying, ‘Okay, this chunk of genome right here somehow corresponds to body size.’ So if you want to tease that apart more, you have to shuffle the deck again. And then shuffle it again.” Keeping your eye on the right card gets difficult. “You really need a lot of samples to get past the noise,” she says, “and that’s a challenge about a project like this. You need a lot of individuals, and that means a lot of money and a lot of time and a lot of mice.”

The Search for a New Island

As the “giant mice” experiment currently stands, the Payseur Lab will, eventually, uncover specific genes that are responsible for the Gough Island mouse’s astounding size, work that could have implications for research on things like human metabolic diseases or even breeding livestock.

“When you look at domesticated animals, size is one of the most important traits because it’s correlated with characteristics like productivity,” Payseur explains. “There’s a lot of interest in CALS in understanding the genetic basis of size variation—in that context it would help select for increased body size and know what genes confer the response. Maybe there’s a more efficient way to ‘build the animal.’”

But if Payseur is to truly unravel the evolutionary mystery of the island rule, he’s going to not only need more time, money and mice—he’s going to need a new island.

The idea is to run the same experiment with another population of large island mice and see if evolutionary patterns emerge. Do some of the same 19 genetic regions his lab has identified show up in those mice, or did they get bigger through a completely different mechanism?

“It would be nice to choose an island because it has similar ecological conditions to Gough that might have driven the same kind of body size increase,” Payseur muses. “But another consideration is, it would be nice to choose an island where the mice have come from a different part of the world. I’m in the throes of figuring that out right now.”

Either way, it’s not a decision that will be made quickly. And the project, which is funded in part by the National Institutes of Health, is slated to run for several more years, meaning that large mice will be calling a UW–Madison lab home for a while.

Gray has already moved on from the project, taking a job as a research scientist at Exact Sciences, a Madisonbased biotech company. Both Nolte and Parmenter realize that they’ll also head elsewhere in their careers before the full story of the Gough Island mice can be translated. But they admit to hoping that they’re still around when the next cardboard box full of large, wild mice arrives in the lab.

“Just knowing that Bret is pursuing a new island population makes us all giddy,” Nolte says.

Payseur shares their excitement, but he knew when he launched the study that he was signing on for what could end up being a career-long project.

“I think that genetics is the most powerful way to answer evolutionary questions,” he says. But getting at answers can be “more complicated than one might imagine,” Payseur admits. “It would be nice to have a simple explanation, but I tend to be attracted to more complicated projects.”

In one respect at least, things might be finally getting a little less complicated for the Payseur Lab: Wherever they turn next for a population of giant mice, the island in question will be a little less remote than Gough. And the mice involved will be a little smaller. And, just maybe, writing the next chapter of this story will be a little bit easier—aided by a key created from the genome of the largest mice on Earth.