Transpacific Animal Dispersals

From Conquering The Pacific: An Unknown Mariner and the Final Great Voyage of the Age of Discovery, by Andrés Reséndez (HarperCollins, 2021), Kindle pp. 25-27:

Dispersals across the Pacific are more daunting still. Some species do exist on both sides of the Pacific Ocean, as we have seen. Marsupials live in the Americas (opossums and shrews) and in Australasia (kangaroos, koalas, Tasmanian devils, etc.). Intriguingly, a tiny arboreal marsupial from South America known as the monito del monte is more closely related to Australian marsupials than to its American cousins. Could this be the first terrestrial mammal to cross the Pacific? Recent research shows that marsupials originated in South America and migrated to Australia tens of millions of years ago, when there was a land connection via Antarctica or at least great proximity among these three landmasses. The same holds true for other lineages distributed on both sides of the Pacific, including birds, frogs, and turtles.

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The only terrestrial vertebrate that seems to have survived a transpacific passage of six thousand miles is an iguana. The vast majority of iguanas are indigenous to the New World. Yet one genus called Brachylophus lives in the South Pacific islands of Fiji and Tonga. How did it get there? A passage from Central or South America would have taken a minimum of six months and more likely a year or more. Like geckos, iguanas are well suited for oceanic dispersals. They are able to obtain water from the plants they eat and possess nasal salt glands and thick skins that protect them from dehydration. Their presence not only on the American continent but also on many surrounding islands demonstrates their ability to travel across stretches of ocean. The Galápagos Islands, for instance, lie about six hundred miles away from the coast of Ecuador and are home to no fewer than three species of land iguanas as well as one marine iguana that lives on land but dives into the ocean to procure food, foraging on seaweed and reaching exposed rocks completely surrounded by water.

Still, it is one thing to drift on logs for a couple of weeks and quite another to endure a six-thousand-mile passage. After several months adrift and no food left, any voyaging iguana would have perished. Nonetheless, some biologists have proposed a possible solution. The stowaways may have spent much of this journey as eggs. Brachylophus has an unusually long incubation period of seven, eight, or even nine months, one of the longest of any iguana. It is possible then that thirty or forty million years ago an unsuspecting group of iguanas, some in the form of eggs, may have dispersed by means of an epic rafting passage in which everything went right. Yet even if Brachylophus was somehow able to cross much of the Pacific, few other terrestrial vertebrates ever did until humans began making inroads in far more recent times.