How rodents the dimensions of bears arrived within the Caribbean

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The history of the Caribbean and the species that inhabit the myriad islands of this part of the ocean is complex. How and when certain groups of animals and plants came to the region is not yet fully understood.

This includes a group of rodents technically known as caviomorphs, which include species such as capybara, guinea pigs, and chinchillas.

At some point in the past, the caviomorphs made their way to the islands of the Caribbean, where they split into at least 33 species, most of which are now extinct. How exactly these species were related to their mainland relatives was unclear.

By researching the ancient DNA from fossils of extinct rodents, a new study has succeeded in unraveling the history of these seafarers.

Dr. Selina Brace, a researcher at the museum, was involved in compiling the history and journey of the rodents, along with colleagues such as Prof. Ian Barnes, Dr. Rosie Woods, who conducted the study as part of her PhD, and Prof. Samuel Turvey from the Institute of Zoology.

“The big thing we found out is that all of the Caribbean species are a single group,” explains Selina. “That means they all have a single common ancestor.”

This shows that after a single arrival approximately 18 million years ago, this group of rodents evolved into large numbers of species living in many different environments. But that’s not all.

“They have also evolved into a number of different sizes, some of which are up to 30 times larger than their relatives,” says Selina.

“This is the largest increase in size in any known living or extinct rodent.”

The paper can be read in full in the journal Molecular Biology and Evolution.

The Caribbean Complex

The geography of the Caribbean is complex as five different tectonic plates meet in the same region. This makes it very difficult to find out where and when different species came to the islands as the country was in such a dramatic change.

One way to find out how animals hopped across the islands is to look at their DNA and use it to determine their relationships. Since most of the species are now extinct, Selina and her colleagues used ancient DNA from the fossils of these rodents to create this image.

But the hot and humid environmental conditions in the Caribbean make things even more complicated.

“It’s hard to work with Caribbean material, especially because it’s so hot,” explains Selina. “Heat is not a friend of DNA, and the hotter the environment, the faster the DNA breaks down. So what remains are these very short fragments that make the job difficult.”

By refining their techniques, the team was able to extract enough DNA from these fossils to compare them to the remaining species that survive in the Caribbean, creating a detailed family tree.

From here they could clearly see a moment when either some rodents or a pregnant person were rafting across open water to live on an island in the archipelago. From there the animals hopped on and switched.

“It is unusual to have just one colonization event,” says Selina. “We have worked on other Caribbean mammals, such as the region’s extinct primates, and they have arrived at not just one, but multiple colonization events.”

In addition, the variety of changes in animal body sizes and shapes is incredible. There are species that lived on the ground and those that lived in trees. The most dramatic differences, however, are those in size: the smallest species only weigh 100 grams, but the largest 200 kilograms, comparable to American black bears.

The process by which a smaller species gets onto an island and evolves into a much larger animal is known as island gigantism and may be due in part to a lack of predators.

“It is also interesting that they have this island gigantism,” continues Selina. “We now know what mainland species these island forms are related to, and we can see that their closest relatives are actually quite small.

“It is unusual for this massive increase to increase.”

Unfortunately, these huge sizes may have contributed to their ultimate extinction. Of a total of 33 species of caviomorphs that once lived in the Caribbean, only 11 survive.

This most likely happened after humans colonized the islands about 5,500 years ago, bringing with them invasive predators and hungry appetites.

Protect the last survivors

Of this earlier diversity, only a handful of native rodent species remain scattered across the islands of the Caribbean. However, their future remains uncertain even today.

“There are 11 species that survive in the Caribbean and the vast majority of them are critically endangered and critically endangered,” explains Selina.

This is due to a combination of factors including habitat destruction and the invasive black rats and mongoose that are common to many islands.

“Now that we’ve shown what an interesting group these rodents are, it really shows that we should put more effort into preserving and protecting these last surviving remains of this absolutely fascinating family,” says Selina.

“Their evolutionary history is quite unusual, so we should work to protect the few surviving species.”